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 alleles 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 specific to sex - linked inheritance). Alleles on all chromosomes except X and Y are autosomal, not related to sex - linked inheritance.

Answer:

alleles that are carried on X and Y chromosomes