Genetics: Mendel's Laws

G. Mendel: pea plant experiments led him to discover the laws of heredity.

Three laws

• First law (uniformity of F₁ hybrids): when pure-line parents (AA × aa) are crossed, the first generation is completely uniform, F₁ (Aa). The dominant trait appears.
• Second law (segregation of traits): from Aa × Aa, phenotype 3:1, genotype 1:2:1.
• Third law (independent assortment of traits): when the dihybrid AaBb × AaBb is crossed, the F₂ phenotype ratio is 9:3:3:1.
• Gametes are "pure": each gamete receives only one allele of each pair.

Law	Cross	Result
I	AA × aa	all Aa in F₁
II	Aa × Aa	phenotype 3:1, genotype 1:2:1
III	AaBb × AaBb	phenotype 9:3:3:1

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Key facts

• Homozygote: AA or aa. Heterozygote: Aa. The dominant trait masks the recessive one.
• The dihybrid AaBb produces 4 types of gametes, each 25%: AB, Ab, aB, ab. F₂ has 16 combinations.
• In incomplete dominance, from Aa × Aa the phenotype and genotype are both 1:2:1.
• Test cross: an organism of unknown genotype is crossed with a homozygous recessive (aa). Aa × aa gives 1:1.
• Number of gamete types: 2ⁿ (n: number of heterozygous pairs).