• Sex linkage is the phenotypic expression of an allele that is dependent on the gender of the individual and is directly tied to the sex chromosomes.
• In mammals the homogametic sex is female (XX) and the heterogametic sex is male (XY), thus the sex-linked genes are carried on the X chromosome.
• Few examples include:

a) White eye in drosophila ( recessive gene X linked)

b) Color blindness in human (X linked recessive)

c) Hemophilia in human (recessive sex-linked allele)

d) Barred feather patterns in chickens (Dominant sex-linked allele)

e) Hypertrichosis (Y linked)

a) Sex limited traits:

• It is expressed in one of the two sexes only.
• Example: feathering pattern in fowl, beard development in human.

b) Sex influenced traits:

• Sex-influenced traits are autosomal traits that are influenced by sex.
• If a male has one recessive allele, he will show that trait, but it will take two recessive for the female to show that same trait.
• One such gene is baldness. A lot of sex-limited traits can determine parental carriers by using a pedigree.
• Eg: baldness in a man, horns in sheep, spotting in cattle.