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Excerpt from Non-Disjunction as Proof of the Chromosome Theory of Heredity
There are now about fifty sex-linked mutations known in Drosophila, and the data collected in their investigation are the most extensive known in experimental breeding. The arrangement of the sex-linked genes in a linear series and the establishment of the relative distances between the loci are based upon over half a million ¿ies.4 The work on non disjunction deals directly with the best known of these sex-linked char acters, and therefore rests upon a very firm Mendelian foundation.
The inheritance of a recessive sex - linked character may be illustrated by the cross of a vermilion female by a wild type (red - eyed) male (see figure The sons are vermilion - eyed like the mother, and the daugh ters are wild type like the father. This criss - cross inheritance is ex plained by the theory that the genes for the sex - linked characters are carried by the X chromosomes. As shown in figure 2, the son derives his single X chromosome from his mother, and shows vermilion eye color because the gene for vermilion was carried by that chromosome. The Y chromosome from the father does not affect the visible characters of the son in any way. Thus a male always shows by its characters what genes are carried by its X chromosome. The daughter receives from the mother an X carrying the vermilion gene, but since the vermilion is recessive to the unmutated gene (red) carried by the X which she re ceives from the father, she will be wild type (red).
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