Human chromosome methodology / edited by Jorge J. Yunis.

Date:
1965
Catalogue details

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

Credit: Human chromosome methodology / edited by Jorge J. Yunis. Source: Wellcome Collection.

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    Identification of Chromosomes 161 somes whose locations are too close to each other in the karyogram cannot produce two peaks in the density distribution. Patau understated his case when he named as critical distance of the two locations the standard deviation of a single-chromosome distribution. Actually, the distance between the means of two normal distributions must be about 2.2 times the standard deviation before any hint of bi- modality will appear in the combined distribution and considerably more before the peaks will be pronounced enough to cause significant concen¬ trations of points in Lejeune's scatter diagram. A distance equal to 2.6 times the standard deviation would probably still be insufficient (Fig. 2). Fig. 2. Two examples of a combined distribution being more or less bimodal, depending on the difference of the means of the two component distributions (see text). Unfortunately, Lejeune did not publish standard deviations, but he did state that the (mean) difference between the homologs of chromosomes 1 and 2 was 5% and 5.7%, respectively. It can be shown that the ex¬ pected difference between homologs is equal to 2 о/л/ж if the distribution is normal with standard deviation o. Hence, we estimate s = 5.35 X \/л/2 = 4.7% as coefficient of variation, which is in very good agree¬ ment with the estimates obtained from measurements by other authors (p. 155). It follows that two chromosomes with about equal arm ratios must differ in length by considerably more than 2.2 X 4.7 = 10% if each is to produce a significant point concentration in the karyogram. It will be seen from Lejeune's (1964) Fig. 5 that at least the chromo¬ somes 10 and 8 and also 6 and X are very far from fulfilling this con¬ dition. Probably all neighboring chromosomes are too close to each other to produce significant point concentrations, but the lack of infor¬ mation on the variation of the centromere index makes a more definite [•—- 2.2аГ-->| 2.6cf --->1 \