Download SEX-RELATED INHERITANCE

SEX-RELATED INHERITANCE
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.
Mary Lyon correctly hypothesized that the Barr Bodies are actually condensed, inactive X
chromosomes: She also said that:
• 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 cell divisions.
The Lyon Hypothesis explains:
•Dosage compensation, that is, why normal males and females have the same amount of
gene products or enzymes coded for by sex-linked genes;
For example, XCf/XCf females have the same amount of clotting factor in their
blood as XCfY males.
•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 sexlinked coat color gene. (XBXb). Patches of tissue where XB remains active are
black; those where XB is 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.
Genomic Imprinting
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 is reprogrammed differently in each sex.
Sex-Limited Traits
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.
Sex-Influenced Traits
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 baldness in humans are all good
examples.
Example:
Genotype
H'H'
H'H
HH
Male
Horns
Horns
hornless
Female
Horns
hornless
hornless
Sex hormones clearly are involved with expression of these alleles, but again, the genes could be
on any chromosome.