Sex Determination - Sex Determination in Human

Sex Determination in Human:

• Genes determining sex in human beings are located on two sex chromosomes, called allosomes.
• The sex determining mechanism in the case of humans is 'XY type'.
• The human diploid cell has 46 chromosomes, i.e. 23 pairs in each cell.
• Out of 23 pairs of chromosomes present, 22 pairs are exactly the same in both males and females; these are the autosomes. Autosomes determine all body characters like colour of hair, skin, colour of eyes, height, etc.
• The human male has a pair of heterosomes called sex chromosomes, and the female has a homologous pair of sex chromosomes. It is represented as XY and XX respectively in males and females.
• One set of chromosomes is inherited from each parent, one from the mother and one from the father.
  

  Sex determination in human beings

• As a result, a human male has 44 + XY chromosomes, while a female has 44 + XX chromosomes.
• During spermatogenesis among males, two types of gametes are produced. 50% of the total sperm produced carry the X-chromosome and the rest 50% have Y-chromosome besides the autosomes. Females, however, produce only one type of ovum with an X-chromosome. 
• There is an equal probability of fertilization of the ovum with the sperm carrying either the X or Y chromosome. e.g. Female gamete (ovum) 22 + X Male gamete (sperm) 22 + X or 22 + Y
• In case the ovum fertilizes with a sperm carrying X-chromosome the zygote develops into a female (XX) and the fertilization of the ovum with the Y-chromosome carrying sperm results in a male offspring. Thus, it is evident that it is the genetic makeup of the sperm that determines the sex of the child. It is also evident that in each pregnancy there is always a 50% probability of either a male or a female child. It is unfortunate that in our society women are blamed for giving birth to female children and have been ostracised and ill-treated because of this false notion.
• This type of sex determination is found in certain insects like Drosophila melanogaster and mammals, including humans.
• As females produce similar gametes, they are 'Homogametic' while males are 'Heterogametic'. Such, is a type of sex determination where in the male gamete decides the sex of the progeny (offspring) is designated as 'Male heterogamety'.
• The Y chromosome carries a gene SRY (sex-determining region), also called testis-determining factor (TDF).
• In humans, the absence of the Y chromosome inevitably leads to female development and this SRY gene is absent in the X chromosome. The gene product of SRY is the testes determining factor (TDF) present in the adult male testis.