 |  | David Reimer and his brother
This is the first family gathering after David Reimer announced his intention to live as a male instead of a female. (David is on the left.) |
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 | | Genes involved in human sex determination
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 | | Head of Drosophila melanogaster
Drosophila, the fruit fly, is a common model organism used in genetic research. It was one of the organisms sequenced in the Human Genome Project. |
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 | | Human chromosomes
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 | | Human chromosomes
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 | | Human chromosomes
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 | | Janet Reimer holding her twins, Bruce and Brian
Brenda and Brian are identical twins. Brenda was born a male and named Bruce. After a botched circumcision, scientists believed it was best to change him into a girl. This case, termed the "Joan/John case" in medical literature, has been used often in the argument for nurture's role in the gender identity. At 14, Brenda shed her upbringing as a female and began to live as a male. He took the name David, and has said that he never felt right as a female, disproving what the scientists believed: that nurture could fully override his genetic makeup. |
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 | | Paternal and maternal lineages
Maternal lineages can be traced through mitochondrial genes, which are inherited by males and females only from the mother. Paternal lineages can be traced through the Y chromosome, which is inherited only by males and only from the father. (M=male and F=female) |
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 | | Pathways leading to sex development in mammals
The genital ridge in an embryo is converted to a bipotential gonad by the products of the LHX9, SF1, and WT1 genes. This gonad develops into an ovary under the influence of the WNT4 and DAX1 gene products; it develops into a testis under the influence of the products of the SRY and SOX9 genes. The ovary produces cells that make estrogen, which causes the Méllerian duct to differentiate into the female genitalia. The testis makes two hormones, anti-Méllerian duct factor (AMH), which causes the Méllerian duct to regress, and testosterone, which causes the Wolffian duct to differentiate into male internal organs. Testosterone is also converted into dihydrotestosterone (DHT), which is required for development of male external genitalia. |
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 | | The evolution of the Y chromosome
The degeneration of the Y occurred in four discrete episodes, beginning about 300 million years ago when a reptile-like ancestor acquired the SRY gene on one of its autosomal chromosomes. Each of the four episodes involved a failure of recombination to occur between the X and the Y chromosomes, resulting in subsequent decay of some genes in the non-recombining region. |
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 | | X and Y chromosomes
The Y chromosome is the smallest chromosome in humans. While most chromosomes have about 1,500 genes, the Y has less than 200. |
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 | | Y chromosome
The Y chromosome is very small compared to the X chromosome. The pseudoautosomal regions at the tips contain the genetic material on the Y that shows similarity to the X chromosome. The SRY gene is located on the p arm of the Y. |
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