Testis determining factor
Encyclopedia
Testis-determining factor (TDF) is a general term for the gene
(or product thereof) that results in maleness in humans and some other species.
Certain genes cause chemical reactions that result in the development of testes. Embryo
s are gonad
ally 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 factor
s, or is a transcription factor itself. Its expression
directly or indirectly causes the development of primary sex cord
s, 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 HY antigen in the control of testicular development, which was later disproven.
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).
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
(or product thereof) that results in maleness in humans and some other species.
Certain genes cause chemical reactions that result in the development of testes. Embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
s are gonad
Gonad
The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...
ally 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
Female
Female is the sex of an organism, or a part of an organism, which produces non-mobile ova .- Defining characteristics :The ova are defined as the larger gametes in a heterogamous reproduction system, while the smaller, usually motile gamete, the spermatozoon, is produced by the male...
.
The TDF is encoded by the SRY
SRY
SRY is a sex-determining gene on the Y chromosome in the therians .This intronless gene encodes a transcription factor that is a member of the SOX gene family of DNA-binding proteins...
gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
located in the Y chromosome
Y chromosome
The Y chromosome is one of the two sex-determining chromosomes in most mammals, including humans. In mammals, it contains the gene SRY, which triggers testis development if present. The human Y chromosome is composed of about 60 million base pairs...
. It is a DNA-binding protein that enhances other transcription factor
Transcription factor
In molecular biology and genetics, a transcription factor is a protein that binds to specific DNA sequences, thereby controlling the flow of genetic information from DNA to mRNA...
s, or is a transcription factor itself. Its expression
Gene expression
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...
directly or indirectly causes the development of primary sex cord
Sex cord
In animal embryology, the gonadal cords or sex cords are structures that develop from the gonadal ridge. After sexual differentiation, in males the sex cords become the testis cords, which help develop and nourish the Sertoli cells, while in females they become the cortical cords.-External...
s, which will later develop to seminiferous tubules. These cords form in the central part of the yet-undifferentiated gonad
Gonad
The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...
, turning it into a testis. The now induced Leydig cells of the testis then starts secreting testosterone
Testosterone
Testosterone is a steroid hormone from the androgen group and is found in mammals, reptiles, birds, and other vertebrates. In mammals, testosterone is primarily secreted in the testes of males and the ovaries of females, although small amounts are also secreted by the adrenal glands...
while the Sertoli cells produce Mullerian Inhibiting Substance.
Older texts discuss the role of the HY antigen in the control of testicular development, which was later disproven.
Role in disease
The TDF gene has some interesting implications. The genetic recombinationGenetic recombination
Genetic recombination is a process by which a molecule of nucleic acid is broken and then joined to a different one. Recombination can occur between similar molecules of DNA, as in homologous recombination, or dissimilar molecules, as in non-homologous end joining. Recombination is a common method...
of Crossing over
Chromosomal crossover
Chromosomal crossover is an exchange of genetic material between homologous chromosomes. It is one of the final phases of genetic recombination, which occurs during prophase I of meiosis in a process called synapsis. Synapsis begins before the synaptonemal complex develops, and is not completed...
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
XX male syndrome
XX male syndrome is a rare sex chromosomal disorder. Usually it is caused by unequal crossing over between X and Y chromosomes during meiosis in the father, which results in the X chromosome containing the normally-male SRY gene...
).
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
Swyer syndrome
Swyer syndrome, or XY gonadal dysgenesis, is a type of hypogonadism in a person whose karyotype is 46,XY. The person is externally female with streak gonads, and left untreated, will not experience puberty...
(46XY, genotypic male but phenotypic female).