Mendel's First Law (Law of Segregation)

What the law states

Gregor Mendel studied inheritance in pea plants and proposed that each characteristic is controlled by a pair of factors, now called alleles. His law of segregation states that the two alleles for a gene separate (segregate) during the formation of gametes, so every gamete receives only one allele of the pair, never both.

Why it happens: the link to meiosis

The physical basis of segregation is meiosis. A body cell such as Tt has two chromosomes carrying the T and t alleles at the same position (locus).

During meiosis, homologous chromosomes separate into different gametes, so half the gametes carry T and half carry t.

• A Tt parent's chromosomes carrying T and t are homologous pairs.
• In meiosis I, the homologous chromosomes are pulled to opposite poles.
• Each gamete produced ends up with only one chromosome of the pair, so only one allele.
• A Tt individual therefore produces two types of gametes, T and t, in equal proportion.

What it means for a cross

Because each parent's gametes carry a single allele, the alleles from both parents combine at random fertilisation to restore the pair in the offspring. This is why a Punnett square works: each gamete cell in the grid carries only one letter, and the offspring genotype is formed by combining one allele from each parent.

How it is examined

SPM questions may ask you to state the law of segregation, explain it using a diagram of meiosis, or explain why a heterozygous parent produces two types of gametes in a genetic diagram.

Worked problem 1 (easy): a heterozygous x homozygous recessive cross

Question. In pea plants, tall (T) is dominant to short (t). A heterozygous tall plant (Tt) is crossed with a short plant (tt).

Using the law of segregation, determine the types of gametes each parent forms and the offspring ratio.

Segregation step. The Tt parent's two alleles segregate during meiosis, so it forms two kinds of gamete, T and t, in equal numbers. The tt parent forms only t gametes.

Worked problem 2 (medium): a heterozygous x heterozygous cross

Question. Two heterozygous tall pea plants (Tt) are crossed. Explain, using segregation, why the offspring appear in a 3 tall : 1 short ratio.

Segregation step. In each parent the T and t alleles segregate, so each parent produces T and t gametes in equal numbers. Fertilisation then combines the gametes at random.

Worked problem 3 (SPM-level): deducing a genotype from segregation

Question. A tall pea plant of unknown genotype is crossed with a short plant (tt). Half the offspring are tall and half are short.

Determine the genotype of the tall parent and explain your answer using the law of segregation.

Reasoning. Some offspring are short (tt), so they must have received a t allele from the tall parent as well as from the short parent. The tall parent must therefore carry a t allele but still look tall, so its genotype is Tt.

Its alleles segregated into T and t gametes in equal numbers, which is why crossing with tt gives a 1 tall : 1 short ratio.

Practice problems

1. A pea plant with the genotype Rr, where round (R) is dominant to wrinkled (r), forms gametes. State the types of gamete it produces and their proportions, and name the process responsible.
2. In guinea pigs, black (B) is dominant to white (b). Cross a heterozygous black guinea pig with a white one, and state the offspring genotype and phenotype ratios.
3. A homozygous tall pea plant (TT) is crossed with a short plant (tt). Explain, using segregation, why no short plants appear in the F1 but reappear in the F2.

Answers to the practice problems

Common misconceptions

Source:SRC-DSKP-EN

Frequently asked questions