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:
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: http://en.wikipedia.org/wiki/Testis_determining_factor.
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.
