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Mendel‘s Law of Segregation Gregor Mendel (1822-1884) 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) Parents phenotype genotype Gametes F1 generation round seeds RR R x wrinkled seeds rr R r r gametes gametes R R r Rr Rr r Rr Rr F1 phenotypes 100% plants producing round seeds F1 genotypes 100% heterozygotes Rr • F1 intercross • Parents phenotype genotype round seeds x round seeds Rr Rr R • Gametes • F2 generation Phenotype r R r gametes gametes R r R RR Rr r Rr rr 75% plants producing round seeds 25% plants producing wrinkled seeds Genotype 25% RR 50% Rr 25% rr Ratio 3:1 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 If plant is heterozygous Rr Parents phenotype round x wrinkled genotype RR rr gametes R R r r Parents phenotype genotype gametes Offspring Offspring gametes gametes R R r Rr Rr r Rr Rr Offspring round x wrinkled Rr rr R r r r gametes gametes R r r Rr rr r Rr rr Offspring phenotype 100% round phenotype 50% round 50% wrinkled Genotype 100% Rr genotype 50% Rr 50% rr Results from Mendel's Experiments F1 Phenotyp e 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 Terminal Flower Position Axial 705 Axial : 224 Terminal 3.15:1 Tall x Dwarf Plants Tall l787 Tall : 227 Dwarf 2.84:1 Parental Cross 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 Mendel‘s Generalization 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 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