Sex differences in chromosomal disorders

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 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.