Mendel‘s Law of

Segregation

Gregor Mendel (1822-1884)Gregor Mendel (1822-1884)

He was a monk andHe was a monk andin his spare time hein his spare time heperformed experiments on performed experiments on garden peas, mice and garden peas, mice and bees in the monastery bees in the monastery gardens. gardens.

He wished to see what He wished to see what would happen when he would happen when he crossed similar organisms crossed similar organisms with different with different

characteristics.characteristics.

Experiments with Pea Plants

- Seed coat colour (grey or white)

- Seed shape (round or wrinkled)- Seed colour (yellow or green)- Pod colour (green or yellow)

- Flower position (axial or terminal)- Pod shape (inflated or

constricted)- Stem length (tall or dwarf)

Cross-Pollination of Purebread Plants

- cross-pollination between true breeding green and yellow pods

- all F1 green

F1 Generation

Gg = heterozygous

F2 Generation

- self-pollination of green F1 plants- ¾ in F2 green,

¼ yellow- 3 : 1 ratio in pod colour in F2

G = dominant = greeng = recessive = yellow

GG, gg = homozygous

Monohybrid inheritanceLet the allele for round seeds be: R (dominant allele)

Let the allele for wrinkled seeds be: r (recessive allele)

Parents phenotype round seeds x wrinkled seeds

genotype RR rr

Gametes

F1 generation

R R r r

F1 phenotypes 100% plants producing round seeds

F1 genotypes 100% heterozygotes Rr

gametes R Rr Rr Rrr Rr Rr

gametes

• F1 intercross

• Parents phenotype round seeds x round seeds

genotype Rr Rr

• Gametes

• F2 generation

R r rR

Phenotype 75% plants producing round seeds

25% plants producing wrinkled seeds

Genotype 25% RR 50% Rr 25% rr

Ratio 3:1 Round seeds: wrinkled seeds

gametesgametes R rR RR Rrr Rr rr

BackcrossTo test whether a plant producing round seeds is homozygous RR or heterozygous Rr it can be crossed with a homozygous rr plant

If plant is homozygous dominant RR

Parents phenotype round x wrinkledgenotype RR rrgametes

Offspring

If plant is heterozygous Rr Parents phenotype round x wrinkledgenotype Rr rrgametes

Offspring

R R r rR r r r

gametesgametes R R

r Rr Rrr Rr Rr

gametesgametes R r

r Rr rrr Rr rr

Offspring

phenotype 100% round

Genotype 100% Rr

Offspring

phenotype 50% round 50% wrinkled

genotype 50% Rr 50% rr

Results from Mendel's Experiments

Parental Cross F1

Phenotype

F2 Phenotypic Ratio F2

Ratio

Round x Wrinkled Seed

Round 5474 Round :1850 Wrinkled

2.96:1

Yellow x Green Seeds

Yellow 6022 Yellow :2001 Green

3.01:1

Axial x TerminalFlower Position

Axial705 Axial :224 Terminal

3.15:1

Tall x Dwarf Plants

Tall l787 Tall :227 Dwarf

2.84:1

Does the observed ratio match the theoretical ratio?

• The theoretical or expected ratio of plants producing round or wrinkled seeds is 3 round :1 wrinkled

• Mendel’s observed ratio was 2.96:1• The discrepancy is due to statistical error• The observed ratio is very rarely the same as the

expected ratio • The larger the sample the more nearly the results

approximate to the theoretical ratio• Statistical tests such as the chi-squared test can be

used to test the closeness of fit of the observed and theoretical ratios

Mendel‘s Generalization

1. Alternative versions of genes account for variations in inherited characters- concept of alleles (G=green, g=yellow)

2. For each character, an organism inherits two genes, one from each parent- two gametes form somatic cells- one allele comes from the mother, one from the father

3. If the two alleles differ, then:- dominant allele is fully expressed in the organism's appearance (phenotype)- recessive allele has no noticeable effect on the organism's appearance (genotype)

4. The two genes for each character segregate during gamete production - ensures variation

Mendel‘s Generalization

Law of Segregation

• 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 

• segregation of alleles occurs during the process of gamete formation (meiosis)

• randomly unite at fertilization