Posts from the ‘Study Material/Notes class XII’ Category

Class XII Biology Sample paper

12th bio 112th bio 212th bio 312th bio 4


Class XII Sample paper

12th bio sp1
12th bio sp2


[84] Table 2.8. Geologic time scale.*

[85] Table 2.8 (continued)

Table 2-8 Geographic time scale (400dpi, 850 kb)geo time scale


Notes of Human Reproduction+Reproductive Health+Genetics+Evolution


 Sexual Reproduction in human beings involves :
            Gametogenesis            : formation of sperm in male and ovum in female.
            Insemination               : transfer of sperm into female genital tract.
            Fertilization                 : fusion of sperm and ovum to form zygote.
            Implantation                : development of blastocyst and its attachment to uterine wall.
            Gestation                     : embryonic development.
            Parturition                   : birth of the baby.
                        A pair of testis
                        Male accessory gland
                        Male accessory duct.
                        External genitalia
 Testis –A pair of oval testis 4-5 cm. long 2-3cm wide are present in a pouch like structure called scrotum. Each testis has 250 compartment called testicular lobules.  Seminiferous tubules are highly coiled tubes present in lobules which contain two types of cells, male germ cell (spermatogania) and sertoli cellsInterstitial space around seminiferous tubules contains interstitial cell or Leydig cells.
                                    Spermatogenic cell        : develop in to sperm
                                    Sertoli cells                  : provide nourishment to developing sperm.
                                    Leyding cell                : secretion of androgen hormone.
Male Accessory Ducts:These include rete testis, vasa efferentia, epididymis and vas deferens. These tubules form a very long & complicated system for storage and transport mature sperms.A pair of vas deferens ascends to the abdomen and loop over urinary bladder.
 Male Accessory Gland :Includes A pair of seminal vesicles. A prostate gland         A pair of bulbo-urethral gland or Cowper’s gland: also help in lubrication of penis.
External genitalia: It consists of penis though which urethra opens out as urethral meatus. It  has a swollen tip called glans penis covered by membrane called foreskin.
a)      A pair of ovaries. b)      A pair of oviduct or fallopian tubesc)      Uterusd)     Cervix e)      Vaginaf)       External genitaliag)      A pair of mammary gland.
 Ovaries: These are primary female sex organ about 2-4cm long almond shaped, located on each side of abdomen to pelvic wall and uterus. Their function is to produce ovum and ovarian hormones i.e. estrogen and progesterone.
 Oviducts or fallopian tubes : It is about 10-12cm long, extending from funnel shaped infundibulum near ovaries to uterus. Infundibulum leads to wider ampulla which is connect through isthmus to uterus on both sides.
 Uterus : It is also called womb. It is inverted pear shape open into vagina through cervical canal. Uterus wall consist of three layers – perimetrium, myometrium and endometrium. Endometrium undergoes cyclic changes during menstrual cycle while myometrium has smooth muscles which help in parturition.
 External genitalia : In consists of mons pubis, labia majora, labia minora, hymen and clitoris.
 Mammary gland : A pair of Mammary glands produce milk during lactation. Glandular tissue of mammary gland is divided into 15-20 mammary lobes, which contain alveoli. Through lactiferous duct, milk is suck out of mammary gland.
 Gametogenesis:  Gametogenesis involves the formation of haploid gametes in gonads. In male, it is spermatogenesis and in female, it is oogenesis.
                                    Primary Spermatocytes
                                                1st Meiotic division
                                    Secondary Spermetocytes
                                                2nd Meiotic division
                                    Primary Oocyte
                                                1st Meiotic division
                                    Secondary Oocyte
                                                2nd Meiotic division

Site Of Occurrence

Site of occurrence : In the
seminiferous tubules of testes
In the ovaries
Yolk is absent Vitellogenesis occurs in growth
Number of Gametes
Number of gametes – One
spermatogonium forms 4 haploid
One Oogonium forms only one ovum
And three polar bodies
Site Completion
Site of completion – It is started
and completed in the testes.
It is started inside the
ovary but completed outside
Size of Gametes
Size of gametes – Smaller than spermatogonia Ovum is much larger than oogonium.
Menstrual cycle :It involves of cyclic changes in the ovaries which culminate into the periodic vaginal bleeding called menstruation. On an average, it is completed in 28 days. It consists of four phases:-
a)      Proliferative phase
b)      Ovulatory phase
c)      Luteal phase
d)     Menstrual phase
Proliferative Phase : It involves growth and proliferation of uterine endometrium, fallopian tubes and vagina while ovulation occurs.
 Ovulatory phase: It is characterized by rupture of Graafian follicle and release of ovum.
Luteal Phase: Characterized by change of empty Graafian follicle into a corpus luteum which secretes progesterone.
 Menstruation Phase : It is followed by Luteal phase, if the ovum remains unfertilized. If fertilization occurs, it is followed by implantation and normal growth of foetus.
Fertilization and Implantation :
            It includes four steps
a)      Approach of sperm to ovum
b)      Penetration of sperm into ovary
c)      Activation of ovum
d)     Fusion of gametic nuclei
 Semen is released in the vagina during copulation. Fertilization is caused by the fusion of sperm and ova in ampullary – isthmic junction. Sperm causes changes in zona pellucida membrane to prevent polyspermy.
Main significance of fertilization is to restore dipliody.
Implantation leads to the beginning of pregnancy. Cleavage is the process in which the zygote undergoes rapid mitotic divisions to form blastula. The cleavage initially forms a solid ball like morula. Morula changes into blastocyst.Blastocyst enters the uterus about 72 hour after fertilization and gets implanted to endometrium of the uterus on to 7th day.
 Blastocyst consists of inner cells and outer trophoblasts. Trophoblasts help its attachment with the endometrium while inner cells differentiate into ectoderm, mesoderm and endoderm. All major organs get formed till the embryo is 12 weeks from these three germ layers.
Pregnancy and Parturition :
Placenta: The Human placenta is formed by chorionic villi of trophoblast surrounded by uterine tissue and material blood for the developing embryo.
Placenta – (its significance)
Helps in exchange of oxygen and CO2 between the mother and the foetus.
Helps in the elimination of nitrogenous wastes of the foetus.

Acts as an endocrine gland and secrets a hormone such as HCG (Human Chorionic Gonadotropin), Human placental lactogen (hPL), estrogen and progesterone.

Parturition and Lactation:

Parturition: The process of vigorous contraction of myometrium to expel the foetus is called parturition.

 Lactation: Colostrum is the first milk that is secreted from the mother’s mammary glands just after the child birth. It is rich in calories and proteins and also contains antibodies to provide passive immunity to the new born infant. Synthesis of milk is stimulated by the prolactin. Its release is stimulated by oxytocin.
REPRODUCTIVE HEALTH:-  It refers to as a total well being in physical, emotional, behavioral and social aspects of reproduction.
AMNIOCENTHESIS:-  The process of  detection of sex of an embryo is known as amniocentesis. It is carried out by testing of amniotic fluid of the developing foetus . It is a process which is misused to which the female child ratio becomes decreases.

CDRI- Central Drug Research Institute.

Population explosion:- Increasing no. of individual per unit area is known as population explosion. It is due to decreased in (MMR) & (IMR).
*MMR-maternal mortality rate.
*IMR-infant mortality rate.
Birth control:-  The regulation of conception by preventive methods or devices to limit the number of offspring is called birth control.
            It can be carried out by three methods:-
1)      Natural method:-  avoiding foetus during the days from 10-17 days after menstrual cycle in which the changes of ovum formation is high. The automatic seizing of menstruation during the lactation makes a condition known as lactational amenorrhea, in which no ovum is formed(less the chance of fertilization). (a) Safe period:- a week before and a week after menses is considered as the safe period for sexual intercourse. The ideas based on the following facts::
      —ovulation occurs on about 14th day(may be 13th to 16th day) of menstruation.
      —ovum survives for about 1-2 days.
      —sperms remain alive for about 3 days.
This method may reduce the chances of pregnancy by about 80%. However, a great care is needed in its use.
          (b) Coitus Interruptus:- this is the oldest method to birth control. It involves withdrawal of the penis from the vagina by the male before ejaculation so that the semen is not deposited in the vagina and there is no fertilization.
         (c) Lactational Amenorrhea:-   it is based on the fact that ovulation and therefore the menstrual cycle do not occur during the period of  intense lactation following child birth(parturition). This method is considered effective only upto a maximum period of six months following parturition and has no side effects.
 2)      Barrier method:–  by avoiding the contacts sperm and ovum by using physical barriers like cervical caps and intrauterine Device. Also by taking various female hormones in the forms of contraceptive peels.      IUDs It is generally made up of copperand which is commonly known as copper-T which is allowed to present inside the uterus which releases the copper ion which sterilized the active sperm and also disturbs the embryo just after sterilization.
 3)            Surgical method:-  this is also known as sterilization. The mature sperm and ovum are not allowed to meet with each other. It advised for the male/female partner as a terminal method to prevent any more pregnancies. Surgical intervention blocks gamete transport and prevents conception.
          Vasectomyà  sterilization procedure in the male.
         Tubectomyà sterilization procedure in the female.
Intentional or voluntary of pregnancy before the foetus becomes viable is called medical termination of pregnancyor induced abortion. It is a process of destroying the developing embryo due to certain imbalance in health of female. During this process the developing embryo is checked by the application of chemicals or physically disturb by the help of needle after this destroyed cells of embryo sucked out from the uterus. It is safe upto 12 weeks of pregnancy.It is one of the most widely used methods of fertility control. Nearly 45 to 50 million induced abortions are performed in a year in all over the world, which account to 1/5th of the total number of conceived pregnancies in a year.
*Why MTP?
     — MTP is done to get rid of unwanted pregnancies due to:
          (a). Casual unprotected intercourse.               (b). Rapes.
          (c). Failure of contraceptive used during coitus.
SEXUALLY TRANSMITTED DISEASES (STDs)These are the disease which is generated due to disturbances of hormonal secretion or also by various physical agents mainly by bacteria and virus, commonly known as STDs or VD or RTI.Examples:- HIV leading to AIDS, gonorrhea, syphilis, genital herpes, chlamydiasis, genital warts, trichomonasis, hepatitis-B are few common STDs.
                    Though all persons are vulnerable to these infections, their incidence is very high among adolescents in the age group of 15-24 years.
Human Immuno Deficiency Virus
Neisseria gonorrhea(bacteria)
Trepanoma palladium(bacteria)
Chlamydia trachomatis
Genital herpes
Herpes Simplex Virus
Hepatitis Virus
Trichomonas vaginalis(protozoan)

(1). Creating awareness to limit the number of sexual partners, particularly in young people.
(2). Use condoms.
(3). Avoid sharing of injection needles, surgical instruments etc.
(4). In case of doubt, immediately specialists must be contacted for early detection and cure of  STDs, get complete treatment if diagnosed with disease.
INFERTILITYThe process of natural failure of fertilization which takes place due to abnormal hormonal secretion or any genetical failure meant for fertilization. It is inability to conceive and produce children inspite of unprotected sexual cohabitation. Such a phenomenon is termed as infertility.
These are the special techniques to overcome infertility or which assist infertile couples to have children.
(1). “Test tube baby” programme.
(2). Gamete intra fallopian transfer (GIFT).
(3). Intra cytoplasmic sperm injection (ICSI).
(4). Artificial insemination technique (AIT).

WHO   à World health organization.
RCH    à Reproductive and child health care programme.
STD     à Sexually transmitted diseases.
CDRI   à Central Drug Research Institute.
MMR   à Maternal mortality rate.
  IMR     à Infant mortality rate.
IUDs   à Intra Uterine Devices.
MTP   à Medical Termination of Pregnancy.
VD     à Venereal Diseases.
RTI     à Reproductive Tract Infection.
AIDS   à Acquired Immuno Deficiency Syndrome.
PID      à Pelvic Inflammatory Diseases.
ART    à Assisted Reproductive technologies.
IVF      à In Vitro Fertilization.
EF        à Embryo Transfer.
ZIFT    à Zygote Intra Fallopian Transfer.
IUT      à Intra Uterine Transfer.
GIFT    à Gamete Intra Fallopian Transfer.
ICSI     à Intra Cytoplasmic Sperm Injection.
AIT      à Artificial Insemination Technique.
IUI       à Intra Uterine Insemination.

     Principles of Inheritance & Variations.

Principles of inheritance and variation.
* Heredity is the transference of characters from one generation to the subsequent generation i.e. parents    to offspring.
*Inheritance is the process by which characters are passes on from parents to progeny.
*Variation means differences between individuals of same species.
*genetics is the branch of biology which deals with the study of inheritance.
Gregor Johann Mendel
*Gregor Johann Mendel (1822 – 1884) is known as father of genetics.
*Mendel Carried out Hybridization experiments on Garden Pea for 7 years (1856 – 1863).
Mendel selected following characters / traits for his experiments.

01. Plant height Tall(T) Dwarf (t)
02. Flower Position Axial (A) Terminal (a)
03. Pod Shape Full / Inflated (F) Constricted (f)
04. Pod Colour Green (G) Yellow (g)
05. Flower Colour / Seed Coat Colour Violet / Red (V or R) Grey White (v or r) / White
06. Seed Shape Round (R) Wrinkle (r)
07. Seed Colour Yellow (Y) Green (y)

Law of Dominance:
a) Characters are controlled by discrete units called factors.
b) Factors occur in pair.
C) In a dissimilar pair of factors one member of the pair dominates (dominant) the other (recessive).
d) In F1 generation only one allele is able to express its effect in the individual.

 Law of Segregation:The two allelomorhic characters of an individual do not get mixed up by they segregate during gamete formation. Each gamete receives only one character of the two allelomorphs and a pair condition is restored by random fusion of gametes during fertilization.
Test Cross: It is a cross between F1 hybrid and the recessive parent.To confirm the purity of F1 hybrid, whether it is homozygous, a test cross is done.
a) If the test cross yields off springs of 50% dominant and 50% recessive character, then the F1 hybrid is heterozygous. It shows (1:1) test ratio for monohybrid cross and (1:1:1:1) for dihybrid cross.
b) If the test cross yields all the dominant character, the F1 hybrid is homozygous.
 Back Cross:
            Which a inter cross is done between two genetically different parents, a hybrid is produced which may be homozygous are heterozygous. To determine the purity of parents & to test genotype of F1 Hybrid, a cross is made between F1 hybrid and on of the parent. Such a cross is known as back cross.

Exception of Principle of Dominance:
 Incomplete Dominance: When two parents are inter crossed with each other, the hybrid that produced does not resembled either of the parents but is midway between the two parents(dominant and recessive parents), i.e., the expression of the character in a hybrid or F1 individual is intermediate on a fine mixture of the expression of the two factors.

            The phenomenon of incomplete dominance occurs in four ‘O’ Clock plant (Mirabilis jalapa) & Snapdragon(Antirrhinum majus) and Andalusian fowls.

 Co-Dominance: when both alleles of a pair are fully expressed in a heterozygote, the genes and trait are said to be co-dominant.

AB Blood Group.
Sickle Cell AnaemiaC) Multiple Alleles: More than two alternate forms of a gene present on the same locus are called multiple alleles.
            Example; Inheritance of blood group.
  Principle of Law of Independent Assortment:
            When two pairs of independent alleles are brought together, they show independent dominant effects. During the formation of gametes the genes of different characteristics are independent of one another.
Chromosomal theory of Inheritance:
            Walter Sutton and Theodor Boveri proposed the Chromosome theory of Inheritance. Main features of chromosome theory of inheritance:
            Nucleus contains chromosomes. Therefore, chromosome must carry the heredity traits.
Gametes contain only one chromosome of a type and only one of two alleles of trait.
The pained condition of both chromosomes as well as Mandelian factors is restored during fertilization.
Each chromosome contains numerous genes & the positions assigned to each gene is called locus. These genes help the organism to develop from the Zygote.
Each chromosome retains its individuality, uniqueness & continuity throughout the life of an organism and from generations to generation. They never get last or mixed up but behave as units.
  Linkage & Recombination:
            A chromosome contains large number of genes in a linear order and these genes belonging to a particular chromosome tend to be inherited together. This tendency of genes to remain together during the process of inheritance is called as linkage. Morgan (1990) proved and defined linkage on the basis of his breading experiments in fruitfy (Drosophila melanogaster).
Morgan etal observed that when two genes in a dihybrid cross were situated on the same chromosome, the proportion of parental gene combination were much higher than the non parental types.
Morgan attributed this to the physical association of the two genes which he referred as linkage and the term recombination to the non-parental combinations different from the parental types.
 Linkage maps or Chromosomes Maps:
A linkage or genetic chromosome map in a linear respiration of the sequence & relative distances of the various genes present in a chromosome. The first chromosome maps were prepared by Sturtevant in 1911 from two chromosome of Drosophila.

Determination of Sex:

Chromosome theory of sex put forwarded by Wilson and Stevens (1950) and named X and Y bodies as sex chromosomes, X and Y.

It is of the following types:
XX – XY type found in human beings.
XX – XO type found in grass hoppers, cockroaches and bugs.
ZW –ZZ type found in birds and some reptiles.
ZO – ZZ type found in butterflies & moths.

14. Genetic Variations:

Mutation – it is the sudden, discontinuous and heritable change which alters the phenotype of an individual. The term “mutation” was coined by Hugo de Vices (1901).
Types of mutation due to change in structure of chromosome.
Chromosomal mutation may alter the structure of chromosomes.
Deletion (Deficiency) – A segment of chromosome separate and lost.
Duplication – It is the phenomenon of having an extra chromosome segment attached to a normal segment.
Inversion – A segment of chromosome separates and rejoins at in an inverted position. It results in the change in sequence of nitrogenous base or genes in chromosome.
Point mutation is the abrupt change in gene structure due to change in a single base pair of DNA due inversion and substitution, without changing the reading of subsequent bases.
Example: Sickle Cell Anaemia.
Frame shift mutation is the elimination or addition of one or two base pairs or a segment of DNA in a gene. It leads to chain of triplet codon on m-DNA that is responsible for polypeptide chain. It results in lateral sifting of entire reading frames from the site of mutation.
            Thalassemia – an inherited blood disorder is an example of frame shift mutation.
15. Pedigree Analysis:
It is a chart to represent the data collected from a family over a number of generations for a certain genetic trait by using international conventional symbol. It reveals the ancestral history of an individual and its possible genotype for a trait.
16. Human Genetic Disorders:
            a) Mendelian Disorders:
            I) Sickle Cell Anaemia: Sickle cell Anaemia is an autosomal hereditary disorder in which
The erythrocytes form biconcave disc to elongated sickle like structure under low oxygen deficiency as during strenuous exercise and at high altitudes.
                                         The disorder or disease is caused by the formation of abnormal haemoglobin
called Haemoglobin-S which differs from normal Haemoglobin-A in only one Amino Acid
– 6th amino acid of B-Chain, glutamic acid is replaced by valine.
II) Phenylketonuria: it is an inborn error of metabolism which is inherited as autosomal recessive trait. The affected individual do not produced phenylalanine hydrolase which converts phenylalanine to tyrosine. So, in the absence of this enzyme, phenylalanine accumulates in the blood & produces toxic effect on CNS resulting irreversible brain damage, severe mental & physical retardation.
III Haemophilia: It is a sex linked disorder and it is also known as bleeder’s disease as the patient will continue to bleed even from a minor cut since he or she does not possess the natural phenomenon of blood clotting due to absence of anti haemophilic globulin or factor VIII and plasma thromboplastin factor IX essential for it.

b) Chromosomal Disorders:
I) Down’s syndrome:              a) Trisomy of chromosome no.21.
b) Short stature, Round head, Furrowed Tongue and retarded Development.
II) Turner’s Syndrome:           a) Absence of one of the X chromosome (karyotype of 45 chromosomes)
b) Sterility, Lack of secondary sex characters.
III) Klinefelter’s Syndrome:   a) Additional copy of X chromosome.
b) Sterility, masculine development with some feminine features & development of breast, i.e., Gynaecomastia.
Off spring: An individual produced by sexual reproduction.
 Alleles: Slightly different or the various forms of a gene.
 Factors: Physical, discrete, definite, stable bodies that carry characters from parents to off springs.
 Homozygous: A pair of homologous chromosomes carries similar alleles of a particular character.
 Heterozygous: A pair of homologous chromosomes carries dissimilar alleles for a particular character.
 Genotype: The genetic constitution of an organism.
 Phenotype: The observable characteristics of an organism.
 F1 generation: The first hybrid generation produced in monohybrid and dihybrid crosses etc.
 F2 Generation: Generation produced on self pollination of the F1 individuals.
 Emasculation: Removal of anthers from a bisexual flower,
before the pollen grains mature.
 Diploid: A condition in which each cell contains a single set of all the type of chromosomes found in an individual.
 Haploid: A condition in which each body cell contains a single set of all the type of chromosomes found in an individual.
 Autosomes: Normal chromosomes other than sex- Chromosomes.
 Recombination: A new grouping of genes which is different from the parental types.
 Gene pool: The sum total of all the genes and their alleles present in an interbreeding population.
 Heterosis: presence of better qualities in a hybrid than either ofthe parent.
 Monosomy: Condition where a particular chromosome ispresent in a single copy in a diploid nucleus.
 Trisomy: Condition where a particular chromosome is present in three copies in three copies in a diploid nucleus.
 Gene: basic unit of heredity.
 Pleiotropy: The ability of a gene to have more than onephenotypic effect.
 Multiple allelism: A gene exists in more than two allelic forms.
 Linked genes: all the genes present on a chromosome.
 Back Cross: A cross performed between hybrid and one of its parents.
 Reciprocal cross: cross involving two types of organisms where the male of one type is crossed with female of the second type and vice versa.

 Genetics: The study of Heredity and variations.

  • Evolution
    Ø  The origin of life is considered a unique event in the in thehistory of universe.
    Ø  Evolutionary Biology: Study of History of life forms on earth including origin of life and evolution of life.
    Ø  Origin of Life: The appearance of life for the first time on the earth.
    Ø  Natural selection: To better fit in an environment, acts over a number of generations and slowly increase the number of proportion of individuals.
    Ø  Artificial selection: Carried out by man to select better breeds of organisms. Viviparous:Animals giving birth to young ones.
    Ø  Big Bang Theory: According to this theory, the whole matter in the beginning in the universe was concentrated in the form of a dense hot fireball and the universe arose from a huge explosion of a very large entity. Each was formed about 4.5 million years back.
    Ø  Genetic Drift: Random changes in gene frequencies in a small population occurring by chance rather than by natural selection.
    Ø  Founder: Original drifted population.
    Ø  Saltation: Single step large mutation
    Ø  Founder effect: When new populations rapidly become genetically distinct, due to large changes in allele frequency and become different species.
    Ø  Adaptive radiation: Common ancestor gives rise to new  species that are adapted to new habitats and ways of life.
    Ø  Gene pool: The sum total of different kinds of genes pooled by all the members of a population.
    Ø  Biogeography: The study of patterns of distribution of plants and animals in different parts of the earth.
    Ø  Theory of special creation: Life was created by supernatural power.Paleontology: Study of fossils.
    Ø  Theory of spontaneous generation: Life originated from nonliving things in a spontaneous manner.
    Ø  Theory of Panspermia: Life migrated to earth from outside heavenly body in the form of the spores.
    Ø  Parallel Evolution: Convergent evolution found in closely related species.
    Ø  Analogous Organs: Organs which have same fundamental structure but different in their structural details and origin.
    Ø  Analogous organs such as wings of bats and grasshoppers.
    Ø  Tendril of Pea (Modified Leaf) and tendril of grape vine (modified stem).
    Ø  Homologous Organs: Organs which have same fundamental structure but different in functions.
    Ø  Homologous Organs such as forelimbs of cattle and human.
    Ø  Thorns and tendrils of Bougainvillea and Cucurbits.
    Ø  Biogeography: It is the study of distribution of plants and animals in different parts of earth.
    Ø  Coacervates: Covacervates are an aggregate of spherical molecules bounded by liquid covering and divide by budding.
    Ø  Adaptive Radiation: It is the process of different species in a given geographical area starting from a point and radiating to the other areas of geography.
    Ø  Natural Selection: The process of selection of the individuals which are well adapted to the changed environmental condition.
    Ø  Divergent Evolution: Development of organs in different groups of organisms along different directions due to adaptation to different needs. e.g.
    Ø  Convergent Evolutions: Development of organs in different groups of organisms for the same function due to their common adaptive needs. E.g.
    Ø  Mimicry: Resemblance of an organism with another on with a non-living object in form colour pattern and behavior to escape notice by predator or prey. The organism that bears resemblance is called a mimic, and the organism or inanimate object to which a mimic resembles is termed model. E.g. Leaf Insect, Stick Insect.
    Oparin-Haldane Theory of Origin of Life
    The experiment of Miller & Urey:
    Urey and Miller (1953) demonstrated that the electrical discharges are heat energy can form the complex organic substances from the mixture of Methane, Ammonia, Water, and Hydrogen. They took glass tubes, flasks, condensers, etc., for their experiment. They created an atmosphere containing Hydrogen, Ammonia Methane and water vapour in one flask and allowed the condensed water in another flask and condenser. They passed the electric sparks from electrodes in the gaseous chamber of flask and heated the another flask containing water. They passed the mixture of these gases through the condenser. After a week, they analyzed the liquid for chemical composition inside the apparatus. They found large number of complex organic compounds such as Acetic Acid, Urea, Fatty Acids and Lactic Acid including Amino Acid like glycine, alanine, and aspartic acid. So they called this process of abiotic synthesis. Other investigators added sugar, purines, and pyrimidines to the list of spontaneously formed chemicals by using UV-light and they found fairly complex building bricks of which living matter is composed of.
    Hardy-Weinberg Principles:
    Hardy-Weinberg principles states:
    Allele frequencies in a population are stable and is constant from generation to generation.The gene pool (total genes and their alleles in a population) remain” constant. This is called genetic equilibrium.Sum total of all the allelic frequencies is 1.
    For example, in a diploid individual, p and q represent the frequency of allele A and allele a. The frequency of AA individuals in a population is simply p2. Similarly, the frequency of aa is q2 and of Aa is 2pq. Hence, p2 + 2pq+ q2 = 1. Thus is a binomial expansion of (p + q )2.
    When frequency, measured, differs from expected values, the difference (direction) indicates the extent of evolutionary change. Disturbance in genetic equilibrium or Hardy Weinberg equilibrium i.e. change of frequency of alleles in a population would then be interpreted as resulting in evolution.
    Different effects of Natural selection on Variation:
    Nature selection is the different reproduction leading to differential contribution of genotypes to the gene pool of the next generation.
    The three different effects of natural selection on variation are—-
    If both the smallest and largest individuals contribute relatively fewer offspring to the next generation than those closer to the average size. Then stabilizing selection is operating. This selection reduces variation but does not change the mean value.
    If the individuals at one extreme of the size distribution contribute, then the mean size of individuals in the population will increase. In this case directional selection is operating.
    When the natural selection favours individuals at both extremes of the distribution, then disruptive election I operating. Thus two peaks are produced in the distribution of a trait.

    Human evolution
    Ramapithecus – It was man like. Few fossils were discovered in Ethiopia and Tanzania.
    Australopithecus – They in East African grasslands. They hunted with stone weapons but ate
    Homo habilis – It was the first human being like creature. Brain capacity was between 650 –
    Homo erectus – Fossils were found in Java. It had the brain capacity of about 900c.c.
    Homo sapiens – Fossils found in Africa and Asia.
    Modem Homo sapiens – Modem man, aroused between 75,000-10,000 years ago. They spread
    all over the globe and learned to cultivate plants and domesticate
    animals. Agriculture started around 10,000 years ago

Molecular basis Of Inheritance

posted Dec 23, 2010 9:12 PM by jaleelkabdul jaleel   [ updated Dec 24, 2010 4:20 AM ]

posted Dec 19, 2010 6:52 PM by jaleelkabdul jaleel   [ updated 13 minutes ago ]

Nucleic Acids: The polynucleotide chains of very high molecular weight are called nucleic acids. They are the genetic material of lining.
1. Deoxyribonucleic Acid (DNA):
# DNA is a long polymer of deoxyribonucleotides.
# the length of DNA is defined as the number of nucleotides present in it.
2. Structure of DNA:
* DNA is formed of number of nucleotides units.
* Each nucleotide has a nitrogenous base, a pentose sugar and inorganic phosphate.
* Nitrogen basis occur in DNA belonging to tw0o groups, purine and pyrimidine.

            * DNA has two double ring of purines (Adenine-A and Guanine-G) and two single ring pyrimidine (Cytosine-C and Thymine-T)
* A nitrogenous base is linked to the pentose sugar through a N-glycosidic linkage to form a nucleoside.
* When a phosphate group is linked to 5 – OH of a nucleoside through phosphodiester linkage, a corresponding nucleotide is formed.
* The two chains are held together by two hydrogen bonds between adenine-A with Thymine-T and by three hydrogen bonds between Guanine-G with Cytosine-C.
* The two DNA chains are antiparallel that is, they run parallel but in opposite directions. In one chain the direction 5’ -> 3’ while in the opposite one it is 3’ -> 5’.
3. Double Helix Model of DNA.
* DNA is double helix and formed of two polynucleotide chains which are coiled with one another in a spiral.
* The nucleotides in a polynucleotide chain are linked together by phosphodiester bond.
* The two chains of DNA have anti parallel polarity 5’ -> 3’ in one and 3’ -> 5’  in other.
* Nitrogen bases of two polynucleotide chains form complementary pairs, A opposite to T & G opposite to C.
* The helix has a constant diameter of 20Ao (2nm) throughout its entire length.
* The pitch of helix in 3.4nm (34Ao) with roughly 10base pairs in each turn. The average distance between base pairs comes to about 0.34nm.

4. DNA as Genetic Model: (Griffith’s Experiment).
* It is the change in the genetic constitution of an organism by picking up genes present in the remains of its dead relatives.
* Transformation was first studied by S.F Griffith in1928 while studying on bacterium Streptococcus pneumoniaecalled pneumococcus.
* The bacteria has two strains – the smooth form(s) secretes a polysaccharide capsule which gives the colonies a smooth appearance and virulent. Another form is non-capsulated which gives the colonies a rough appearance and is not virulent (R)
*Griffith tested the virulence of the two strains by injecting the live R-type and live S-type separately into mice, he found that R-type bacteria were non-pathogenic while the S-type caused the death in the mice.
*Heat killed S-type bacteria into mice and they survived equally well
* In the last, he injected a mixture of heat killed “S” & live “R” simultaneously, the mice died with the symptoms of pneumonia. Living type S bacteria were recovered from their bodies.


This happened because something from the dead bacteria had entered the live ones & made them virulent.
   * He concluded that the R-strain bacteria had been transformed by the heat killed S-type, which must be due to the transfer of the genetic material (Transforming Principle).

5. Hershey and Chase experiment.

Alfred Hershey and Marth Chase (1952) indicated that DNA is the genetic material and not the protein.
* Viruses that infect bacteria is called bacteriophage.
* The bacteriophage attaches to the bacteria & its genetic material enters the bacterial cell by dissolving the cell wall of bacteria.
* They grew some viruses on a medium that contained radioactive phosphorus
   and other on medium that contained radioactive Sulphur.
* Virus grown in the presence of radioactive Phosphorus contained radioactive DNA but not radio active protein because DNA contains phosphorus but protein does not.
* Similarly viruses grown on radioactive Sulphur contained radioactive protein but not radioactive DNA because DNA dose not contain Sulphur.
* Radioactive phases were allowed to attach to E.coli bacteria then as the infection preceded the viral coats were removed from bacteria by agitating them in a blender. The virus particles were separated from the bacteria by spinning them in a centrifuge.
* Bacteria that were infected with virus had radioactive DNA were radioactive, indicating that DNA was the material that passed from the virus to bacteria. Bacteria that were infected with the viruses that had radioactive proteins were not radioactive. This indicates that proteins did not enter the bacteria from the  viruses. DNA is therefore the genetic material that is passed from virus to bacteria.

6. Mode of DNA Replication
DNA replication occurs during S-phase of cell cycle.
Mechanism of DNA replication is as follows:

* The starting point where replication of DNA begins at a specific point were inter wind DNA segments start unwinding called origin of replication.
* Since the two strand cannot be separated in its entire length (due to very high energy requirement) replication occurs with small opening of the DNA-helix; the Y shaped structure formed is called replication fork.
* The DNA-dependent DNA-polymerases catalyze polymerization of the nucleotides only 5’ -> 3’.
* Consequently on one of the template strands (with 3’ -> 5’ polarity) the synthesis of DNA is continuous, while on the other template strand (with  polarity 5’ -> 3’), the synthesis of DNA is discontinuous, i.e. short stretches of
   DNA are synthesized known as Okazaki fragments.
* The discontinuously synthesized fragments are later joined by the enzyme  DNA-ligase.
* As one strand grows continuously while the other strand is formed discontinuously DNA replication is semi discontinuous.

7. Transcription
* The process of copying genetic information from one strand of the DNA into RNA is called transcription.

* Principles of complementarities govern the process of transcription; the exception is that uracil is incorporated instead of thymine opposite adenine of template.
* Only a segment of DNA is transcribed and that only one of two strands is copied.
* Both the DNA strands cannot be copied in transcription because that will produce two types of proteins, one with correct sequence of amino acids and the other with reverse sequence of amino acids.
* Further if two complementary RNA’s are produced simultaneously, they would have a tendency to form double stranded RNA resulting in non translation of coded information into proteins. The whole exercise of transcription would then appear futile.
* Transcription Unit. The segment of DNA that takes part in transcription unit it has three components:
            1) A promoter,
            2) The Structural gene, and
            3) A terminator.
Besides a promoter, eukaryotes also require an enhancer.
* Promoter is located upstream of structural gene. By convention it is called 5′ end (of coding strand which is 3’ end of template strand).
*Terminator region is present down stream of structural gene at the 3’ end (of coding strand which is actually 5’ end of the template strand).
* Structural gene is component of that DNA strand which has 3’ – > 5’ polarity (as transcription occur only in 5’ <– 3’ direction). This strand of DNA is called template strand.
* The other strand which has a polarity of 5’<– 3’ is displaced during transcription. This non-template strand which does not take part in transcription is also called coding strand or plus(+) because genetic code present in this strand is similar to genetic code (based on mRNA) except that uracil is replaced by thymine.

8. Genetic Code:

The relations hip between the sequence of amino acids in a polypeptide and nucleotide sequence of DNA or mRNA is called genetic code. There are 64 codons in the genetic code. Out of 64 codons UAA, UAG & UGA do not represent to any amino acid and are called non-sense or termination codons.
Salient features of genetic code:
•         It is triplet (made of 3 letters).
•         Non-Ambiguous & specific codons. One codon codes for only one amino acid and
•         not any other.
•         The code is degenerative. More than one codon can code for same amino acid. Example, Phenylalanine has two and arginine has six codons.
•         The code is comma less. There is no punctuation between any of the codon triplets.
•         The code is non-overlapping. It means that same latter cannot be used for two different codons.
•         The code is nearly universal. Exceptions are these in mitochondrial codons, and in some protozoa’s

•         AUG has dual functions. It codes for Methionine (met), and it also act as initiating codon.

9. Protein Synthesis:
Mechanism of Protein synthesis in E.coli is given below:

•         The entire process of protein synthesis can be divided into two stages- Transcription and Translation.
•         Amino acids are regarded as building blocks of proteins.
•         Carboxyl group (-COOH) of one amino acid is bonded with amino group (-NH2) of another amino acid by a Peptide bond.
•         Several amino acids are linked by Peptide Bonds to form Poly Peptide chain.
•         The process of transmission of Genetic information from template strand of DNA to m-RNA is called Transcription .The site of transcription is nucleus.
•         It requires DNA template, nucleotide triphosphates, RNA polymerase, & Mg or Mn ions.
•         Transcription begins with uncoiling of the two strands of DNA. One strand of DNA acts as a template for m –RNA formation. By the action of RNA polymerase m-RNA is formed according to the triplet code of DNA by copying process.
•         m–RNA comes out of the nucleus and attaches itself to the ribosomes.
•         Attachment of m- RNA TO RIBOSOMES
•         Activation of amino acids
•         Initiation of amino acids
•         Polypeptide chain elongation

•         Polypeptide chain termination
•         Arrangement of amino acids in specific sequence & formation of polypeptide chain according to the information present in m-RNA is called translation.
•         Site of translation is ribosomes.
•         Translation is a complex process which involves the participation of ribosomes, m-RNA, t-RNA& amino acids
Attachment of mRNA to ribosomes

•         mRNA binds with 30 s ribosomal sub unit with its initiation codon AUG:
•         At times GUG also acts as initiation codon.
•         The group of ribosomes  present on the mRNA molecule is called poly ribosome


•         Amino acids are scattered in the cytoplasm , they are in inactive state.

•         They are activated by  specific aminoacyl t-RNA Synthetase enzymes,ATP is also required

•         Amino acids      +   ATP                                             AA – AMP complex +PP
Amino acyl t-RNAsynthetase

•         These enzymes possess two binding sites one for amino acid and the other for its t-RNA.
•         Initiation of polypeptide chain
•         It requires amino acyl t- RNA complex ,m-RNA with initiation codon,30 s and 50 s subunits of ribosomes,GTP& initiation factors.
•         With the help of anticodon UAC, t-RNA carries FORMYL METHIONINE and attaches to AUG codon of m- RNA and forms initiation complex .

•         Both are linked together with hydrogen bonds.
•         t-RNA is linked to m- RNA at decoding site.

 POLYPEPTIDE CHAIN ELONGATIONFormyl methionine moves from decoding site. A second amino acid is brought to A site by another aminoacyl t-RNA.A peptide bond is formed between the amino acids
Water is formed in this reaction .The movement of dipeptide from A site to P site is called Translocation.
•         Terminator or non sense codons (UAA, UAG, &UGA) present on the m-RNA strand signal the termination of polypeptide chain.

•         The last m-RNA and t-RNA are also set free.
•         Ribosomes dissociates into 30 S and 50 S subunits.

 10. The Lac-Operon:

* The Lac-Operon consists of one regulatory gene (i), three structural genes(z,y,a), operator gene(o) & promoter gene(p).
* In Lac-Operon the regulator gene is called i-gene because it produces an inhibitor or repressor. The repressor binds to operator gene and turn off the Operon. It exerts a negative control over the working of structural genes.
Structural genes are those genes which actually synthesize mRNAs. The Lac-Operon of E-Coli contains three structural genes (z, y, a).
* The ‘z’ gene codes for b-galctosides which is primarily responsible for the hydrolysis of the disaccharide, lactose into its monomeric units galactose and glucose. The ‘y’ gene codes for permease, which increases permeability of the cell to b-galctosides. The ‘a’ gene encodes a transacetylase.
Operator gene is a gene which directory controls the synthesis of mRNAs over the structural gene. It is switched off by the presence of a repressor. An inducer can take away the repressor and switch on the gene. When switch on the structural gene synthesis of polypeptide chain, i.e., transcription and translation occur.
Promoter gene acts as an initiation signal which functions as recognition centre for RNA polymerase provided the operator gene switch on. RNA polymerase is bound to the promoter gene. When the operator gene is functional, the polymerase moves over it and it reaches the structural genes to perform transcription.
The repressor of the operon is synthesized from the i-gene. The repressor protein binds to the operator region of the operon and presents RNA polymerase from transcribing the operon. In the presence of an inducer such as lactose or allolactose, the repressor is inactivated by interaction with the inducer. This allows RNA polymerase access to the promoter and transcription proceeds.

Inducer regulator switching on of the operon. The inducer of Lac-operon of Escherichia coli is lactose (actually allolactose).
Human Genome Project (HGP)

Human Genome Project was called a mega project. The project was a 13-years project coordinated by the U.S Department of Energy and the National Institute of Health. The Project was completed in 2003. HGP was closely associated with the rapid development of a new area in biology called as Bioinformatics.

 Goals of HGP

Some of the important goals of HGP were as follows:

(i)           Identify all the approximately 20,000-25.000 genes in human DNA;
(ii)         Determine the sequences of the 3 billion chemical base pairs that make up human DNA;
(iii)       Store this information in databases;
(iv)       Improve tools for data analysis;
(v)          Transfer related technologies to other sectors. such as industries;
(vi)       Address the ethical, Legal, and social issues (ELSI) that may arise from the project. 
Salient Features of Human Genome Project

1.            Human genome has 3164.7 billion nucleotide bases.
2.            The average gene size is 3000 base pairs with sizes vary much. The largest gene being dystrophin at 2.4 million base pairs.
3.            The human genome consists of about 30,000 gene much lower than previous estimates to contain 80,000 to 1,40,000 genes.
4.            The function of over 50% of discovered genes is unknown.
5.            Less than 2% of the genome code for proteins.
1.                  99.9% of the nucleotide bases are exactly similar in all human beings.
2.                  At about 1.4 million locations occur single nucleotide differences called SNPs (snips) or single nucleotide polymorphism. They have the potential to help find chromosomal locations for diseases associated sequences and tracing human history.
3.                  Repetitive sequences are nucleotide sequences that are repeated many times, sometimes 100 to 1000 times. They have the direct coding functions but provide information’s as to chromosome structure, dynamics and evolutions.
4.                  Chromosomes I has most genes (2968) and Y has the fewest (231).
   Applications of Future Challenges of HGP
 1.                  Detection of Cancers: Efforts are in progress to determine genes that will change cancerous cells to normal.
2.                  Healthy Living: It will indicate prospects for a healthier living, designer drugs, genetically modified diets and finally our genetic identity.
3.                  Knowledge of Interactions: it will be possible to study how various genes and proteins work together in an interconnected network.
4.                  Study of tissues: all the genes or transcripts in a particular tissue, organ or tumor can be analyzed to know the cost of effect produced in it.
                     DNA Finger Printing
1.                  Alee Jeffrey’s (1984) invited the DNA fingerprinting technique at Leicester University. U K. Dr. V. K. Kashyap and Dr. Lalji Singh started the finger printing technology in India.
2.                  DNA Finger printing is a technique to identify a person on the basis of his / her DNA specificity by the means of their digital / palmer print. Each person has a unique DNA finger print.
Principles of DNA Printing:
1.                  Based pairs of DNA by their differences, about 0.1% or 3 X 106 base pairs (out of 3 X 106  bp) provided individuality to each human being. DNA fingerprinting involves identifying differences in specific regions in DNA sequence called as Repetitive DNA because in these sequences a small stretch of DNA is repeated many times. These repetitive DNA are separated from the bulk genomic DNA as different peaks during density gradient centrifugation. The bulk DNA forms a major peak and the other small peaks are referred to as Satellite DNA
2.                  Depending upon length, base composition and numbers of repetitive units, satellite DNAs has subcategorizes like micro satellites and minisatellites.
3.                  Satellite DNAs show polymorphism.
4.                  If a variant at a locus is present with a frequency of more than 0.01 populations, it is called DNA polymorphism.
5.                  Variations occur due to mutations, therefore, DNA polymorphism is the occurrence of mutations in a population at high frequency.
6.                  Short nucleotide repeats in the DNA are very specific in each individual and vary in number from person to person but are inherited. These are Variable Number Tandem Repeats (VNTRs) also called minisatellites.
7.                  Each individual inherits these repeats from his / her parents which are used as genetic markers in a personal identity test.
Techniques of DNA Finger Printing:
1.                  The DNA is extracted from the nuclei of white blood cells or of spermatozoa or of the hair follicle, skin, bone, saliva, etc.
2.                  The DNA molecules are first digested with help of enzyme restriction endo-nuclease that cuts them into fragments. The fragments of DNA also contain the VNTRs.
3.                  The fragments are separated according to size by gel electrophoresis.
4.                  VNTRs are multiplied through PCR technique.
5.                  The separated fragments of DNA in the gel are copied on to a nylon paper by Southern Blotting Technique.
6.                  Special DNA-probes are made in the laboratories which contain repeated sequence of bases complementary to those on VNTRs. These probes are made radioactivity by labeling with radioactive isotopes. The radioactive DNA-probes bind to the repeat sequences on the nylon membrane.
7.                  An X-ray film is exposed to the nylon membrane to mark the places where the radioactive DNA has bound to the DNA fragments. These places are marked as dark bands when X-ray film is developed. This is known as autoradiography.
8.                  The dark bands on X-ray film represent the DNA fingerprints (DNA Profiles)
Application of DNA fingerprinting:
1.                  In forensic laboratories for identification of criminals.
2.                  Paternity disputes to find real parents can be solved by DNA fingerprinting.
3.                  It is useful in determining population and genetic diversities.
4.                  It is used to study the breeding patterns of animals facing the danger of extinction.
  • Codon: Sequence of three nitrogenous bases on mRNA, that code for a particular amino acid.
  • Anticodon: Sequence of three nitrogenous bases on tRNA that
  • is complementary to the codon for the particular amino acid.
  •  Bacteriophage: Viruses which infect bacteria.
  • Operon: A transcriptionally regulated system consisting of structural gene, operator gene, promoter gene and regulator gene.
  • Nucleotide: A complex biomolecule formed by the condensation of pentose sugar, base and one to three phosphoric acids.
  • Polymer: A large biomolecule formed by the linking small units the monomers.
  • Inducer: A chemical substance which regulate switching on and off the operator gene besides also act as substrate.
  • Splicing: Process of removal of introns and joining of exons in HnRNA.
  • Template: A strand which acts as model for the synthesis of a new strand.
  • Nucleosome: A repeating unit of chromatin structure formed by
  • mistune octamer and wrapped around DNA.
  • Central Dogma: Unidirectional flow of genetic information from
  • DNA — RNA——-Proteins.
  • Promotor: A site, at which RNA polymerase is held.
  • Coding Strand: Strand of DNA which does not code for anything.
  • Translation: Process of polymerization of amino acids to form a peptide under the direction of base sequences in m-RNA.
  • Introns: Non-coding segments of eukaryotic structural gene.
  • Exons: Coding segments of eukaryotic structural gene.
  • Euchromatin: Light stained loosely packed transcriptionally active chromatin.
  • Heterochromatin: Dark stained compactly packed, inactive.
  • Semi conservative: Type of replication in which DNA molecule has one parental and one newly synthesized strand.
  • Genetic Code: A triplet base sequence regulating the sequence  of amino acids in a polypeptide

Double Fertilisation in Angiosperms

Double Fertilisation

After entering one of the synergids, the pollen tube releases the two male gametes into the cytoplasm of the synergid. One of the male gametes moves towards the egg cell and fuses with its nucleus thus completing the syngamy. This results in the formation of a diploid cell, the zygote. The other male gamete moves towards the two polar nuclei located in the central cell and fuses with them to produce a triploid primary endosperm nucleus (PEN) (Figure 13a). As this involves the fusion of three haploid nuclei it is termed triple fusion. Since two types of fusions, syngamy and triple fusion take place in an embryo sac the phenomenon is termed double fertilisation, an event unique to flowering plants. The central cell after triple fusion becomes the primary endosperm cell (PEC) and develops into the endosperm while the zygote develops into an embryo.

(a) Fertilised embryo sac showing zygote and Primary Endosperm Nucleus (PEN); (b) Stages in embryo development in a dicot [shown in reduced size as compared to (a)]



The Theory of Evolution

In science, theories are statements or models that have been tested and confirmed many times.

Theories have some important properties:

  • They explain a wide variety of data and observations
  • They can be used to make predictions
  • They are not absolute, they serve as a model of understanding the world and can be changed as the world view changes

In science, the term “Theory” does not express doubt.

In science, the term theory is used to represent ideas and explanations that have been confirmed through tests and observations

The theory of evolution remains one of the most useful theories in biology because it explains many questions and observations.

Some questions that can be answered by evolution.

  • Why do so many different animals have the same structures, the arm bones in a human are the same bones as a flipper in a whale?
  • Why do organisms have structures they no longer use, like the appendix in a human? Non functioning wings in penguins
  • Why are there bones and fossil evidence of creatures that no longer exist? What happened to these creatures?
  • Why do so many organisms’ morphology and anatomy follow the same plan?
  • Why is the sequence of DNA very similar in some groups of organisms but not in others?
  • Why do the embryos of animals look very similar at an early stage?
The Theory of Evolution is considered a Unifying Theory of Biology, because it answers many of these questions and offers and explanation for the data.

Lamarke’s Theory of Acquired Characteristics

Some thought that you would gain or lose features if you overused or didn’t use them, and you could pass these new traits onto your offspring.

This was known as the Inheritance of Acquired Characteristics

  • A lizard that didn’t use it legs would eventually not have legs and its offspring wouldn’t have legs
  • A giraffe stretched its neck to reach higher leaves, and this stretched neck would be a trait inherited by its offspring

Lamarke’s Theory was eventually discarded – PROVEN TO BE WRONG!

Why? Logically it doesn’t work. Imagine if you were in a car accident and had a leg amputed. This does not mean that your children will only have one leg. Features gained during life are not passed on to children.

Darwin’s Theory of Evolution by Natural Selection

Darwin was a naturalist who observed many species. He is famous for his trips to the Galapagos Islands, his observations of the finches (and other animals) and the book he wrote: “The Origin of Species:

1. Variation exists among individuals in a species.
2. Individuals of species will compete for resources (food and space)
3. Some competition would lead to the death of some individuals while others would survive
4. Individuals that had advantageous variations are more likely to survive and reproduce.

This process he describes came to be known as Natural Selection
The favorable variations are called Adaptations

Darwin’s Finches: 

Darwin noted that all the finches on the galapagos island looked about the same except for the shape of their beak. His observations lead to the conclusion that all the finches were descendents of the same original population. The shape of the beaks were adaptations for eating a particular type of food (Ex. long beaks were used for eating insects, short for seeds)

Evidence of Evolution

1. Fossil Evidence

  • If today’s species came from ancient species, the we should be able to find remains of those species that no longer exist.
  • We have tons of fossils of creatures that no longer exist but bear striking resemblance to creatures that do exist today.
  • Carbon dating–gives an age of a sample based on the amount of radioactive carbon is in a sample.
  • Fossil record-–creates a geologic time scale.

2. Evidence from Living Organism

  • Evidence of Common Ancestry –Hawaiin Honeycreeper
  • Homologous Structures–structures that are embryologically similar, but have different functions, the wing of a bird and the forearm of a human
  • Vestigial Organs–seemingly functionless parts, snakes have tiny pelvic and limb bones, humans have a tail bone
  • Biochemistry and DNA
  • Embryological development–Embryos of different species develop almost identically
  • Observation of species change (wolves/dogs, peppered moths)

Mendel’s Genetics

Mendel’s Genetics

photo of 3 domesticated horses pulling a plow

Hybridized domesticated horses

For thousands of years farmers and herders have been selectively breeding their plants and animals to produce more useful hybrids click this icon to hear the preceding term pronounced.   It was somewhat of a hit or miss process since the actual mechanisms governing inheritance were unknown.  Knowledge of these genetic mechanisms finally came as a result of careful laboratory breeding experiments carried out over the last century and a half.

photo of Gregor Mendel 

Gregor Mendel

By the 1890’s, the invention of better microscopes allowed biologists to discover the basic facts of cell division and sexual reproduction.  The focus ofgenetics click this icon to hear the preceding term pronounced research then shifted to understanding what really happens in the transmission of hereditary traits from parents to children.  A number of hypotheses were suggested to explain heredity, but Gregor Mendel click this icon to hear the name pronounced, a little known Central European monk, was the only one who got it more or less right.  His ideas had been published in 1866 but largely went unrecognized until 1900, which was long after his death.  His early adult life was spent in relative obscurity doing basic genetics research and teaching high school mathematics, physics, and Greek in Brno (now in the Czech Republic).  In his later years, he became the abbot of his monastery and put aside his scientific work.

photo of 3 pea pods and peas

Common edible peas

While Mendel’s research was with plants, the basic underlying principles of heredityclick this icon to hear the preceding term pronounced that he discovered also apply to people and other animals because the mechanisms of heredity are essentially the same for all complex life forms.

Through the selective cross-breeding of common pea plants (Pisum sativum) over many generations, Mendel discovered that certain traits show up in offspring without any blending of parent characteristics.  For instance, the pea flowers are either purple or white–intermediate colors do not appear in the offspring of cross-pollinated pea plants.  Mendel observed seven traits that are easily recognized and apparently only occur in one of two forms:

1.    flower color is purple or white 5.    seed color is yellow or green
2. flower position is axil or terminal        6. pod shape is inflated or constricted
3. stem length is long or short 7. pod color is yellow or green
4. seed shape is round or wrinkled

This observation that these traits do not show up in offspring plants with intermediate forms was critically important because the leading theory in biology at the time was that inherited traits blend from generation to generation.  Most of the leading scientists in the 19th century accepted this “blending theory.”  Charles Darwin proposed another equally wrong theory known as “pangenesis” click this icon to hear the name pronounced.  This held that hereditary “particles” in our bodies are affected by the things we do during our lifetime.  These modified particles were thought to migrate via blood to the reproductive cells and subsequently could be inherited by the next generation.  This was essentially a variation of Lamarck’s incorrect idea of the “inheritance of acquired characteristics.”

Mendel picked common garden pea plants for the focus of his research because they can be grown easily in large numbers and their reproduction can be manipulated.  Pea plants have both male and female reproductive organs.  As a result, they can either self-pollinate themselves or cross-pollinate with another plant.  In his experiments, Mendel was able to selectively cross-pollinatepurebred click this icon to hear the preceding term pronounced plants with particular traits and observe the outcome over many generations.  This was the basis for his conclusions about the nature of genetic inheritance.

structures of
drawing of a flower cross-section showing both male and female sexual structures

In cross-pollinating plants that either produce yellow or green pea seeds exclusively, Mendel found that the first offspring generation (f1) always has yellow seeds.   However, the following generation (f2) consistently has a 3:1 ratio of yellow to green.

diagram showing the result of cross-pollination in the first 2 offspring generations--in generation f1 all are yellow peas but in generation f2 the ratio of yellow to green peas is 3 to 1

This 3:1 ratio occurs in later generations as well.   Mendel realized that this was the key to understanding the basic mechanisms of inheritance.

diagram showing the result of cross-pollination in the 3rd offspring generation--the offspring of the 2nd generation green peas are all green, the offspring of one third of the 2nd generation yellow peas are all yellow, the offspring of the other 2nd generation yellow peas are green or yellow in a 3 to 1 ratio

He came to three important conclusions from these experimental results:

1.   that the inheritance of each trait is determined by “units” or “factors” that are passed on to descendents unchanged      (these units are now called genes click this icon to hear the preceding term pronounced)
2. that an individual inherits one such unit from each parent for each trait
3. that a trait may not show up in an individual but can still be passed on to the next generation.

It is important to realize that, in this experiment, the starting parent plants were homozygous click this icon to hear the preceding term pronounced for pea seed color.  That is to say, they each had two identical forms (or alleles click this icon to hear the preceding term pronounced) of the gene for this trait–2 yellows or 2 greens.  The plants in the f1 generation were all heterozygous click this icon to hear the preceding term pronounced.   In other words, they each had inherited two different alleles–one from each parent plant.  It becomes clearer when we look at the actual genetic makeup, or genotype click this icon to hear the preceding term pronounced, of the pea plants instead of only the phenotype click this icon to hear the preceding term pronounced, or observable physical characteristics.

diagram of genotypes of pea plants in 3 generations after cross-pollination

Note that each of the f1 generation plants (shown above) inherited a Y allele from one parent and a G allele from the other.  When the f1 plants breed, each has an equal chance of passing on either Y or G alleles to each offspring.

With all of the seven pea plant traits that Mendel examined, one form appeared dominant over the other, which is to say it masked the presence of the other allele.  For example, when the genotype for pea seed color is YG (heterozygous), the phenotype is yellow.  However, the dominant yellow allele does not alter the recessive green one in any way.   Both alleles can be passed on to the next generation unchanged.

Mendel’s observations from these experiments can be summarized in two principles:

1.   the principle of segregation
2. the principle of independent assortment

According to the principle of segregation, for any particular trait, the pair of alleles of each parent separate and only one allele passes from each parent on to an offspring.  Which allele in a parent’s pair of alleles is inherited is a matter of chance.  We now know that this segregation of alleles occurs during the process of sex cell formation (i.e., meiosis click this icon to hear the preceding term pronounced).

illustration of the segregation of alleles in the production of sex cells

Segregation of alleles in the production of sex cells

According to the principle of independent assortment, different pairs of alleles are passed to offspring independently of each other.  The result is that new combinations of genes present in neither parent are possible.  For example, a pea plant’s inheritance of the ability to produce purple flowers instead of white ones does not make it more likely that it will also inherit the ability to produce yellow pea seeds in contrast to green ones.  Likewise, the principle of independent assortment explains why the human inheritance of a particular eye color does not increase or decrease the likelihood of having 6 fingers on each hand.  Today, we know this is due to the fact that the genes for independently assorted traits are located on different chromosomes click this icon to hear the preceding term pronounced.

These two principles of inheritance, along with the understanding of unit inheritance and dominance, were the beginnings of our modern science of genetics.  However, Mendel did not realize that there are exceptions to these rules.  Some of these exceptions will be explored in the third section of this tutorial and in the Synthetic Theory of Evolution tutorial.

By focusing on Mendel as the father of genetics, modern biology often forgets that his experimental results also disproved Lamarck’s theory of the inheritance of acquired characteristics described in the Early Theories of Evolution tutorial.  Mendel rarely gets credit for this because his work remained essentially unknown until long after Lamarck’s ideas were widely rejected as being improbable.