Question

which combination of alleles are always involved in the inheritance of a sex - linked trait?
heterozygous alleles
homozygous alleles
alleles that are carried on x and y chromosomes
alleles that are carried on all chromosomes except the x and y chromosomes

Explanation:

Sex - linked traits are determined by genes located on the sex chromosomes (X and Y). Heterozygous and homozygous alleles refer to the state of alleles on autosomes or sex chromosomes in general (not specifically related to the definition of sex - linked). Alleles on all chromosomes except X and Y are autosomal alleles. Sex - linked traits specifically involve alleles on X and Y chromosomes.

Answer:

alleles that are carried on X and Y chromosomes