In addition to their role in determining sex, the sex chromosomes have genes for many characters.
Genes located on a sex chromosome are called sex linked genes.
In humans the term usually refers to X-linked characters: genes located only on X chromosomes.
Fathers can pass X-linked alleles to their daughters, but not sons.
Mothers can pass sex-linked alleles to both sons and daughters.
If a sex linked trait is due to a recessive allele:
A female will express the phenotype only if she is homozygous recessive.
If a male receives the recessive allele from his mother he will express the phenotype.
Far more males have disorders that are inherited as sex linked recessives than females.
Examples: Colour blindness
X chromosome has a locus for colour vision with two alleles:
XN = Normal colour vision
Xn = Red-green colour blindness
Y chromosome does not have a colour vision locus.
If a male receives the Xn allele he will have impaired colour vision, whereas a female with XNXn will not.
Phenotypes Carrier Female x Normal Male
Genotypes XNXn XNY
Normal Female : Carrier Female : Normal Male : Colour blind Male
1 : 1 : 1 : 1
Complete WS5: Sex-Linked Genes
Complete WS6: Extension questions