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= Sex reversal = From Wikipedia, the free encyclopedia

Sex reversal is the phenomenon whereby organisms which are genetically a certain sex develop as the opposite sex. This can be caused by many factors, including temperature, pollutants, and other environmental conditions. Herbicides for example, one of the many pollutants caused by humans, can act as estrogen promoters or inhibitors. This respectively increases or decreases the number of female offspring through controlling aromatase.[1][2] The exact mechanism for sex reversal however is still unknown.

Sex reversal can occur in humans, mice, fish, and many other organisms.

Fish
Sex determination in fish is dictated by chromosomes. Many species of fish however change sex later on in life. This can occur by fusion of sex chromosomes with autosomes. These fusions can result in a multiple sex chromosome system. Fish that undergo this process as a part of normal development include tilapia, goby fish, and many more. Sex reversal in fish can be induced by hormones, steroids, temperature, social interactions, or other environmental conditions. Human waste effluent can include estrogens and androgens, sex hormones that influence development of female or male characteristics. When juvenile three-spined sticklebacks were exposed to these hormones in water, they expressed both ovarian and testicular tissue in the gonads, a phenomenon known as intersex.

Amphibians
Amphibians have three sex chromosomes: X, Y, and Z. It is nearly impossible however to determine the sex of an amphibian by looking at these chromosomes. This is because many other factors are important for sex determination, such as temperature and sex hormones. Amphibians treated with estradiol, a female sex hormone, undergo male-to-female sex reversal. Tadpoles of a species of wood frog, Lithobates sylvaticus, undergo sex reversal when exposed to different temperatures. High temperatures result in male and intersex tadpoles. This female-to-male sex reversal induced by high temperatures is common in many species of amphibians.

T-Associated Sex Reversal in Mice
A deletion mutation called a hairpin tail (Thp) on chromosome 17 causes male mice to develop ovaries or an intermediate between ovaries and testes, known as ovotestes. Included in this deletion is the gene that encodes the MAP3K4 enzyme, important in the regulation and development of cellular processes. Without MAP3K4, a gene on the Y chromosome called Sry goes unregulated, preventing male organs from developing. MAPK is also involved in human gonadal development, indicating that this may be a cause behind human sex reversal as well.

Humans
Sex reversal in humans interferes with normal sexual development. The results of this are classified as disorders of sexual development (DSD) and are a form of gender dysplasia, an inconsistency between chromosomal and gonadal sex. XY gonadal dysgenesis is the most common human occurrence of sex reversal. With this disorder, individuals that are genetically male usually lack testes and develop female genitalia. XX gonadal dysgenesis is more rare, occurring in approximately one in every 20,000 people. This occurs when an individual that is genetically female develops as a male. Although appearing male, these people typically experience symptoms including enlarged breast tissue, decreased male sex hormones, or infertility.

Implications
The ratio of males to females within a population is known as the sex ratio. The ideal sex ratio for most sexually reproducing populations is 1:1. Since each offspring requires both a mother and a father, a sex ratio of 1:1 maximizes the number of potential offspring. Abnormal sex reversal within a population can cause the sex ratio to change and become unstable. Number of potential offspring therefore decreases, potentially causing populations to decline. Increasing pollutants that increase sex reversal and throw off sex ratio interferes with the well-being of many species.

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