M endel‘s Law of Segregation

Gregor Mendel ( ) He was a monk and in his spare time he performed experiments on garden peas, mice and bees in the monastery gardens. He wished to see what would happen when he crossed similar organisms with different 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 = green g = recessive = yellow GG, gg = homozygous

Monohybrid inheritance Let the allele for round seeds be: R (dominant allele) Let the allele for wrinkled seeds be: r (recessive allele) Parentsphenotype round seeds x wrinkled seeds genotypeRR rr Gametes F1 generation RRrr F1 phenotypes100% plants producing round seeds F1 genotypes100% heterozygotes Rr

F1 intercross Parents phenotype round seeds x round seeds genotype Rr Rr Gametes F2 generation RrrR Phenotype75% plants producing round seeds 25% plants producing wrinkled seeds Genotype25% RR 50% Rr 25% rr Ratio3:1 Round seeds: wrinkled seeds

Backcross To 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 wrinkled genotype RR rr gametes Offspring If plant is heterozygous Rr Parents phenotype round x wrinkled genotype Rr rr gametes Offspring RRrr Rrrr phenotype 100% round Genotype 100% Rr Offspring phenotype 50% round 50% wrinkled genotype 50% Rr 50% rr

Results from Mendel's Experiments Parental Cross F 1 Phenotyp e F 2 Phenotypic Ratio F 2 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 Terminal Flower Position Axial 705 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