Barr bodies; the Lyon Hypothesis and dosage compensation
In 1949, Murray Barr found that staining cat cells with Feulgen, a
stain
that binds to DNA, often resulted in the appearance of a "drumstick" in
interphase nuclei, but only if the cat was female. Later it was learned that
cells of normal human females also have a "Barr body" and that Triplo X
females have 2 Barr bodies, per cell, Turner's none etc.
• Every X > 1 is condensed and inactive in gene expression;
• X-inactivation occurs early in development;
• When it occurs, which X is inactivated in any cell is chance;
• The same X remains inactive in all daughter cells in subsequent
The Lyon Hypothesisexplains:
•Dosage compensation, that is, why normal males and females have
the same amount of gene products or enzymes coded for by sex-
linked genes;
•Why females heterozygous for an X-linked gene can vary greatly in
expression for the gene;
•Why Tortoise Shell and Calico cats are females:
Tortoise shells (a mix of black and yellow-orange hair) are
heterozygous for a sex-linked coat color gene. (XBXb). Patches of
tissue where XBremains active are black; those where XBis
condensed and inactive are orange.
Calico cats also express a dominant gene for white spotting.
Male kittens of tortoise-shell cats are either black or orange.
(Klinefelters has been seen in cats)
A similar situation affecting the absence of sweat glands in various
parts of the body is seen in human females heterozygous for
anhidrotic ectodermic dysplasia.
Identical twin females can have quite different patterns of
expression.
There are now good X-specific and Y-specific probes for detecting the
number of X and Y chromosomes in any cell.
Small regions of chromosomes other than the X and Y are specifically
inactivated during male and female gametogenesis; different regions are
condensed in oogenesis than in spermatogenesis. The inactivated regions
are not expressed in the fetus, so if the normally "active" gene(s) donated
by the other parent is/are defective, an aberrant phenotype may result.
Human examples include Prader-Willi and Angelman syndromes.
Both are related to failure to express genes on chromosome 15.
In Prader-Willi, which causes mental retardation, uncontrollable
hunger, and thus obesity, the region of Chromosome 15 that is
normally active only in male gametes is missing or non-functional.
In some cases, the child is found to have 2 copies of Chromosome 15
from the mother (both copies inactive) and none from the father
(uniparental disomy).
In Angelman's, the region of chromosome 15 that is normally active
in the egg is defective, resulting in a phenotype often referred to as
"happy puppet".
Tracking inheritance of "imprinted genes" can be difficult since the
normal gene is only expressed in one sex, and the effect does not
always persist to adulthood.
Many traits, especially secondary sex characteristics, are only expressed in
one sex. This does not mean that the genes are necessarily located on the
X or Y chromosomes or that the genes are not present in the other sex. A
good example is lactation, which is limited to females. However, as any
dairy farmer knows, daughters of "good bulls" give more milk.
Traits that are dominant in one sex but recessive in the other are said to
be sex influenced.
Horns in sheep, coat color in Ayrshire cattle and pattern baldnessin
humans are all good examples.
When dealing with these traits, the legend must show the sex as well as
the phenotype to describe the mode of inheritance. Note the difference in
heterozygous males and females in the example below:
Sex hormones clearly are involved with expression of these alleles, but
again, the genes could be on any chromosome.
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