The crucial gene on the Y chromosome that has this testis determining function is called Sry, for “sex-determining region of Y.” Remarkably, when this gene is introduced into the genome of an XX mouse zygote, the transgenic embryo produced develops as a male, even though it lacks all of the other genes on the Y chromosome ( Figure 20-17). The Y chromosome influences the sex of the individual by inducing the somatic cells of the genital ridge to develop into a testis instead of an ovary. The sperm that fertilizes the egg determines the sex of the resulting zygote: eggs have a single X chromosome, whereas the sperm can have either an X or a Y. Individuals with a Y chromosome develop as males no matter how many X chromosomes they have, whereas individuals without a Y chromosome develop as females, even if they have only one X chromosome. The Y chromosome is the determining factor. Female mammals have two X chromosomes in all of their somatic cells, whereas males have one X and one Y. We now know that the sex of a mammal is determined by its sex chromosomes, rather than by the environment (although for some animals, such as crocodiles and many fish, the opposite is true). The Sry Gene on the Y Chromosome Can Redirect a Female Embryo to Become a MaleĪristotle believed that the temperature of the male during sexual intercourse determined the sex of offspring: the higher the temperature, the greater the chance of producing a male. As the primordial germ cells migrate through the embryo, they are signaled to survive, proliferate, and migrate by various extracellular proteins produced by adjacent somatic cells. They then actively migrate through the gut to their final destination in the developing gonads ( Figure 20-16). In the next few days, these cells proliferate and are swept back into the embryo proper along with the invaginating hindgut. In mice, for example, 1 week after fertilization, about 50 cells in tissue lying outside the embryo proper are induced by their neighbors to become primordial germ cells. A small group of cells in the early mammalian embryo is induced to become primordial germ cells by signals produced by neighboring cells. In mammals, by contrast, the egg is more symmetrical, and the cells produced by the first few divisions of the fertilized egg are all totipotent-that is, they can give rise to any of the cell types in the body, including germ cells. When the egg is fertilized and divides repeatedly to produce the cells of the early embryo, the cells that inherit specific molecules localized in a particular region of the egg cytoplasm become primordial germ cells. In most animals, including many vertebrates, the unfertilized egg is asymmetrical, with different regions of cytoplasm containing different sets of mRNA and protein molecules (discussed in Chapter 21). Primordial Germ Cells Migrate into the Developing Gonad
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