The two alleles of a gene pair are located on -

The correct answer is Option (3) → Homologous Chromosomes

Alleles are the different forms of one gene, such as T (tall) and t (dwarf). A diploid cell has its chromosomes in pairs, one of each pair from each parent.

Option (1) Sex Chromosomes — Incorrect. The sex chromosomes are only one pair, X and Y in humans, and they decide the sex. Most genes do not lie on them, so this cannot be the general rule.

Option (2) Heterologous Chromosome — Incorrect. Chromosomes that do not form a pair are non-homologous. They carry different genes, so the two alleles of one gene are not found on them.

Option (3) Homologous Chromosomes — Correct. Homologous chromosomes are the two members of a pair, alike in size, shape and the genes they carry. The two alleles of a gene pair sit at the same site on the two homologous chromosomes. When the homologues separate in meiosis, the alleles separate too.

Option (4) Autosomes only — Incorrect. The word “only” makes this wrong. Genes on the X chromosome also have two alleles in a female, one on each X.

NCERT Extract:
Chromosomal Theory of Inheritance – …