Saturday, February 26, 2011

Are We There Yet? (The Joy of Maps)

Before I go into too much depth, I'll give credit where credit is due.

We've recently been studying the madness of chromosome mapping over the past few days, so I thought I'd give a quick overview. So...here we go!

Chromosomes can be viewed as similar to a thumb. A thumb has two separate regions, split by a knuckle, one of which is clearly longer than the middle. Similarly, a chromosome has two arms, a p arm (shorter) and a q arm (longer). These arms are split by a notch known as the centromere.

Bizarre analogies aside, let's take some time to look at a specific gene on the X chromosome: Xq28.

Let's look at what each subset of this cytogenetic locus means:

X

This simply means that this particular gene (MRX28) is located on the X chromosome (more on the significance of this later!).

q

This means that the gene is on the longer of the two arms of the chromosome--the q arm.

28

This means that this gene is on the band labeled 28. Chromosomes, when stained, show different bands. This is caused by the differing ways in which the DNA is wrapped.

Now, I chose a gene on the X chromosome for a reason. Genes on this particular chromosome are known as "x-linked." Abnormalities on the X chromosome are always apparent in males because there is not a dominant allele on the X chromosome to mask the presence of the mutant allele. For an example of this, consider color blindness. Men are color blind for red and green more often than women because the gene(s?) for detecting red and green light is on the X chromosome, and men only have one copy of the X chromosome, so defections are not masked.

The MRX28 gene I mentioned earlier is one that has been linked to mental retardation. Here are some examples of other x-linked genes and the symptoms mutations carry:

COL4A5 (Xq22)

This is the gene that causes Alport's syndrome. This syndrome damages the various blood passages within the kidneys, which leads to urine in the blood and less effective filtering by the kidneys.

ATP7A (Xq21.1)

This particular gene causes Menkes syndrome, in which the body cannot absorb enough copper. This can affect the structure of many organs within the body (including skin, hair, and nerves) and often leads to a low body temperature and bleeding in the brain.

MECP2 (Xq28)

This gene is the root of Rett syndrome. This is usually found in girls because, although a defective X chromosome can make it to a boy, the boy will not survive. A girl, however, because she has two X chromosomes, is typically strong enough to live with the syndrome. Symptoms of Retts include problems breathing, seizures, and loss of sleep.

On a slightly cheerier note, I would like to relate an accomplishment of mine that is directly related to this. While I was having my hair cut, I made a joke about already going bald. The lady cutting my hair then asked if my mother's father was bald. Although my mother was adopted, I began to realize something: the gene that causes baldness is probably on the X chromosome. (I haven't actually researched this, but it seems likely.) This would also explain why women tend not to go bald--women have two X chromosomes, so they would not suffer the symptoms of a defective X chromosome as often.

Is that right? 

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