Studies by C. Ford, P. Jacobs, and co-workers in 1959 demonstrated the importance of genes on the X and Y chromosomes as sexual determinants by documenting the sexual phenotypes of humans with abnormal chromo-meiosis cell division somal constitutions. Errors in meiosis can produce sperm or egg cells that that forms eggs or have abnormal numbers of sex chromosomes. Upon fertilization, these cells develop into embryos with their own aberrant sex chromosome dosage.

Cells in individuals with Turner's syndrome contain a single copy of the X chromosome and no copies of the Y chromosome. These "XO" individuals develop as females, although their ovaries are nonfunctional. This demonstrates that just a single copy of the X chromosome is sufficient to direct most female sex development.

The reciprocal condition, "YO," with no X chromosome present, has not been documented in humans, as X chromosomes contain genes necessary for an embryo's survival. Individuals with Klinefelter's syndrome phenotypic related to (XXY), however, develop as phenotypic males, although they produce no sperm. This indicates that a single copy of the Y chromosome is sufficient to override the female developmental program and promote most male development.

Such observations led to speculation that the Y chromosome contains a "testis-determining factor" necessary to activate development of the male

? gonads. Several genes on the Y chromosome were suggested as possible testis-determining factors but ultimately rejected. In 1990 Peter Goodfel-low and coworkers at the Human Genetics Laboratory in London, England, the observable characteristics of an organism studied a group of sex-reversed XX males. Such individuals develop as phenotypic males despite being XX individuals.

The researchers discovered that the XX males had a small segment from a Y chromosome incorporated into one of their X chromosomes. This same segment was found to be missing from the Y chromosome of a group of sex-reversed XY individuals, who developed as phenotypic females. The segment acted as a testis-determining factor, as its presence was correlated with the activation of male development. DNA sequencing of the segment identified a gene that was named "SRY" from the description "sex-determining region of the Y chromosome."

SRYs Function. Studies in mice have supported SRYs role as a primary determinant of male development. The mouse homolog of SRY (Sry) is expressed in developing gonads in males but not females. It is present during but not after testis differentiation. Finally, experiments have been conducted where introducing the SRY gene into the genomic DNA of embryonic female (XX) mice causes some of them to develop as males.

Despite the clear causal relationship between this gene and male development, the specific mechanisms involved are unclear. The SRY protein is transcription messen- similar to "high-mobility group" proteins, which regulate the transcription of other genes. Its structure contains a domain that can bind to specific target DNA sequences. Mutations to this domain, which reduce SRY's ability to bind correctly to DNA, are frequently observed in XY individuals that develop as females.

SRY could conceivably function by activating genes involved in testicle differentiation, by repressing genes involved in ovary development, or by ger RNA formation from a DNA sequence

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