File:Contributions to the genetics of Drosophila melanogaster (1919) (20501308158).jpg

Title: Contributions to the genetics of Drosophila melanogaster
Identifier: contributionsto00carn (find matches)
Year: 1919 (1910s)
Authors: Carnegie Institution of Washington; Morgan, Thomas Hunt, 1866-1945; Bridges, Calvin B. (Calvin Blackman), 1889-1938; Sturtevant, A. H. (Alfred Henry), 1891-1970
Subjects: Drosophila melanogaster; Heredity; Karyokinesis
Publisher: Washington, Carnegie Institution of Washington
Contributing Library: NCSU Libraries
Digitizing Sponsor: NCSU Libraries

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26 THE ORIGIN OF GYNANDROMORPHS. SOMATIC MOSAICS. Somatic mosaics can be accounted for by autosomal elimination in the same way that gynandromorphs are accounted for by X-chromo- somal elimination. Somatic mosaics might also be expected to arise from binucleated eggs and to be as often found as are gynandromorphs with the same origin. As a matter of fact, we have found only one certain case, which is less than expected on the latter view. The case is as follows: -^ The grandmother was spineless (third-chromosome recessive) and the grandfather was spread (another third-chromosome recessive). The daughters and sons were wild-type. A pair of these gave a 2:1:1:0 ratio, as expected, because of no crossing over in the male. One of the granddaughters (No. 561, Oct. 3, 1914, text-fig. 7) was a mosaic of spineless and not-spineless. The left side of the thorax and abdomen and the left wing and the middle and last left leg were spineless. The rest of the female (including all of the head and left foreleg) had long bristles and hairs of the wild type. Simple eUmination of the third chromosome from the spread parent would explain this case were it not that the existence of an individual lacking an autosome is doubtful, because none have as yet appeared through autosomal non-disjunction. On the alternative view of a binucleated egg, one nucleus contained the spineless third chromosome, the other a spread-bearing chromosome; both nuclei were fertilized by X sperm bearing the spineless X chromosome, and gave the female spineless on the left side and wild-type on the right side. The fact that the overwhelming number of hy- brid mosaics are gynandromorphs, involving there- fore the sex chromosome, can not be explained as due to failure to discover autosomal mosaics if they occurred. In most of our cases these would be just as striking as in the cases where the sex chromo- somes are involved. Evidently some peculiarity in the separation of the halves of the sex chromosomes makes the elimination of one of the daughter halves more probable than in the case of other chro- mosomes. Such a supposition is, of course, in harmony with the pecu- liar behavior of the sex chromosome at the reduction division of the male, at least when it lags on the spindle. On the other hand, when it does divide, as in the female, no such peculiarity is recorded, and it is this reduction, rather than the former one, that we need for com-. parison.
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Text-figure 7.

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