MadSci Network: Medicine

Re: On inherited hair loss

Date: Wed Apr 26 00:10:56 2000
Posted By: Mike Crawford, Postdoctoral Fellow, Biology
Area of science: Medicine
ID: 956531565.Me

First, a little about genetics--

We humans receive one copy of 23 'regular' (or somatic) chromosomes from each parent-so we have 
two copies (total of 46) in each cell.  In these chromosomes, we all carry gene defects that may cause diseases or other disorders,
but these potential problems are almost always backed up by a normal copy of the gene obtained from the other parent--hence we
show no sign of the disorder.

An exception to the above rule is the sex chromosome (X or Y).  The mother has two 'Xs' and will always give her kid an 'X'.
The father has one 'X' and a 'Y' and will give the kid one or the other.  The father, therefore, determines the sex of the 
child.  If he gives you 'X' then you're 'XX'--a GIRL, if he gives you 'Y' then you're 'XY'--a BOY.

"X-linked" disorders are defects in one of the thousands of genes found on the 'X' chromosome.  These disorders can range
from severe mental retardation to more harmless things like color blindness or PATTERN BALDNESS.  The reason that males
are more susceptible to an X-linked disorder is that when mom gives her son an 'X' carrying the defective gene, dad gives the
son a 'Y', which cannot back up the defective 'X'.  Women seldom get these disorders since the chances of getting two
bad 'Xs' for the exact same gene are quite slim.

So as far as whose fault it is, blame both parents--Mom for giving it to you, and dad for not backing you up. 

Now another point should be made about where to look for the bald 'X' in a mother's family.
You have to look at EITHER  the (1) mother's father OR (2) her brothers:

(1) Grandpa has the bald 'X' (and is bald) and passed it to your mom/her sisters-who give it to half of their sons.
(remember mom's brothers would only get the 'Y' and would not be bald in this case).

(2) Grandpa has a good 'X', but Grandma has bald 'X'/good 'X'  and gives a 50/50 ratio to kids, so half of mom's brothers
are bald.

If you are grandson of all this, then in scenario (1) you have 50% chance at baldness, scenario (2), a 25% chance.

Both (1) AND (2) are possible in the same family, but would be rare and might lead to bald women in the family (depending on
other factors such as testosterone levels)

Hope this helps.
Michael Crawford

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