Sex Determination

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.

Sex Chromosome Inheritance in Humans

What are Chromosomes?

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.

Type of Sex Chromosomes in Males and Females

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 offspring are formed. When the ovum is fertilized by Y containing sperm, male offspring are formed. Hence the sex of the child is determined by the type of sex chromosome contributed by the father.

Role of X Chromosomes

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, the 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.

Role of Y Chromosomes

In 1905, Nettie Stevens identified Y chromosomes as a sex-determining chromosome while working on mealworms. She proposed 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.

Sex Chromosomal Defects in Human Beings

Numerical abnormalities of X and Y chromosomes are responsible for causing several conditions in human beings. Some of them are discussed below.

1. 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.

2. 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.

3. 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.

4. XYY Syndrome: A condition where 44+XYY karyotype gives rise to male individuals with increased stature, learning problems etc.

Process of Determination of Sex of a Foetus – Amniocentesis

The sex of the foetus can be determined before birth by the process called Amniocentesis. The fetus floats in the amniotic sac filled with amniotic fluid in the uterus. The cells from the fetus 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 fetus. 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.

Chromosomal Mechanism of Sex Determination

As we know, once the egg of a female gets fertilized by the sperm of a male, it results in the formation of zygote. This zygote is the first cell of a new life and this cell contains the content of both the male and female cells. And this zygote will divide again and again until it eventually forms a baby.

In sexually reproducing organisms besides morphological and behavioral differences between male and female sexes, the chromosomal difference also occurs. A German biologist, Henking (1891) firstly noted that half of the spermatozoa contained an extra chromosome which was called by him is X body. The significance of X body was not known till 1902. In 1902 Mcclung suggested that the X body was involved in some way in determination of sex. Later on it was well known that the X body was a chromosome involved in sex determination so it is called sex chromosome.

In human females all chromosomes are paired and are in equal size. In the mail all the chromosomes are paired but the chromosome prepared with X chromosome was distinctly smaller and was called Y chromosome.

In dioecious diploid organisms following two systems of chromosomal determination of sex have been recognised:

• Heterogametic males

• Heterogametic females

Heterogametic Males : in this type, the female sex has two X chromosomes, while the male has only one X chromosome. Therefore, during the gametogenesis male produces two types of gametes, 50% with X chromosome while 50% without X chromosome.

So a sex which produces two different type of gametes is called heterogametic sex, while sex which produces similar type of gametes is called homogametic sex. The heterogametic male are of following two types:

• XX-XO Type: in certain insects such as Hemiptera and Orthoptera, the female has two X chromosomes while male has only one X chromosome.

• XX-XY Type: In mammals, certain insects and some angiosperm plants, the female has two X chromosomes, while male has one X chromosome and one Y chromosome.

Heterogametic Females : in this type, male sex has two sex chromosomes (X chromosomes), while the female has only one X- chromosome. Therefore, during gametogenesis females produce two types of eggs/ova, 50% with X chromosome while 50% without X chromosome.

• ZO-ZZ Type: in birds, moths and butterflies, the female has one Z chromosome while male has two Z chromosomes.

• ZW-ZZ Type: In fishes, reptiles, some birds and insects, the female has one Z chromosome and one W chromosome, while male have two Z chromosomes.

Environmental Sex Determination in Bonellia

In some organisms the environment influences the determination of sex,such as temperature, population size of species, sex of other individuals etc.

In Marine worm bonellia, Maine bonellia lives inside the cloaca of females like a parasite. The female produces those eggs which are being fertilized by the male. The eggs hatch into the Larva. Dost Lava settles on the proboscis of female, develops into male. If the larva does not find any female to settle, it attaches to the substratum and develops into a female.

Important Questions

1. What Is sex determination?

Ans: The process of determining an offspring's biological sex and consequently, the sexual characteristics they will acquire, is known as sex determination. According to the sex chromosome combinations that they inherit from their parents, humans normally develop as either males or females.

2. How to determine the sex of baby during pregnancy?

Ans: There are several ways to determine the sex of the baby during pregnancy which include Noninvasive Prenatal Testing (NIPT), Amniocentesis, Chorionic Villus Sampling (CVS) and Obstetric Ultrasonography.

Practice Questions

1. What is the type of sex chromosome found in human females?

• XY Type

• XX Type

• XO Type

• None of the above

2. Which of the following is a non-invasive technique used for prenatal sex determination?

• CVS

• Amniocentesis

• NIPT

• All of the above

Answers:

1. (b)

2. (c)

List of Related Articles

• Basics of Genetics

• Molecular Basis of Inheritance

• Chromosomal Disorders in Humans

Conclusion

There are several theories and mechanisms regarding the determination of the sex of an organism, yet there are several aspects like regulatory pathways to be determined in this field. This is because most of the genes that regulate the secondary sexual characteristics of a male and a female are located on autosomes rather than sex chromosomes and these characteristics depend on the extent to which those genes are expressed.