In most animal species, chromosomes can be categorized as two types: autosomes and sex chromosomes. Autosomes are non-sex chromosomes that are the same number and kind between sexes. Sex chromosomes determine if the individual is male or female. Sex chromosomes in the human female are XX; those of the male are XY. Males produce X-containing and Y-containing gametes; therefore males determine the sex of offspring. Besides genes that determine sex, sex chromosomes carry many genes for traits unrelated to sex. X-linked gene is any gene located on $\mathrm{X}$ chromosome; used to describe genes on $\mathrm{X}$ chromosome that are missing on the Y chromosome. Human skin, hair, and eye colour are also polygenic traits because they are influenced by more than one allele at different loci. The result is the perception of continuous gradation in the expression of these traits.