Thursday, 22 March 2012

Inheritance of Male Pattern Baldness

The Genetics of Baldness - Stephen Connor (s4291526)


What is Male Pattern Baldness (MPB)?
Male pattern baldness (MPB) is a condition which most men (and some women) will face during their lives. MPB causes in people a receding hair line and their hair will become a lot thinner. In the later stages of MPB complete baldness around the crown and possibly the middle section of the top of the head can occur. But what causes male pattern baldness?

What causes MPB?
Testosterone is a steroid hormone that affects many different areas of the body and their functions, however it is not often realized that testosterone is also the cause of hair loss. When testosterone is in the presence of an enzyme called 5-alpha-reductase, the enzyme will break down testosterone into dihydrotestosterone, which is often referred to as DHT.

Male pattern baldness occurs as a result of hair follicles being sensitive to DHT. If a hair follicle is sensitive to DHT it will miniaturize and eventually no hair will grow where that hair follicle grew. It can be seen that through the process of miniaturizing and loss of hair how the symptoms of MPB are so (Receding hair line, thinning of hair). (WebMD 2010)

How does a person’s genetics affect their chance of having MPB?
In 2005, German researches discovered a gene on the X-chromosome that affects baldness (Dr Barry Starr 2006). This gene (androgen-receptor gene, AR) instructs the making of androgen-receptors (also known as dihydrotestosterone receptors).   If the AR gene allows too many androgen-receptors to be made in the scalp or in hair follicles it can result in more testosterone being on the scalp. This in turn results in the creation of more dihydrotestosterone which then leads to greater loss of hair.
Some people who have MPB have an AR gene that performs normally. This means the AR gene is not the only factor that influences the development of MPB. Two independent studies were conducted to discover a common factor that people with MPB had that people without MPB did not have. Both studies came to the conclusion that people suffering with MPB have changes on their chromosome 20 compared to those without MPB.
Everyone has two chromosome 20s, one from each parent. This allows for one chromosome 20 to be unchanged even though the other is changed. This shows that there are three different combinations of chromosome 20s a person can have (and therefore three different likelihoods of having MPB):

1.       Two unchanged chromosome 20s (0 times more likely to develop MPB)
2.       One unchanged and one changed chromosome 20 (3.7 times more likely to develop MPB)
3.       Two changed chromosome 20s (6.1 times more likely to develop MPB)
(Melinda Beck 2008)

Not just one factor affects the likeliness of developing male pattern baldness.  An AR gene that allows for too many androgen-receptors to be in the scalp or hair follicles will increase the chances of developing MPB dramatically. Certain variations to the chromosome 20 also increase the probability of having MPB. Variations on one chromosome 20 will increase the chance of having MPB by 3.7 times and having variations on both chromosome 20s will increase the chance by 6.1 times.



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