The two alleles of a gene pair are located on which of the following sites of chromosomes?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Homologous sites on homologous chromosomes.
Chromosomes occur in pairs. The two members of a pair are homologous chromosomes: one comes from each parent, and both carry the same genes in the same order. The two alleles of a gene (such as T and t for height) sit at the same place on the two homologous chromosomes. When homologous chromosomes separate in meiosis, the two alleles go to different gametes; this explains Mendel’s law of segregation.
Option (1) Homologous sites on heterologous chromosomes. — Incorrect. Heterologous (non-homologous) chromosomes carry different sets of genes. The partner allele of a gene is not found on another, non-homologous chromosome.
Option (2) Heterologous sites on homologous chromosomes. — Incorrect. Different positions on homologous chromosomes hold different genes. The two alleles of one gene must be at matching positions.
Option (3) Homologous sites on homologous chromosomes. — Correct. The two alleles are at homologous sites on homologous chromosomes, that is, at the same position on the two chromosomes of a pair. That is why they pair up and then separate during meiosis.
Option (4) Heterologous sites on heterologous chromosomes. — Incorrect. Different positions on different chromosomes belong to two separate genes, not to the two alleles of one gene.
NCERT Extract:
Chromosomal Theory of Inheritance – “… Recall that you have studied the behaviour of chromosomes during mitosis (equational division) and during meiosis (reduction division). The important things to remember are that chromosomes as well as genes occur in pairs. The two alleles of a gene pair are located on homologous sites on homologous chromosomes.”