Sex-influenced Sex-influenced TraitsTraits

• Previously, we learned about sex-linked traits, controlled by sex-chromosomes. (x-linked)

• We will now discuss sex-influenced traits, controlled by autosomal chromosomes

What is a sex-influenced trait?

• A sex-influenced trait is controlled by a pair of alleles found on the autosomes. (Not on sex chromosomes)

• Its expression is influenced by gender (presence of hormones such as: estrogen, progesterone, testosterone, etc.).

• Sex-influenced traits vary in the degree of the phenotypic expression.

Example: Pattern Baldness

• It can occur in both males and females, but is influenced by the hormone testosterone.

The combination of alleles lead to different

phenotypic expressions depending on gender.

What is the probability that you will be bald if your father is homozygous and balding, and your mother is homozygous

and not balding?

Father = B’B’ x Mother = BB

• All offspring are BB’

• If you are male, then you will be bald.

• If you are female, then you will not be bald.

Actually, It’s not that simple…

• Many human traits are controlled by several different alleles and environmental factors. These traits are polygenic, such as height. (Actually the man at the right is a pituitary giant at 7’9” due to an uncontrolled amount of growth hormone)

• This means that we cannot fully explain pattern baldness using sex-influenced inheritance. It is not easy to explain that diversity found in different ages of onset, as well as the severity of baldness.

Another Example: Singing voice• The genotype that causes males to have a

tenor voice (highest male singing voice), causes females to have a contralto voice (the deepest female singing voice).

• The genotype that causes males to have deep bass voices, is the same genotype that causes females to have high soprano voices.

Peacock plumage The plumage of a male peacock is highly

decorated and colored, while the plumage of a female peahen is dull by comparison.

A variety of peacock feathers