. The physical basis of heredity . Fig. 92.—Normal female and male groups of chromosomes of the vinegar fly, with theXXY female group below. XXY individuals (Fig. 92). It has been shown by cyto-logical examination that such females do actually containan additional Z-ehromosome. The four possible ways inwhich these three chromosomes might be expected tobehave at the reduction division when the polar bodies SEX-CHROMOSOMES AND INHERITANCE 201 are given off by the egg are shown in the next diagram(Fig. 93). One X may go out of the egg, and the other Xand the Y stay in; or one X may stay in the eg
. The physical basis of heredity . Fig. 92.—Normal female and male groups of chromosomes of the vinegar fly, with theXXY female group below. XXY individuals (Fig. 92). It has been shown by cyto-logical examination that such females do actually containan additional Z-ehromosome. The four possible ways inwhich these three chromosomes might be expected tobehave at the reduction division when the polar bodies SEX-CHROMOSOMES AND INHERITANCE 201 are given off by the egg are shown in the next diagram(Fig. 93). One X may go out of the egg, and the other Xand the Y stay in; or one X may stay in the egg and theother X and the Y go out. In these two cases, X and Xmay be thought of as members of a pair that conjugate, asin the normal female, and then separate, and chance alonedetermines whether the Y stays in or passes out. AgainY may go out of the egg and X and X stay in; or X and X POLARBODY EGGS SPERM. 4 Fia. 93.—Non-disjunction. In the upper part of the figure the four possible modes ofelimination of the sex-chromosome from XXY eggs are shown; the results of their fertili-zation by an X-bearing sperm of the male is shown below. go out and Y stay in. Here X and Y may be supposed tobe members of the conjugating pair, and the free X goesto the same pole as the X that conjugated. In the diagram, each of these four types of eggs isrepresented as fertilized by an X-bearing sperm. In orderto make the outcome more apparent the original XXYfemale may be supposed to have had white eyes (clearXs) and the male that fertilized her red eyes (here repre-sented by the black X carrying the gene for red eyes). Four classes of individuals are expected: (1) Red-evedfemales (XXY); (2) red-eyed females (XX); (3) red-eyed 202 PHYSICAL BASIS OF HEREDITY females {XXX) that die, and (4) red-eyed males (XY).The last are exceptional, since white-eyed females nor-mally never produce anything but white-eyed s
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