Sex-Influenced Traits
Male and Female Pattern Baldness
Where we are heading….
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In the previous section, we learned
about sex-linked traits that were
controlled by alleles found on the sexchromosomes.
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In this section, we will discuss sexinfluenced traits, traits that are
controlled by alleles on autosomal
chromosomes.
What is a sex-influenced trait?

A sex-influenced trait is a trait controlled by a pair
of alleles found on the autosomal chromosomes
(pairs 1 through 22) but it’s phenotypic
expression is influenced by the presence of
certain hormones.
Estrogen, Progesterone, Testosterone, etc.

Sex-influenced traits can be seen in BOTH
sexes, but will vary in frequency between the
sexes, or in the degree of the phenotypic
expression.
Example of Sex-Influenced Traits

Pattern Baldness
– Pattern Baldness can occur in both males
and females, however it is much more
common in males.
– Why is this?
Because the pattern baldness trait is
influenced by the hormone testosterone.
Pattern Baldness

The combination of alleles for pattern
baldness will lead to different phenotypic
expressions depending on the sex of the
individual.

For example: Let B represent the non-bald allele
- BB genotype: non-bald in both sexes
- bb genotype: bald in both sexes
- Bb genotype: bald in men; non-bald in females
Pattern Baldness

The “B” allele acts as a dominant allele
in the heterozygous genotype in
females, but acts as a recessive allele
in the heterozygous genotype of the
male.
Pattern Baldness

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What would this bald guy’s genotype
be?
Bb or bb
What about his balding mother?
bb
Try this out!

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’s genotype: bb
X
Mother’s genotype: BB
All offspring are Bb
If you are a male, you will be bald! If you are a
female, you will not demonstrate pattern baldness.
You can solve using Monohybrid
crosses



Complete the simple cross and then express
the phenotype based on whether we are
talking about males or females.
Ex. A heterozygous balding male reproduces
with a heterozygous normal female.
Do the cross and determine the phenotypic
rations for males and females
Let’s try a few problems involving
sex influenced traits…

A male homozygous for clubfoot reproduces
with a normal homozygous female. What are
the genotypes and phenotypes of their
children if testosterone alters the phenotypic
expression of the trait in the heterozygous
expression?

Let F represent normal feet
Let f represent clubfoot.
Let’s try a few problems…

Two heterozygous individuals get married
and have lots of children. The father suffers
with a painful condition called gout. What are
the parent’s genotypes? What are the
genotypes and phenotypes of the children?

Let G represent no gout, and g represent
gout.
How about this one:

Rheumatoid arthritis occurs more
often in females than males due to
the presence of estrogen. A
heterozygous woman marries a
heterozygous male. RR would cause
the condition in both sexes. A
homozygous recessive, rr, genotype
would prevent the disorder in both
sexes