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extensively. Primary sex determination in these animals does not involve the Y chromosome but instead is determined by the ratio of the number autosomal describes a of X chromosomes to autosomal (nonsex) chromosomes. By examining chromosome other individuals with unusual numbers of various chromosomes it has been than the X and Y sex- . . . . , . . r .

determining chromo- determined that in Drosophila those with one or fewer X chromosomes per somes diploid autosome set develop as males while those with two or more X

chromosomes develop as females. Individuals with intermediate ratios such as those with two X chromosomes and a triploid set of autosomes develop as intersexes with both male and female characteristics. Although this ratio serves as the primary determinant of sex in both of these organisms, the specific gene products that influence this ratio assessment are different, demonstrating that different molecular mechanisms can be used for a similar purpose.

There is also significant variability in the strategies by which the outcome of sex determination is communicated to the various tissues that undergo sexual differentiation. In humans and most other mammals, the presence or absence of SRY protein in cells of the gonad specifies their sexual differentiation and which hormones are secreted by the gonad to direct the sexual differentiation of most other cells in the individual.

hormones molecules In Drosophila hormones have little effect on sexual differentiation.

?re|eased by œN to Instead, with only a few exceptions, each cell decides its sex independently influence another of other cells and tissues. This cell autonomous mechanism is demonstrated in experimentally produced mosaic organisms called gynandro-morphs ("male-female forms") in which some cells are XX (female) and others XO (male). Such individuals develop into adults with a mix of male and female cell types that match each cell's genotype. The lack of evolutionary conservation of sex-determining mechanisms among animals is particularly interesting because of the similarities that exist in other major switch genes for basic developmental processes. see also Androgen Insensitivity Syndrome; Gene Expression: Overview of Control; Nondisjunction; Transcription Factors; X Chromosome; Y Chromosome.

Jeffrey T. Villinski and William Mattox

Bibliography

Berta, Philippe, et al. "Genetic Evidence Equating SRY and the Testis-Determining Factor." Nature 348 (1990): 448-450.

Cline, Thomas W., and Barbara J. Meyer. "Vive la Difference: Males vs. Females in Flies vs. Worms." Annual Reviews of Genetics 30 (1996): 637-702.

Gilbert, Scott F. Developmental Biology, 6th ed. Sunderland, MA: Sinauer Associates, 2000.

Hodgkin, Jonathan. "Genetic Sex Determination Mechanisms and Evolution." BioEssays 4 (1992): 253-261.

Sinclair, Andrew H., et al. "A Gene from the Human Sex-Determining Region Encodes a Protein with Homology to a Conserved DNA-Binding Motif." Nature 346 (1990): 240-224.

Zarkower, David. "Establishing Sexual Dimorphism: Conservation amidst Diversity?" Nature Reviews Genetics 3 (2001): 175-185.

Sex-Linked Inheritance See Inheritance Patterns

Sexual Orientation

The biological basis of sexual orientation (heterosexuality, homosexuality, or bisexuality) has long been a topic of controversy in both science and society. A growing body of research supports the view that genetics and the environment work together to determine sexual orientation. Some issues remain unclear. First, how much of sexual orientation is genetic and how much is shaped by environmental influences, including family, society, and culture? Second, is sexual orientation a fixed trait, or is it subject to environmental influence and changeable over time? Two types of genetic studies, classical family/twin/adoption studies and biological/molecular studies, support multiple genetic and environmental determinants in male and female sexual orientation.

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