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Modern Genetic Analysis Chapter 16. The Genetic Basis of Development
Binary Fate Decisions: Pathways of Sex Determination

In many species, sex determination is associated with the inheritance of a heteromorphic chromosome pair in one sex. However, this does not imply that all such species have evolved from a common ancestor that possessed such a heteromorphic sex chromosome set. Rather, XX-XY sex determination mechanisms appear to have arisen independently many times in evolution. The XX-XY sex chromosomes of flies and mammals arose independently and, as we will see, the underlying mechanisms for sex determination are quite different. The main features of sex determination mechanisms in flies and mammals are contrasted in Table 16-1.

In both flies and mammals, many areas of the body display sexually dimorphic characteristics; that is, these areas differ in males and in females. For example, in Drosophila, the two sexes differ in the structure of the sex organs themselves and in pigmentation of the abdomen. The sex determination mechanisms and how they ensure expression of such sexually dimorphic characteristics will be the main focus of the discussions that follow.

© 1999 by W. H. Freeman and Company. All rights reserved.