Question

Why is the ratio in pleiotropy is 2:1? Explain it with an example.

Answer 1

Hint: Pleiotropy occurs when one gene affects two or more apparently unrelated phenotypic traits. A gene that exhibits multiple phenotypic expressions is called a pleiotropic gene.

Complete answer:
i) When a single gene controls two (or more) different traits, it is called the pleiotropic gene, and this phenomenon is called pleiotropic or pleiotropic. The ratio is 2:1 rather than 3:1.
ii) According to Mendel's Unity Character Principle, one gene (factor) controls one character (trait), but sometimes a single gene produces two related or unrelated phenotypic expressions.
iii) For example, sickle cell anaemia is caused by the Hbs gene. Normal or healthy gene is the dominant HbA gene.
iv) Carriers (heterozygous – HbA/HbS) show signs of mild anaemia as their RBCs become sickle-shaped (half-moon-shaped) in oxygen deficiency. They are said to have a sickle cell trait and are normal under normal conditions.
v) Homozygotes with a recessive Hbs gene, however, die of fatal anaemia.
vi) Thus, the sickle cell anaemia gene is lethal in a homozygous condition and produces a sickle cell trait in a heterozygous carrier.
vii) A single gene produces two different expressions.
viii) Marriage between two carriers will produce normal, anaemic sickle cell and carrier children at 1:2:1 ratio. But sickle cell anaemics that are homozygous to the HbS gene will die, as HbS is a lethal gene that causes the bearer to die. Thus, sickle cell anemia dies leaving carriers and normals in a ratio of 2:1.

When yellow mice were crossed with normal agouti (brown) mice, half yellow and half brown offspring were produced. This suggested that the yellow mice were heterozygous, and that the yellow allele, AY, was dominant in the agouti allele, A. But when two yellow mice crossed with each other, the yellow and brown offspring produced a ratio of 2:1 and the yellow offspring did not grow true (were heterozygous) 2yellow : 1 brown among survivors.

	AY	A
AY	AYAY(dies as embryo)	AYA
A	AyA	AA

Note: The geneticist Gregor Mendel, known for his famous studies with pea plants, first noticed Pleiotropy. Mendel noticed that the color of the plant flower (white or purple) was always related to the color of the leaf axil (a plant stem area consisting of an angle between the leaf and the upper part of the stem) and the seed coat.