5 Mendel's first law (law of segregation)

When two purebred individuals that differ in one pair of contrasting traits are crossed:

• all individuals of the first generation (F₁) show only one of the parents' traits, the dominant one (100%);
• when F₁ individuals are crossed with each other, both traits appear in F₂ in the ratio 3 (dominant) : 1 (recessive).

Explanation.

• Each trait is controlled by a pair of alleles.
• During gamete formation the two alleles of the pair separate (segregate), so each gamete carries only one allele.
• At fertilisation, an allele from the father and one from the mother come together to form a new pair.

Example: seed colour (Y = yellow, y = green)

P: pure yellow (YY) × green (yy) Gametes: Y and y F₁: all Yy (yellow), 100% F₁ × F₁: Yy × Yy; gametes Y, y (each parent)

Punnett square:

F₂ genotypes: 1 YY : 2 Yy : 1 yy. F₂ phenotypes: 3 yellow : 1 green. Probability of green seeds = 1/4 = 25%.