Sex-Influenced Traits
Sex-influenced traits appear more often in one sex than the other. Although these traits may
appear more often in males than in females, they are not sex-linked, because they do not appear
on the sex chromosomes. The genes for sex-influenced traits are on the autosomes not the sex
chromosomes. Sex-influenced traits are influenced by both genes and hormones, not just genes.
As a result, an allele in one sex can be dominant because of the interactions between genes and
hormones, while the same gene in the other sex is recessive because they do not have the same
hormone interactions.
Example: Baldness
Males: B = dominant allele for baldness
b = recessive allele for normal amount of hair
Female: B = dominant allele for normal amount of hair
b = recessive allele for baldness
Hormone: Testosterone is responsible for the phenotypic expression of the different alleles. In
males, the testosterone causes the allele for baldness to be dominant over the normal allele
whereas in females, the minimal amount of testosterone has the opposite effect.
Sex-Linked Traits
Humans have 46 chromosomes. (Camels have 75 chromosomes in case you were wondering.
This will definitely be on your final exam! ). Non-sex chromosomes are called autosomes.
The human body has 44 autosomes and 2 sex chromosomes.
What are the chances of a couple having a boy? What are the chances of a couple having a girl?
Draw a punnett square using X and Y as the alleles contributed by the mom and dad.
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Genes that are carried on the sex chromosomes are the genes responsible for the sex
linked traits.
Most sex linked traits are determined by genes found on the X chromosome, not the Y
chromosome. This is because the Y chromosome bears very few genes in comparison to
the X chromosome which bears a normal complement of genes.
If you are male and have one gene on the X chromosome, it is expressed whether it is
dominant or recessive.
Many times females can be carriers of a gene that causes a disorder, but not necessarily
have the disorder. This is because one of the two X’s is normal and suppresses the
recessive trait.
Males tend to get the disease or disorder that is carried on a sex chromosome.
Example of sex-linked trait: the gene that codes for eye color in fruit flies is found on the X
chromosome. The allele that codes for red (XR) is dominant to the allele that codes for white (Xr).
The Y chromosome has no allele for eye color.
Cross a male with white eye color with a female that is homozygous dominant for red eye color.
genotype of the mother:
genotype of the father:
Draw your punnett square below:
What percent of the offspring will have red eyes?
Will there be any white-eyed females?
Who is the carrier of the white-eyed gene in the F1 generation?
What would happen if you crossed a white-eyed female with a red-eyed male? Explain.
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Other examples of sex-linked traits are hemophilia and colorblindness.
Hemophilia is a blood disorder that prevents the synthesis of a factor required for clotting.
The allele responsible for hemophilia is recessive (Xh), and the normal allele is dominant (XH).
The Y chromosome does not contain a comparable gene, so hemophilia is considered sex-linked.
SAMPLE PROBLEM:
A man with hemophilia marries a homozygous normal woman. Predict the genotypes and
phenotypes of their offspring.
Step 1: What are the genotypes of the parents? Mom:
Dad:
Step 2: Construct a punnett square to show all of the possible offspring genotypes.
What percentage of their children will have hemophilia?
What percentage of their children will be carriers?
Can a male be a carrier of an X-linked disease? Explain.