Slide 1
Let's look at some examples of X-linked traits in humans as well
as a phenomenon known as X-inactivation.
Two examples of sex-linked traits in humans are red-green colorblindness and hemophilia. The genes that control these traits are found on the X chromosome. Both conditions are recessive so both conditions are rare in females (remember, females need to inherit two copies of the recessive allele to have the trait whereas males only need to inherit a single copy so it is statistically more likely for a male to express the trait).
Red-green colorblindness is the result of a mutation in a gene on the X chromosome that controls a person's ability to distinguish red and green pigments. This condition is rare in women (in the United States about .4% of women have the condition). It is relatively common in males however, with about 7% of the population exhibiting the trait.
Hemophilia A is a blood clotting disorder that is due to a mutation in a gene on the X chromosome. The affected person does not make the proper blood clotting protein and suffers uncontrollable bleeding when injured. This is a very serious condition and historically was a disease that was lethal during childhood (today we have medicines that can help with the symptoms of the disease but it is still a very serious condition).
Slide 2
Genes code for proteins. Therefore, females should have
double the amount of protein for X-linked genes that males do.
But - they don�t. Which is a good thing because double the amount of protein can be lethal.
So, why don�t they?
Slide 3
A phenomenon known as X-inactivation explains why female mammals
don't have double the amount of protein for X-linked genes.
It turns out that only one X-chromosome remains active in each female cell. The other X-chromosome becomes inactivated. The inactivated chromosome condenses into a structure known as a Barr body. This condensed chromosome essentially hides the genes on the chromosome so they cannot be expressed to produce protein.
This inactivation occurs early in development and in humans and many other mammals it is random - meaning that it is equally likely that the X inherited from m will be inactivated as it is that the X inhertied from dad will be inactivated.
This inactivation process is genetically controlled and is due to a gene referred to as "XIST". The XIST gene is found on the X chromosome and codes for its own inactivation. Only one X chromosome expresses the gene and becomes condensed into a Barr body. You can see a a barr body in this image.
This process of X-inactivation has an important consequence for females that are heterozygous because it can lead to a mosaic phenotype - this means a phenotype in which parts of the individual express the gene on one X chromosome and other parts of her body will express the other X's genes. A nice example of this is calico coloration in cats.
Slide 4
Calico cats have patches of orange fur and patches of gray or
black fur (the white coloration is due to a different gene). The gene
that controls orange vs gray is found on the X chromosome. If a female
is heterozygous, this means that she has the allele for orange color on one X
and the allele for gray color on the other. The parts of her coat that
are gray have the X with the gray allele activated and the X with the orange
allele inactivated. The parts of her coat that are orange have the X
with the orange allele activated and the X with the gray allele
inactivated. She has a mosaic or patchy phenotype.
A human example of this phenomenon is a condition known as ecotdermal dysplasia - the recessive allele for this character results in no production of sweat glands. A heterozygous female will have patches of skin that produce sweat glands and patches of skin that do not produce sweat glands. For the woman, this does not typically cause any serious health problems. However, if she passes the recessive allele for the condition to a son - he will be born with the ability to produce sweat glands. This is a very serious condition because sweating is one of the body's most important means of controlling temperature.
While X inactivation is mainly a phenomenon in females - it can occur in males. If a male inherits two copies of the X chromosome and has the chromosome complement of XXY (a condition known as Klinefelter's syndrome) one of his X chromosomes will become inactivated. While rare, it is possible to find a calico male cat.