Testis determining factor
From The Art and Popular Culture Encyclopedia
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Testis-determining factor (TDF) is a protein signal that results in maleness in humans and some other species.
Certain genes cause chemical reactions that result in the development of testes. Embryos are gonadally identical, regardless of genetic sex, until a certain point in development; then the testis-determining factor causes male sex organs to develop, whereas lack of this factor will cause the embryo to develop as physically female.
The TDF is encoded by the SRY gene located in the Y chromosome. It is a DNA-binding protein that enhances other transcription factors, or is a transcription factor itself. Its expression directly or indirectly causes the development of primary sex cords, which will later develop to seminiferous tubules. These cords form in the central part of the yet-undifferentiated gonad, turning it into a testis. The now induced Leydig cells of the testis then starts secreting testosterone while the Sertoli cells produce Mullerian Inhibiting Substance.
Older texts discuss the role of the H-Y antigen in the control of testicular development, which was later disproven.
Role in disease
The TDF gene has some interesting implications. The genetic recombination of Crossing over can cause the gene to be transferred on to the X chromosome. In this case, the X chromosome will initiate testis development; so, regardless of whether the person has a Y chromosome, the person will turn into a male. Though everything else will be developed as if it were a female (other sex-related alleles), the apparent sex will be male (a syndrome known as XX male syndrome).
On the converse, such a cross-over event also can result in a Y chromosome that is missing the Sex-Determining Region (SRY), which contains the TDF, replaced with the corresponding sequence from the end of the X chromosome. Individuals that inherit this Y chromosome will develop as females, despite having the normal male chromosomal set of one X and one Y. This is called Swyer syndrome (46XY, genotypic male but phenotypic female).
