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Unformatted text preview: nt assortment. Sex differences often correlate with the presence of sex chromosomes. C.E. McClung was the first to suggest (1901) that the differences between males and females were due to the inheritance of specific chromosomes.
When genes are located on different chromosomes, they are assorted independent of one another. An YyRr individual, for example, gives rise to gamete YR, Yr, yR, and yr in equal proportions (25% each) because the chromosomes on which t he genes are located are assorted independently during meiosis. In the cross YyRr x yyrr, therefore, an equal proportion of the four possible phenotypes will be produced if Y is dominant to y and R is dominant to r. 8 Sex-determination systems (Figure 3.18): X-Y system (mammals). The male is the heterogametic sex and the female is the homogametic sex. X-O system (insects). The male has one sex chromosome and the female has two sex chromosomes. X:Y ratio (insects, Drosophila). The ratio of X chromosomes to the number of autosomal sets determin...
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