SRY (Sex Determining Region Y)
The sex determining region Y (SRY) gene is located on the Y chromosome. SRY is the main genetic switch for the sexual development of the human male. If the SRY gene is present in a developing embryo, typically it will become male.
The development of sex in a human depends on the presence or absence of an Y chromosome. Chromosomes are the structures in our cells that contain genes. Genes instruct the body on how to grow and develop by making proteins. For example, genes (and the proteins they make) are responsible for what color hair or eyes a person may have, how tall they will be, and what color skin they will have. Genes also direct the development of organs, such as the heart and brain. Genes are constructed out of DNA, deoxyribonucleic acid. DNA is found in the shape of a double helix, like a twisted ladder. The DNA contains the "letters" of the genetic code that make up the "words" or genes that govern the development of the body. The genes are found in the "books" or chromosomes in the cells.
Normally, there are 46 chromosomes, or 23 pairs, in each cell. The first 22 pairs are the same in men and women and are called the autosomes. The last pair, the sex chromosomes, consists of two X chromosomes in females (XX) and an X and an Y chromosome in males (XY). These 23 pairs of chromosomes contain approximately 35,000 genes.
Human males differ from human females in the fact that they have an Y chromosome and females do not. Scientists thought there must be a gene on the Y chromosome that is responsible for determining maleness. The gene for determining maleness was called TDF for testis determining factor. In 1990, the SRY gene was found and scientist believed it was the TDF gene they had been looking for. The evidence scientists had to show SRY was indeed TDF included the fact that is was located on the Y chromosome. When SRY was found in individuals with two X chromosomes (normally females) these individuals had male physical features. Furthermore, some individuals with XY sex chromosomes that had female physical features had mutations or alterations in their SRY gene. Finally, experiments were done on mice that showed a male mouse would develop when SRY was put into a chromosomally female embryo. This evidence proved that SRY is the TDF gene that triggers the pathway of a developing embryo to become male. While the SRY gene triggers the pathway to the development of a male, it is not the only gene responsible for sexual development. Most likely, the SRY gene serves to regulate the activity of other genes in this pathway.
Men and women both have 23 pairs of chromosomes—22 pairs of autosomes and one pair of sex chromosomes (either XX in females or XY in males). The SRY gene is located on the Y chromosome. When a man and woman have a child, it is the man's chromosomes that determine if the baby will be male or female. This is because the baby inherits one of its sex chromosomes from the mother and one from the father. The mother has only X chromosomes to pass on, while the father can pass on either his X chromosome or his Y chromosome. If he passes on his X chromosome, the baby will be female. If he passes on his Y chromosome (with the SRY gene) the baby will be male. Statistically, each pregnancy has a 50% chance of being female and a 50% chance of being male. The Y chromosome is the smallest human chromosome and the SRY region contains a very small number of genes.
Signs and symptoms
Individuals with point mutations or deletions of the SRY gene have a condition known as gonadal dysgenesis, XY female type, also called Swyer syndrome. At birth the individuals with the XY female type of gonadal dysgenesis appear to be normal females (with female inner and outer genitalia), however, they do not develop secondary sexual characteristics at puberty, do not menstruate, and have "streak" (undeveloped) gonads. They
have normal stature and an increased incidence of certain neoplasms (gonadoblastoma and germinoma).
In normal human sexual development, there are two stages called determination and differentiation. Determination occurs at conception when a sperm from a man fertilizes an egg from a woman. If the sperm has an Y chromosome, the conception will eventually become male. If no Y chromosome is present, the conception will become female.
Though the determination of sex occurs at conception, the differentiation of the developing gonads (future ovaries in the female and testes in the males) does not occur until about seven weeks. Until that time, the gonads look the same in both sexes and are called undifferentiated or indifferent. At this point in development, the embryo has two sets of ducts: the Mullerian ducts that form the fallopian tubes, uterus and upper vagina in females and the Wolffian ducts that form the epididymis, vas deferens, and seminal vesicles in males.
In embryos with SRY present, the undifferentiated gonads will develop into the male testes. The testes produce two hormones that cause the differentiation into maleness. Mullerian inhibiting substance (MIS), also called anti-mullerian hormone (AMH), causes the Mullerian ducts to regress and the Wolffian ducts develop into the internal male structures. Testosterone also helps with the development of the Wolffian ducts and causes the external genitals to become male.
When SRY is not present, the pathway of sexual development is shifted into female development. The undifferentiated gonads become ovaries. The Mullerian ducts develop into the internal female structures and the Wolffian ducts regress. The external genitals do not masculinize and become female.
SRY and male development
As of 2001, how the SRY gene causes an undifferentiated gonad to become a testis and eventually determine the maleness of a developing embryo is not completely understood. What scientists believe happens is that SRY is responsible for "triggering" a pathway of other genes
It is also thought that a threshold exists that must be met at a very specific time for SRY to trigger this pathway. This means that enough SRY protein must be made early in development (before seven weeks) to turn an undifferentiated gonad into a testis. If enough SRY is not present or if it is present too late in development, the gonad will shift into the female pathway.
Other genes in sex development
Several other genes have been found that are involved in the development of human sex, including the gene SOX9. Mutations or alterations in this gene can cause a condition called camptomelic dysplasia. People with camptomelic dysplasia have bone and cartilage changes. SOX9 alterations also cause male to female sex reversal in most affected individuals (male chromosomes and female features). As of 2001, it is not known how SRY, SOX9, and other genes in the sexual developmental pathway interact to turn an undifferentiated gonad into a testis or an ovary.
Zenteno, J.C., et al. "Clinical Expression and SRY Gene Analysis in XY Subjects Lacking Gonadal Tissue." American Journal of Medical Genetics 99 (March 15, 2001): 244-47.
"Sex-determining Region Y." Online Mendelian Inheritance in Man. <http://www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=480000>.
Carin Lea Beltz, MS