Sex-influenced
Traits
• 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 = BB x Mother = B’B’
• 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