Which statement correctly defines an autosome?

• A. Any chromosome that is not a sex chromosome
• B. The sex chromosome
• C. A chromosome that carries only genes for traits
• D. A chromosome that is always X

Answer: (A)

Autosomes are all chromosomes that are not involved in determining sex. In humans, there are 22 pairs of autosomes and 1 pair of sex chromosomes, so autosomes carry the vast majority of genes for a wide range of traits and appear in homologous pairs.

This definition fits the statement because an autosome is defined by what it is not—the sex chromosome. The other ideas mix up what autosomes do or contain: a chromosome that carries only genes for traits isn’t accurate, since chromosomes include many non-coding regions and regulatory sequences as well. And a chromosome that is always X is incorrect because sex chromosomes can be X or Y, while autosomes are the non-sex chromosomes.