Mendel's Laws > The Law of Segregation (Law of Purity of Gametes)

Law of segregation states that, when a pair of allelomorphs are brought together in the hybrid (F₁), they remain together in the hybrid without blending but separate complete and pure during gamete formation. 

1. Each pair of alleles separates during gamete formation, with one going into each gamete.
2. No blending occurs; alleles remain pure and distinct.
3. Gametes fuse randomly during fertilisation to form a zygote.

or

When hybrid (F₁) forms gametes, the alleles segregate from each other and enter in different gametes.

"During gamete formation, the two alleles of a gene pair separate from each other. Each gamete receives only ONE allele — it is pure, never a blend of both."

• Also called: Law of Purity of Gametes
• Mendel's 2nd Law of Inheritance (derived from monohybrid cross)
• It is the only universally accepted Mendelian law that has no exceptions.

How Mendel Discovered It?

Mendel crossed true-breeding tall (TT) and true-breeding dwarf (tt) pea plants for 7 years (1856–1863):

The law is physically explained by Meiosis I:

1. Diploid parent cell has two alleles on homologous chromosomes (e.g., T on one, t on the other)
2. During Anaphase I of meiosis → homologous chromosomes pulled to opposite poles
3. Each resulting haploid gamete gets only ONE allele (either T or t)
4. Gametes are therefore "pure" - never carry both alleles.

Parent (Tt) → Meiosis I → [T] gamete (50%) + [t] gamete (50%)

Punnett Square - F₁ × F₁ Cross (Tt × Tt)

• Genotypic ratio → 1 : 2 : 1
• Phenotypic ratio → 3 : 1 (Tall : Dwarf)