Question

which of the following statements about the non - homologous regions of the x and y chromosomes is true?
a. they contain genes unique to either x or y and do not cross over.
b. they contain identical genes shared between x and y.
c. they are inactive during meiosis.
d. they frequently cross over during meiosis.

Explanation:

• Option A: Non - homologous regions of X and Y chromosomes contain unique genes. For example, the X chromosome has genes like those related to color vision (in humans) that are not present in the non - homologous part of the Y chromosome in the same form, and these regions do not cross over frequently (if at all in a non - recombination sense in the non - homologous way).
• Option B: Non - homologous regions do not have identical genes shared between X and Y. Homologous regions (pseudoautosomal regions) have some shared genes, but non - homologous regions have unique gene content.
• Option C: Non - homologous regions can be active. For example, the SRY gene on the Y chromosome (in a non - homologous region) is very active in sex determination during development.
• Option D: Non - homologous regions do not frequently cross over. Crossing over mainly occurs in the homologous (pseudoautosomal) regions of the X and Y chromosomes.

Answer:

A. They contain genes unique to either X or Y and do not cross over.