Sex determination is a complex system of genes, proteins coded by them and their functions on various target organs that cumulatively determine the development of sexual characteristics in an organism.
The process of sex determination in an organism starts with the development of the male and female reproductive structures right at the stage of embryonic development. The sex determination system is switched on in an undifferentiated zygote depending on the type of sex chromosomes present in it. What follows is a cascade of events comprising protein synthesis and activation and muting of genes and synthesis of various proteins (factors) that act differently on various organs and lead to their sex-specific development.
Chromosomes are rod-shaped structures which become clearly visible during cell division in the cell. They are highly supercoiled DNA molecules that remain coiled around histone and non-histone proteins. The chromosomes are present as less coiled thread-like structures called chromatin in the nucleus of a non-dividing cell forming a network called chromatin reticulum. The chromosome number in a species remains constant. However, chromosomal aberrations are sometimes seen that create abnormalities in human beings.
There are 46 chromosomes in the human cells. Out of these 44 chromosomes are autosomes and have genes for characters which are not related to the formation of reproductive organs. The rest of the two are called sex chromosomes. There are two types of sex chromosomes. X and Y. Females have 44 autosomes and 2 X chromosomes and males have 44 autosomes and one X and one Y chromosome. Hence all the gametes produced in females (ova) contain X chromosomes, while sperms can be of two types X containing and Y containing. When the X containing sperm fertilizes the ova, female offsprings are formed. When the ovum is fertilized by Y containing sperms male offsprings are formed. Hence the sex of the child is determined by the type of sex chromosome contributed by the father.
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The x chromosome is made up of 155 million DNA base pairs and accounts for 5% of the DNA in a cell. The total number of genes present in the X chromosomes is nearly 800 to 900. In 1901 Clarence Erwin McClung suggested for the first time that X chromosomes were involved in sex determination. The gene BMP15 lies in the short arm of the X chromosome and helps in the development of ovaries in females. Gene ZFX has been shown to impact the number of germ cell production in other mammals with less effect in humans. Apart from these genes, X chromosome also contains genes for bodily characters like coloured vision, the gene for anti-haemophilic factor and a number of genes involved in the development of mental abilities of a person.
Other than these, there are hundreds of genes which are not involved in sex determination in females. Human males have only one X chromosome while human females have two. Hence, in the early stages of development, one of the two X chromosomes is permanently inactivated in the somatic cells so that gene expression of one X chromosome out of the two is restricted after the successful development of the sex organs in females. This ensures that the number of proteins produced by the gene expression of X chromosomes in human males and females remains the same. This phenomenon is known as dosage compensation. The process of inactivation is called lyonization and the inactivated X chromosome in the form of facultative heterochromatin is known as the Barr body.
In 1905, Nettie Stevens identified Y chromosomes as a sex-determining chromosome while working on mealworms. She proposed that the theory of C.E. McClung identifying X chromosome as the sex-determining chromosome was wrong. The Y chromosomes contain the SRY gene. SRY or the sex-determining region of Y, also known as the Testis Determining Factor (TDF) produces a protein which initiates testis development during embryogenesis in placental mammals and marsupials. The primordial gonad during embryogenesis is bipotential and can be converted into testes or ovaries. The male-specific transcription factors are activated by the TDF so that the bipotential cells of the gonad develop into Sertoli cells. The cells which have functional TDF proteins differentiate to form the cells of the testis.
Numerical abnormalities of X and Y chromosomes are responsible for causing several conditions in human beings. Some of them are discussed below.
Klinefelter Syndrome: A condition where 44+XXY karyotype gives rise to male individuals with a reproductive disability, tall structure, low IQ learning and reading disabilities.
Triple X Syndrome: A condition where 44+XXX karyotype gives rise to female individuals who are fertile, with average IQ and stature more than that of an average female.
Turner’s Syndrome: A condition where 44+X0 karyotype gives rise to female individuals with short structure. They are infertile as they have ill-developed reproductive parts.
XYY Syndrome: A condition where 44+XYY karyotype gives rise to male individuals with increased stature, learning problems etc.
The sex of the foetus can be determined before birth by the process called Amniocentesis. The foetus floats in the amniotic sac filled with amniotic fluid in the uterus. The cells from the foetus shed off and float in the amniotic fluid. A needle is used to draw a small volume of this fluid guided by ultrasound. Karyotyping of the cells of this fluid helps in the determination of the presence of X and Y chromosomes in the cells of the foetus. This process is however banned in India considering the increased cases of female foeticides in different regions. Thus sex determination before birth is a punishable offence and can lead to imprisonment up to seven years and a fine of up to 5 lakhs.
1. What is the Role of the Y Chromosome in Sex Determination in Human Beings?
Answer: Sex is determined by the type of sperm fertilizing the ovum to form a zygote. There are two types of sex chromosomes in man X and Y. When a zygote formed by the union of an ovum, universally containing X chromosomes and a sperm containing Y chromosome, a male individual is formed. The Y chromosome contains the SRY or the sex-determining region of Y, also known as the Testis Determining Factor (TDF). This gene produces a protein which initiates testis development during embryogenesis in placental mammals. The primordial gonad which is bipotential gets converted into testes when the TDF gene is activated in the developing embryo. Thus Y chromosome from the father is responsible for sex determination of a child.
2. What are the different Types of Disorders Seen in Man Due to Sex Chromosomal Aberrations?
Answer: During the process of gametogenesis by meiosis, non-disjunction of chromosomes in anaphase may take place. This may lead to the production of ova or sperms containing more than one sex chromosome. When these gametes combine with normal gametes or those which also have been formed due to non-disjunction, chromosomal aberrations lead to conditions some of which are discussed below.
Klinefelter syndrome: A condition where 44+XXY karyotype gives rise to male individuals with a reproductive disability, tall structure, low IQ learning and reading disabilities.
Triple X syndrome: A condition where 44+XXX karyotype gives rise to female individuals who are fertile, with average IQ and stature more than that of an average female.
Turner’s syndrome: A condition where 44+X0 karyotype gives rise to female individuals with short structure. They are infertile as they have ill-developed reproductive parts.
XYY syndrome: A condition where 44+XYY karyotype gives rise to male individuals with increased stature, learning problems etc.
3. What is Amniocentesis?
Answer: The foetus develops in a sac called the amniotic sac which remains filled with a fluid called the amniotic fluid in the uterus. The cells of the foetus shed off and remain floating in this fluid. To determine the sex of a foetus, this amniotic fluid is collected with a syringe guided by ultrasound. When the karyotype of the cells in this fluid is analyzed, the sex of the foetus can be determined. This process is banned in India owing to the increasing cases of female foeticides.
4. How Many Types of Sex Can be Determined?
Answer: Sex chromosomes can take different forms and sex determination can happen in many ways. Depending on these two factors, five main biological sexes that can be determined. These are- male, female, hermaphrodite, male pseudohermaphrodite (individuals having testes and a few female genitalia but no ovary), and female pseudoherma[hrodite (individuals having ovaries and a few male genitalia but no testis).