How many alleles do individuals have for a gene?

• A. Two – one on each chromosome; may be the same or different.
• B. Three or more, depending on the gene.
• C. One, since one chromosome is inactive.
• D. Four, corresponding to the four base pairs.

Answer: (A)

For a gene, individuals typically have two alleles. This comes from having two copies of each autosome (one inherited from each parent), so there is one allele at that gene’s locus on each chromosome, giving two copies in total. Those two alleles can be the same (homozygous) or different (heterozygous). The idea that there could be three or more, or that one allele exists because one chromosome is inactive, isn’t accurate for the number of gene copies an individual carries. Also, the number four has nothing to do with alleles; it refers to nucleotide bases, not how many versions of a gene you possess.