1. Sex-Influenced Traits
Male and Female Pattern Baldness

2. Where we are heading….
In the previous section, we learned about sex-linked traits that were controlled by alleles found on the sex-chromosomes.
In this section, we will discuss sex-influenced traits, traits that are controlled by alleles on autosomal chromosomes.

3. 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.
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.
Estrogen, Progesterone, Testosterone, etc.

4. 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.

5. 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

6. 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.

7. Pattern Baldness What would this bald guy’s genotype be? Bb or bb
What about his balding mother? bb

8. 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.

9. 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

10. 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.

11. 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.

12. 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

13. If only it were that simple!
Hair is controlled by many different factors including several alleles and environmental factors. We will discuss this more when we study polygenic traits.
This means that we cannot fully explain pattern baldness using sex-influenced inheritance. It is not easy to explain the diversity found in different ages of onset, as well as severity of baldness.

14. Another example: Singing voice!
The genotype that causes males to have a high tenor voice causes females to have a deep contralto voice. The genotype that causes males to have deep bass voices is the same genotype that causes females to have high soprano voices!