Medical genetics / edited by Victor A. McKusick and Robert Claiborne.

Date:
[1973]
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Medical genetics / edited by Victor A. McKusick and Robert Claiborne. Source: Wellcome Collection.

    29/328 (page 7)
    CHROMOSOMAL ABNORMALITIES I: AUTOSOMAL DEFECTS Normal Meiosis Melotic Nondisjunction (in First Division) Normal Zygote Meiotic nondisjunction is believed to account for most trisomies of the autosomes. If at the first (reduction) division a pair of ho¬ mologous chromosomes fails to separate, the result is an imbal¬ ance in each of the daughter cells: one has both chromosomes, the other has neither. The imbalance is carried over to the second Trisomie Zygote Monosomie Zygote division. If the cell containing the two chromosomes is fertilized by a normal gamete, the result is a trisomie zygote. If the cell lacking a chromosome is fertilized by a normal gamete, the result is a monosomy, which, when it occurs in the autosomes, appears to be incompatible with life, perhaps even with implantation. From a clinical standpoint, none of the trisomies can be called common; Down's syndrome, the most frequent of them, affects perhaps one infant in 500. Embryologically, however, they are very common indeed, appearing in something like 20% of early abortuses examined. One might expect, there¬ fore, that the monosomies, where there is one chromosome too few rather than one too many, should be equally common, the more so in that the mechanisms thought to produce trisomy (which I shall discuss in a moment ) should in theory produce an approximately equal number of mon¬ osomies. In fact, however, there is no clinical example of an autosomal mon¬ osomy, apart from one doubtful case of monosomy 21, which I and some other geneticists believe to represent a somewhat different chromosomal dis¬ order. Indeed, so far as I know, mon¬ osomy has never been observed even in early abortion material. The obvi¬ ous, and I think the correct, conclusion is that the monosomic zygote never succeeds in implanting itself; it may not survive fertilization. In this area, by the way, the contrast between auto¬ somes and sex chromosomes is much less marked; individuals (or abor¬ tuses) with a single Y chromosome have never been observed, and while the single X condition is a well-known clinical entity (Turner's syndrome) the great majority of such individuals perish before birth. Trisomies are produced by a num- 7