Nondisjunction and the Chromosome Theory of Inheritance

When Morgan crossed his original white-eyed male with homozygous red-eyed females, all 1237 of the progeny had red eyes, except for three white-eyed males. As already mentioned, Morgan attributed these white-eyed F1 males to the occurrence of further mutations. However, flies with these unexpected phenotypes continued to appear in his crosses. Although uncommon, they appeared far too often to be due to mutation. Calvin Bridges, one of Morgan's students, set out to investigate the genetic basis of these exceptions.

Bridges found that, when he crossed a white-eyed female (XwXw) with a red-eyed male (X+Y), about 2.5% of the male offspring had red eyes and about 2.5% of the female offspring had white eyes ( FIGURE 4.15a). In this cross, every male fly should inherit its mother's X chromosome and should be XwY with white eyes. Every female fly should inherit a dominant red-eye allele on its father's X chromosome, along with a white-eyed allele on its mother's X chromosome; thus, all the female progeny should be X+Xw and have red eyes. The appearance of red-eyed males and white-eyed females in this cross was therefore unexpected.

To explain this result, Bridges hypothesized that, occasionally, the two X chromosomes in females fail to separate during anaphase I of meiosis. Bridges termed this failure of chromosomes to separate nondisjunction. When nondisjunction occurs, some of the eggs receive two copies of the X chromosome and others do not receive an X chromosome ( FIGURE 4.15b). If these eggs are fertilized by sperm from a red-eyed male, four combinations of sex chromosomes are produced. When an egg carrying two X chromosomes is fertilized by a Y-bearing sperm, the resulting zygote is XwXwY. Sex in Drosophila is determined by the X:A ratio (see Table 4.1); in this case the X:A ratio is 1.0, so the XwXwY zygote develops into a white-eyed female. An egg with two X chromosomes that is fertilized by an X-bearing sperm produces XwXwX+, which usually dies. An egg with no X chromosome that is fertilized by an X-bearing sperm produces X+O, which develops into a red-eyed male. If the egg with no X chromosome is fertilized by a Y-bearing sperm, the resulting zygote with only a Y chromosome and no X chromosome dies. Rare nondis-junction of the X chromosomes among white-eyed females therefore produces a few red-eyed males and white-eyed females, which is exactly what Bridges found in his crosses.

Bridges's hypothesis predicted that the white-eyed females would possess two X chromosomes and one Y and that red-eyed males would possess a single X chromosome. To verify his hypothesis, Bridges examined the chromosomes of his flies and found precisely what he predicted. The significance of Bridges's study was not that it explained

(a) White-eyed female and red-eyed male

(b) White-eyed female and red-eyed male with nondisjunction

(a) White-eyed female and red-eyed male

Fj generation

Conclusion: /2 red-eyed females and normal separation of chromosomes results in 1/2 white-eyed males.

P generation

Nondisjunction in meiosis

White-eyed female female

Xw Xw ri

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