Sex determination

Introduction

    When asking about your sex, you can easily identify yourself as either male or female. What about why? Perhaps it is just because you are told that you are of that sex since you were a child. 

    Generally speaking, sex is determined by the possession of certain sexual characteristics which are governed by sex genes. These sex genes reside in sex chromosomes in one's body and in human, there are two types of sex chromosomes, X and Y. For human, XY are found in a male and XX in a female. However, there may be a condition when a baby girl, who possesses female genitalia (vagina), to have XY sex chromosomes. She will not menstruate and her breast will not develop as her grows. This is the case when one's genetic sex does not match with its phenotypic sex. So should we consider the 'girl' male or female?

    Apart from looking at one's appearance, sex can be determined more specifically in the following four ways. 

Human sexual identity

    In fact, in humans, both male and female embryos have the same structures until the 7th week after fertilization. There must be difference in their sex genes in a baby boy or a baby girl's body to initiate sexual differentiation. But before we discuss anything further, let' s first look at how to determine our genetic sex.

Chromosomal sex

    The genetic composition of male and female is almost the same, each possessing a diploid number (44 for human) of autosomes, body chromosomes. Therefore the remaining one pair of sex chromosomes, allosomes, contributes to sex difference. These sex chromosomes can be either X or Y. X chromosome is the one bearing many genes, more than 1,000 for human and longer, and Y chromosome bears only a few genes and is shorter. Hence about one fourth of the alleles on the Y chromosome have their corresponding alleles on the X chromosome. This region is the pseudoautosomal region where crossing over between X chromosome and Y chromosome can occur. There are genes on the Y chromosome that determines maleness. This was discovered by a group of scientists when they found out that XX male had a tiny piece of Y chromosome, and XY female actually had a small part of the Y chromosome missing. The part of Y chromosome missing in XY female and that present in XX male is actually identical.

    For humans, when a female produces ova (eggs) during meiosis, each of the ova receives an X-chromosome. When a male produces sperms during meiosis, about half of the sperms receive an X chromosome and the other half receive an Y chromosome. During fertilization, if an X-carrying sperm penetrates into the ovum, the resulting zygote (XX) will develop into a female. Likewise, if an Y-carrying sperm penetrates into the ovum, the resulting zygote (XY) will develop into a male. 

    In mammals, X inactivation occurs early in development of a female. It is a process in which one X chromosome, either the paternal or the maternal one, in each cell is inactivated. As a result, a female will have some cells in which genes inherited in the X chromosome from her father become expressed while other cells have the genes inherited in X chromosome from her mother become expressed. The process is believed to be controlled by a gene called XIST. However some genes escape the inactivation and remain active. Once an X chromosome is inactivated in a certain cell, all the cells formed from the divisions of that cell will have the same inactivated X chromosome. For homozygous sex-linked alleles, X inactivation will have no effect because no matter which X chromosome is inactivated, the same other allele can be expressed. That is, X inactivation will only affect heterozygous sex-linked alleles. 

    The sex with the same sex chromosomes (XX) is called the homogametic sex, while the sex with two different sex chromosomes (XY) is called heterogametic sex. The fact that male is the heterogametic sex and the female is the homogametic sex only applies to some animals, including all mammals. 

Gonadal sex

    As we have mentioned, human male and female have the same structure until the 7th week after fertilization. There is in fact a male-determining region on the Y chromosome, which causes the development of testes. This region is called SRY, which means sex-determining region of the Y. The product of this SRY region is a kind of protein that controls over other genes to start the development of male structures in the embryo resulting in changing the undifferentiated gonads to testes. The testes will then secrete two kinds of male hormones that start to mold a male. However, if the SRY region is absent, the undifferentiated gonads will develop into ovaries.

Phenotypic sex 

    When the testis is developed, sustentacular cells in it secrete anti- Mullerian hormone to prohibit the development of Mullerian ducts to uterus, fallopian tubes and vagina. Interstitial cells also secrete the male hormone, testosterone to stimulate the growth of internal male reproductive organs, including epididymis, vasa deferentia, seminal vesicles and ejaculatory ducts. Some testosterone changes to dihydro-testosterone (DHT) to initiate the development of external male features, such as urethra, scrotum and penis.

    Sometimes, although testes are present and anti- Mullerian hormone is produced, leading to the degeneration of female structures, a block in testosterone synthesis or cells not able to receive testosterone, the fetus cannot develop external male structures. 'He' will be regarded as a 'girl' with XY chromosomes. However, when 'she' reaches puberty, testosterone may lead to masculinization. 'She' will not menstruate and her breast will not develop.

Psychological sex

    It is difficult to tell why you feel that you belong to a certain sex, and why you are heterosexual, homosexual or bisexual. Perhaps it is the environment in which you grow up creates your sense of belonging to a certain gender. If you are a boy, you are taught to behave and dress up like a male. If you are a girl, you are told that you should remain lady-like and put on your 'feminine clothes'. Most people in the society have a sense that their marriage partner should be of the opposite sex, that is they are heterosexual. This is considered as the law of nature, which makes homosexual and bisexual usually socially unacceptable. 

    Some scientists believed that homosexuality is determined half genetically and half psychologically. It is supported by the result of studies showing that identical twins are more likely to be homosexual. It is also discovered that an individual may have the sense of being homosexual since he/she is a little child. As a result, some believe that homosexuality is partly an inborn feeling and should only be considered a variant among the common heterosexual population, rather than being an aberration. Of course there are also psychological factors causing one to become homosexual.