Sex differences in chromosomal disorders
There are several aspects
of chromosomal aberrations that have to do with sex differences. One is that any chromosomal aberration of the
X chromosome, short of its entire
deletion, will affect boys more than girls.
This is because, in girls, the other X chromosome can take over some of
the functions lost, or compensate some
of the effects of abnormal emplacement of genetic material on one of the X
chromosomes. In boys, the tiny Y chromosome is very different from
the large X chromosome, and is unable to
compensate for misalignments, deletions, adjunctions or mutations of genetic
material on the X chromosome. The X
chromosome is about as large as the 11th autosome -which by definition is the
eleventh largest. A human being can do
without the Y chromosome (Turner syndrome) and live a peaceful fulfilling life,
given minimal medical treatment.
However, the Y monosomy is not viable,
and this seems to be the case in all mammalian species. In other words, humans, like every mammalian species studied,
cannot survive without at least one X chromosome. Another intriguing sex difference is that one
of the most frequent autosomal aberrations, trisomy-21, affects boys nearly
twice as often as it does girls.
Theoretically, this ought not occur. The twenty-first chromosome pair is the same
in men and women. The failure in
disjunction during parental meiosis responsible for the aberration must somehow
have to do with a protective factor situated on the X chromosome or a
pathological factor situated on the Y chromosome. What little evidence we have seems to point
in the direction of the second hypothesis (see the section on chromosomal
aberrations in chapter 5). Curiously,
Edwards syndrome (trisomy 18) seems to affect the female sex more than the male
sex: male/female ratios of 1:3 have been
reported in the literature. This shows how complicated genetics really is. Genes situated at various sites on various
chromosomes serve not only to produce changes in pieces of the body but also to influence the activity, and sometimes even the integrity of pieces of
chromosomes that can be situated elsewhere on the chromosome, or even on another chromosome!
The age effect
consisting of increasing risk of meiotic errors as a function of
parental age is very different depending
on the sex of the parent. Most of the
age effect in question comes from the mother's age. Since a female neonate is born with her
ova, when she in turn conceives, she does so with ova that are as old as
she. The older these ova are, the more they are susceptible to
accidents, because they have simply
started deteriorating as have all other parts of the body. Fathers are not born with the sperm used for
procreation. New sperm cells are born
every day, more or less so depending on a man's sexual activity. In principle then, the father should contribute less risk from
the aging factor. It has been estimated, depending on the investigation's
method, that about 75 to 95% of the aging effect on chromosomal aberration
comes from the mother and 5 to 25% from the father. The paternal contribution to the aging
component of risk for chromosomal aberration can be explained by the fact that
meiosis does not occur in a vacuum. It
involves numerous cellular and molecular processes, which even though the sperm may be young and
fresh, may cause meiotic accidents.
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