MadSci Network: Genetics
Query:

Re: Does both heredity genes act on the same time?

Date: Sun Feb 26 12:58:52 2006
Posted By: Alex Brands, Post-doc/Fellow, Biological ciences, Lehigh University
Area of science: Genetics
ID: 1135862240.Ge
Message:

Hi Michael,

Many genes exist in various “alleles”, or forms.  The difference between
alleles is due to slight differences in the exact sequence of the gene,
resulting in a slightly different gene product.  As you alluded to, for
each gene, you inherit a copy from your mother and a copy from your father.
 In most cases, both copies are expressed, but one may be “dominant”, while
the other is “recessive”.  This means that, when you look at or test the
individual, the effects of the dominant allele is readily seen, while the
effect of the recessive allele is not.   For example, dark eye color is
typically dominant over light eye color.  Why?  With no eye pigment, the
colored portion of the eye appears light red.  If an individual only has
alleles for light eye color, you can think of those alleles as staining the
eye a light color.  However, if the individual has an allele for dark eye
color, that allele will stain the eye dark, and the eye will be dark even
if the other allele is for light color.  Imagine you spilled a drop of
orange juice on a white tablecloth….it will leave a light orange spot.  If
you also spill a drop of dark purple grape juice on the same spot, it will
turn purple.  The orange stain is still there, but you can’t really see it
since the grape juice stain is so dark.

In some instances, an allele has incomplete dominance over another.  For
example, the flowers of certain plants can come in one of three colors,
white, red, and pink.  The gene that controls this trait come is two
alleles, white and red, and the product of that gene is responsible for the
synthesis of red pigment.  The white allele is basically non-functional, so
if the plant has two white alleles, it cannot make red pigment, and the
flowers will be white.  If it has two red alleles, it can make lots of red
pigment, and the flowers are red.  If it has one red and one white allele,
it can only make half as much red pigment, and the flowers are pink (light
red).  In this case, the red allele is incompletely dominant over the white
allele.

There is a phenomenon called “imprinting”, in which case only one copy is
actually expressed at all, while the other copy is shut down.  There is an
excellent description of genetic imprinting here:
 http://www.genetics.com.au/factsheet/14.htm

Now, to get more specific about your question concerning baldness, there is
an important exception to the notion that you inherit one copy of each gene
from each parent.  Humans have 46 chromosomes, arranged in pairs.  For
chromosomes 1 through 22, the pairs are virtually identical, except for the
small variations between alleles.  However, the 23rd pair are the sex
chromosomes, which determine of you come out to be a male or female.  In
this case, women have two copies of the “X” chromosome, while men have one
“X” chromosome and one “Y” chromosome.  The X and Y are very different from
one another.  Most notably, the Y chromosome is a tiny little thing with
something in the neighborhood of 100 genes, while the X chromosome is quite
large with something around 1000 genes.  Men have one X and one Y
chromosome, while women have two X chromosomes.  This means that women only
have an X chromosome to pass on to their children, while men could pass on
an X or a Y.  If they pass on a Y, the child is male.  Therefore, all men
have inherited their X chromosome from their mother.  Since the X
chromosome has hundreds of more genes than the Y chromosome, all those
extra genes are present as only a single copy in men (since they only have
one X chromosome).  All of those genes, then, can be thought of as
“dominant”, since there is no other copy around.

As (bad) luck would have it, one of the genes affecting male pattern
baldness appears to be on the X chromosome.  That means that in men, this
gene is *only* inherited from their mother!  However, this does not explain
all cases of male pattern baldness, and there are probably many other genes
located on other chromosomes that play a role.  If your mother’s family has
a history of male pattern baldness, there is a very good chance you
inherited that tendency through her X chromosome.

Unfortunately, I could not find much information on the genetics of white
hair, although many sources say that there is a strong genetic component.

Alex Brands
Lehigh University



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