SEX INFLUENCE TRAIT

15.   SEX INFLUENCE TRAIT

The sex-influenced trait is autosomal. It has a different intensity of expression in two sexes. For e.g. Baldness is a phenotypic characteristics or trait that is manifested in male, this expression depends on oestrogen or androgens and controlled by a single gene that is dominant in males but recessive in female.
For example, hair growth patterns have 2 alleles, one allele produces hair to grow over the head and one that causes the pattern of baldness. A heterozygous male is bald but the heterozygous female is not. Pattern baldness can occur in both males and females. It is much more common in males because the pattern baldness trait is influenced by testosterone hormone.
In pattern Baldness, the gene has two alleles, "Bald" and "Non-bald". The product of these genes is highly influenced by the testosterone hormone. In high concentration of testosterone, the baldness allele has a powerful influence and in the low concentration of testosterone, the allele is quite ineffectual. Male and Female both have testosterone, but males have a too much higher concentration of this hormone than females. That means in males, the baldness allele behaves like a dominant allele and female like a recessive allele. 
In this pedigree, only single-sex is affected by trait. A parent has a dominant effect and another parent has a recessive effect.

(A)    The trait is dominant in males and recessive in females.
(B)    Assume all disorders are homozygotes.
(C)    DD is always affected.
(D)    Dd is affected in males, but normal in females. 
Examples :  (1)    Baldness in human            

Dominant in male and recessive in the female are known as "sex influenced" trait.
Note: Sex influenced autosomal genes (cannot be X or Y linked) are more easily expressed in one gender than another for e.g. In male the gene combination (RR/Rr/rR) will express a trait, but a female will only express the trait with (RR) and not (Rr/rR).