Zoology · Ch 10 — Principles of Inheritance and Variation
Law of Segregation
Law of Segregation
The Law of Segregation
The Law of Segregation is Mendel’s first law. It answers a puzzle that troubled biologists for a long time: if a tall plant and a short plant are crossed, why does the first generation (F₁) show only tall plants, yet the second generation (F₂) brings back short plants again? The answer lies in how alleles behave during gamete formation.
Mendel realised that alleles do not blend or mix together. If they did, the F₂ generation would never recover the original short character — it would be lost forever. But Mendel saw that both tall and short plants reappeared in the F₂ in a clear 3:1 ratio. This told him that the two alleles for a trait remain separate and distinct inside the organism, even when one of them is not visible in the F₁.
The key idea is this: during the formation of gametes (eggs and sperm), the two alleles of a gene pair separate from each other. Each gamete receives only one allele from the pair. This is the core of the law — alleles segregate during gamete formation.
Law of Segregation: The two alleles of a gene pair separate (segregate) from each other during gamete formation, so that each gamete carries only one allele for each trait.
How segregation works in different parents
- Homozygous parent (e.g., TT or tt): Both alleles are identical. All gametes produced by such a parent carry the same allele. A TT plant produces only T-bearing gametes; a tt plant produces only t-bearing gametes.
- Heterozygous parent (e.g., Tt): The two alleles are different. During gamete formation, they segregate equally. Half the gametes receive the dominant allele (T) and half receive the recessive allele (t). So a heterozygous parent produces two kinds of gametes in equal proportion.
Why the F₁ shows only one character
In a cross between a homozygous tall (TT) and a homozygous short (tt) plant, the F₁ offspring are all Tt — heterozygous. Because T (tall) is dominant over t (short), the plant appears tall. But the recessive allele t is not destroyed; it is simply hidden. The F₁ plant still carries both alleles, and they remain separate.
Why the F₂ recovers both characters
When two F₁ heterozygotes (Tt × Tt) are crossed, each produces T and t gametes in equal numbers. Random fusion of these gametes gives the following combinations in the F₂:
| Gametes from one parent | T | t |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
The result is:
- TT (homozygous tall) — 1 out of 4
- Tt (heterozygous tall) — 2 out of 4
- tt (homozygous short) — 1 out of 4 …