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I asked a question related to the third sex, and I came to know that its always possible to categorize a human to male or female with presence of Y chromosome.

Now, I have another question. Is there a way to say if someone has Y chromosome only with external physical test? More specifically:

  • Someone has a penis, can we say that s/he will have Y chromosome?
  • Some one has Y chromosome, can we say that he will have a penis?
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up vote 7 down vote accepted

No, an external physical examination would be inconclusive. The reason is the TDF gene. To be more specific, if a person has XY and the gene is not active then the subject would have a female appearance. Also we cannot conclude that a person has a Y chromosome even if it has penis because that gene could be transferred on the X chromosome.

Here is a reference:

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Isn't it the SRY gene which expresses the TDF transcription factor? – LanceLafontaine Aug 27 '12 at 13:42
Sometimes, the area which encodes this protein is referred as TDF gene, although I am not sure if this terminology is abusive. . Also see here a alias for the SRY gene is TDF. – avra Aug 27 '12 at 14:37

An interesting condition is androgen insentivity syndrome. In AIS, the body is unable to respond to androgens, the male sex hormone, of which testosterone is the best known. The most common cause is a defective androgen receptor, and most of these diseases are X-linked. In extreme cases, complete androgen insensitivy syndrome (CAIS) can occur. Genetic males (XY) with CAIS appear outwardly as females due to the lack of androgens that would normally lead to masculinzation. However, the SRY gene (sex-determining region Y) that encodes TDF still causes testes to develop, although they are usually internal. Individuals with CAIS generally identify as females.

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