Sex-Influenced Traits

By : Faizan Ilyas

Sex-Influenced Traits

Basic Terminologies

Alleles : An allele or allel, is one of a number of alternative forms of the same gene.

Autosomal chromosomes : The autosomal chromosomes do not determine the sex of offspring. Rather, they contain many genes that code for the production of thousands of proteins.

  Sex chromosomes : The sex chromosomes determine the sex of offspring. Sex chromosomes determine whether the offspring are male or female.

Lipoma : A lipoma is a benign tumor of fat cells that can cause rubbery tumors of varying size beneath the skin. A benign tumor is a mass of cells that lacks the ability to invade neighboring tissue .

Heterozygous : The genetics term heterozygous refers to a pair of genes where one is dominant and one is recessive.

Homozygous : The genetics term homozygous refers to a pair of genes where both genes are dominant or both are recessive.

Testosterone : It is a steroid hormone which is found in mammals, reptiles, birds and other vertebrates. In mammals, testosterone is secreted primarily in the testicles of males and the ovaries of females. It promotes secondary sexual characteristics such as increased muscle, bone mass, and the growth of body hair.

Where we are heading….

Sex-linked traits are controlled by alleles found on the sex-chromosomes.

Sex-influenced traits 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.

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.

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 in humans (sometimes called “male pattern baldness,” though the condition isn’t restricted to males). This gene has two alleles, “bald” and “non-bald.” The behaviors of the products of these genes are highly influenced by the hormones in the individual, particularly by the hormone testosterone.

. In the presence of high levels of testosterone, the baldness allele has a very powerful influence. In the presence of low levels of testosterone, this allele is quite ineffectual. All humans have testosterone, but males have much higher levels of this hormone than females do. The result is that in males, the baldness allele behaves like a dominant allele, while in females it behaves like a recessive allele.

Sex-Influenced Traits

Male and Female Pattern Baldness

Basic Symbols

Sex-Influenced Trait Assume that the trait is

dominant in males but recessive in females.

Assume all outsiders are homozygotes.

Thus: – DD is always affected– dd is always normal– Dd is affected in males, but

normal in females

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

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