Mendelian Genetics

Mendel 1862

Mendel 1868 Mendel 1880
1862 1868 1880

Genetic Terminology:

  • Trait – any characteristic that can be passed from parent to offspring
  • Heredity – passing of traits from parent to offspring
  • Genetics – study of heredity
  • Alleles – two forms of a gene (dominant & recessive)
  • Dominant – stronger of two genes expressed in the hybrid; represented by a capital letter (R)
  • Recessive – gene that shows up less often in a cross; represented by a lowercase letter (r)
  • Genotype – gene combination for a trait (e.g. RR, Rr, rr)
  • Phenotype – the physical feature resulting from a genotype (e.g. tall, short)
  • Homozygous genotype – gene combination involving 2 dominant or 2 recessive genes (e.g. RR or rr); also called pure 
  • Heterozygous genotype – gene combination of one dominant & one recessive allele    (e.g. Rr); also called hybrid
  • Monohybrid cross – cross involving a single trait
  • Dihybrid cross – cross involving two traits
  • Punnett Square – used to solve genetics problems

Blending Concept of Inheritance:

  • Accepted before Mendel’s experiments
  • Theory stated that offspring would have traits intermediate between those of its parents such as red & white flowers producing pink
  • The appearance of red or white flowers again was consider instability in genetic material
  • Blending theory was of no help to Charles Darwin’s theory of evolution 
  • Blending theory did not account for variation and could not explain species diversity
  • Particulate theory of Inheritance, proposed by Mendel, accounted for variation in a population generation after generation
  • Mendel’s work was unrecognized until 1900

Gregor Mendel:

  • Austrian monk

  • Studied science & math at the University of Vienna

  • Formulated the laws of heredity in the early 1860’s

  • Did a statistical study of  traits in garden peas over an eight year period.

Why peas, Pisum sativum?

  • Can be grown in a small area

  • Produce lots of offspring

  • Produce pure plants when allowed to self-pollinate several generations

  • Can be artificially cross-pollinate

Picture of Pisum sativum
GARDEN PEA

Mendel’s Experiments:

  • Mendel studied simple traits from 22 varieties of  pea plants (seed color & shape, pod color & shape, etc.)

  • Mendel traced the inheritance of individual traits & kept careful records of numbers of offspring

  • He used his math principles of probability to interpret results

  • Mendel studied pea traits, each of which had a dominant & a recessive form (alleles)

  • The dominant (shows up most often) gene or allele is represented with a capital letter, & the recessive gene with alower case of that same letter (e.g. B, b)

  • Mendel’s traits included:

            a. Seed shape —  Round (R) or Wrinkled (r)
            b. Seed Color —- Yellow (Y) or  Green (y)
            c. Pod Shape — Smooth (S) or wrinkled (s)
            d. Pod Color —  Green (G) or Yellow (g)
            e. Seed Coat Color —  Gray (G) or White (g)
            f. Flower position — Axial (A) or Terminal (a)
            g. Plant Height — Tall (T) or Short (t)
            h. Flower color — Purple (P) or white (p)


  •  Mendel produced pure strains by allowing the plants to self-pollinate for several generations
  • These strains were called the Parental generation or P1 strain
  • Mendel cross-pollinated two strains and tracked each trait through two
    generations (e.g. TT  x  tt )
     

                  Trait – plant height

                  Alleles – T tall, t short

    P1 cross    TT  x  tt

    genotype      —    Tt
    t t phenotype    —    Tall
    T Tt Tt genotypic ratio –all alike
    T Tt Tt phenotypic ratio- all alike

  • The offspring of this cross were all hybrids showing only the dominant trait & were called the First Filial or F1generation

  • Mendel then crossed two of his F1 plants and tracked their traits; known as an F1 cross

              Trait – plant height

              Alleles – T tall, t short

F1 cross    Tt  x  Tt

genotype      —    TT, Tt, tt
T t phenotype    —    Tall & short
T TT Tt genotypic ratio —1:2:1
t Tt tt phenotypic ratio- 3:1

  • When 2 hybrids were crossed, 75% (3/4) of the offspring showed the dominant trait & 25% (1/4) showed the recessive trait; always a 3:1 ratio

  • The offspring of this cross were called the F2 generation

  • Mendel then crossed a pure & a hybrid from his Fgeneration; known as an F2 or test cross

Trait   –  Plant Height

Alleles – T  tall, t  short

F2 cross       TT  x Tt

F2 cross       tt  x Tt

T t T t
T TT Tt t Tt tt
T TT Tt t Tt tt

          genotype – TT, Tt

          genotype – tt, Tt

          phenotype  –  Tall

          phenotype  –  Tall & short

          genotypic ratio  – 1:1

          genotypic ratio  – 1:1

          phenotypic ratio – all alike

          phenotypic ratio – 1:1
  • 50% (1/2) of the offspring in a test cross showed the same genotype of one parent & the other 50% showed the genotype of the other parent; always a 1:1 ratio

Problems: Work the P1, F1, and both F2 crosses for all of the other pea plant traits & be sure to include genotypes, phenotypes, genotypic & phenotypic ratios.

  • Mendel also crossed plants that differed in two characteristics (Dihybrid Crosses)
    such as 
    seed shape & seed color

  • In the P1 cross, RRYY  x  rryy, all of the F1 offspring showed only the dominant form for both traits; all hybrids, RrYy

Traits:      Seed Shape & Seed Color

Alleles:     R round                Y yellow
r wrinkled             y green

 P1 Cross:     RRYY          x     r r yy  

      

ry

Genotype:

     RrYy

RY

RrYy

Phenotype:

     Round yellow seed

Genotypic ratio:

     All alike

Phenotypic ratio:

     All Alike
  • When Mendel crossed 2 hybrid plants (F1 cross), he got the following results

Traits:       Seed Shape & Seed Color

Alleles:     R round                Y yellow
r wrinkled             y green

     F1 Cross:     RrYy           x     RrYy                    

RY Ry rY ry
RY
RRYY

RRYy

RrYY

RrYy
Ry
RRYy

RRyy

RrYy

Rryy
rY
RrYY

RrYy

r rYY

r rYy
ry
RrYy

Rryy

r rYy

r ryy

Genotypes Genotypic Ratios

Phenotypes

Phenotypic Ratios

RRYY 1

Round yellow seed

9

RRYy 2
RrYY 2
RrYy 4
RRyy 1

Round green seed

3
Rryy 2
r rYY 1

Wrinkled yellow seed

3
r rYy 2
r ryy 1

Wrinkled green seed

1

Problems: Choose two other pea plant traits and work the P1 and F1 dihybrid crosses. Be sure to show the trait, alleles, genotypes, phenotypes, and all ratios. 

Results of Mendel’s Experiments:

  • Inheritable factors or genes are responsible for all heritable characteristics

  • Phenotype is based on Genotype

  • Each trait is based on two genes, one from the mother and the other from the father

  • True-breeding individuals are homozygous ( both alleles) are the same

  • Law of Dominance states that when different alleles for a characteristic are inherited (heterozygous), the trait of only one (the dominant one) will be expressed. The recessive trait’s phenotype only appears in true-breeding (homozygous) individuals

Trait: Pod Color
Genotypes: Phenotype:
GG Green Pod
Gg Green Pod
gg Yellow Pod
  • Law of Segregation states that each genetic trait is produced by a pair of alleles which separate (segregate) during reproduction

Rr

R r
  • Law of Independent Assortment states that each factor (gene) is distributed (assorted) randomly and independently ofone another in the formation of gametes

RrYy

RY Ry rY ry

Other Patterns of Inheritance:

  • Incomplete dominance occurs in the heterozygous or hybrid genotype where the 2 alleles blend to give a different phenotype

  • Flower color in snapdragons shows incomplete dominance whenever a red flower is crossed with a white flower to produce pink flowers

  • In some populations, multiple alleles (3 or more) may determine a trait such as in ABO Blood type

  • Alleles A & B are dominant, while O is recessive

Genotype Phenotype
IOIO Type O
IAIO Type A
IAIA Type A
IBIO Type B
IBIB Type B
IAIB Type AB
  • Polygenic inheritance occurs whenever many variations in the resulting phenotypes such as in hair, skin, & eye color

  • The expression of a gene is also influenced by environmental factors (example: seasonal change in fur color)