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What is Mendel's law of segregation?

Class 12 BiologyNCERT chapter: Principles of Inheritance and Variationइसे हिंदी में पढ़ें

Exam-ready answer

Mendel's law of segregation states that during gamete formation, the two factors (alleles) of a pair segregate from each other, so a gamete receives only one of the two. The alleles do not blend. A homozygous parent forms one kind of gamete, while a heterozygous parent forms two kinds in equal proportion.

Explanation

Mendel crossed true-breeding tall (TT) and dwarf (tt) pea plants. All F₁ plants were tall (Tt), yet when they were self-pollinated, dwarf plants reappeared in the F₂ generation. This showed that the factor for dwarfness had neither blended nor been lost in F₁; it stayed intact and separated out again. The law of segregation is based on this observation.

A heterozygous Tt plant produces two kinds of gametes, T and t, in equal proportion (50 per cent each), because the two alleles go into different gametes. At fertilisation these gametes combine at random. The Punnett square for Tt × Tt gives TT, Tt and tt in the genotypic ratio 1:2:1, and tall and dwarf plants in the phenotypic ratio 3:1.

Mendel did not know about chromosomes. It was later noticed that chromosomes also occur in pairs and separate during meiosis, just like Mendel's factors. The two alleles of a gene lie at the same locus on homologous chromosomes, and the separation of homologous chromosomes during meiosis is the physical basis of segregation. The law is also called the law of purity of gametes.

Points to remember

  1. Law of segregation: alleles of a pair separate during gamete formation; each gamete gets only one allele.
  2. Alleles do not blend; the recessive trait hidden in F₁ reappears unchanged in F₂.
  3. Homozygous parent (TT or tt) → one kind of gamete; heterozygous parent (Tt) → T and t in equal proportion.
  4. Monohybrid cross Tt × Tt: phenotypic ratio 3:1, genotypic ratio 1:2:1.
  5. Physical basis: separation of homologous chromosomes during meiosis.

Exam tip

State the law in one line, then show the monohybrid cross TT × tt with a Punnett square for F₂ and write both ratios, 3:1 and 1:2:1.

Questions students also ask

Why is the law of segregation called the law of purity of gametes?
Each gamete carries only one allele of a pair, never both and never a blended form. Because a gamete is 'pure' for one allele of each gene, the law of segregation is also known as the law of purity of gametes.
How is the law of dominance different from the law of segregation?
The law of dominance explains why only one trait appears in the F₁ heterozygote: the dominant allele masks the recessive one. The law of segregation explains how both alleles separate during gamete formation, which is why the recessive trait reappears in F₂.
How does a test cross support segregation?
In a test cross, an F₁ tall plant (Tt) is crossed with the recessive dwarf parent (tt). The progeny show tall and dwarf plants in a 1:1 ratio, which shows that the F₁ plant formed T and t gametes in equal numbers, exactly as the law of segregation predicts.

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