What is 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.
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.
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:
(b)
(c)
List of Related Articles
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.
FAQs on Sex Determination
1. What is sex determination in biology?
Sex determination is the biological mechanism that establishes the sex of an organism, defining whether it will develop the sexual characteristics of a male or a female. This process is typically governed by sex chromosomes, which are specific chromosomes that carry the genes controlling sexual development. However, in some species, environmental factors can also influence or determine the sex.
2. How is the sex of a human child determined?
In humans, sex is determined by the XX-XY system. Females have two X chromosomes (XX), making them homogametic as all their eggs (ova) contain a single X chromosome. Males have one X and one Y chromosome (XY), making them heterogametic. They produce two types of sperm: 50% carry the X chromosome and 50% carry the Y chromosome. The sex of the child is determined by which type of sperm fertilizes the egg:
- If an X-carrying sperm fertilizes the egg (X), the zygote will be XX, developing into a female.
- If a Y-carrying sperm fertilizes the egg (X), the zygote will be XY, developing into a male.
3. What are the different types of chromosomal sex determination systems in animals?
There are several chromosomal systems for sex determination found across the animal kingdom. The main types include:
- XX-XY System: Found in humans, most mammals, and some insects. Females are homogametic (XX) and males are heterogametic (XY).
- ZW-ZZ System: Found in birds, some reptiles, and butterflies. In this system, the roles are reversed: males are homogametic (ZZ) and females are heterogametic (ZW). The ovum determines the sex of the offspring.
- XX-XO System: Found in certain insects like grasshoppers and cockroaches. Females have two X chromosomes (XX), while males have only one X chromosome and no corresponding sex chromosome (XO). Males produce two types of gametes, one with an X chromosome and one with none.
4. What are the major chromosomal disorders related to human sex chromosomes?
Anomalies in the number of sex chromosomes, often due to non-disjunction during meiosis, can lead to specific genetic disorders. The most common examples include:
- Klinefelter Syndrome: Occurs in males with an extra X chromosome, resulting in a 44+XXY karyotype. Individuals may have underdeveloped testes, reduced fertility, and some female-like characteristics.
- Turner's Syndrome: Occurs in females who have only one X chromosome, resulting in a 44+X0 karyotype. Individuals are typically short in stature, have underdeveloped ovaries, and are sterile.
- Triple X Syndrome: Females have an extra X chromosome (44+XXX). They are usually fertile and may have no significant symptoms, though some experience learning difficulties.
5. Why is the male parent responsible for the child's sex in humans, not the female parent?
The male parent is responsible because human males are the heterogametic sex, meaning they produce two different types of sex gametes (sperm). Half of their sperm contain an X chromosome, and the other half contain a Y chromosome. In contrast, human females are the homogametic sex; all their eggs contain only one type of sex chromosome—the X chromosome. Since the female can only contribute an X chromosome, the sex of the child exclusively depends on whether the fertilizing sperm carries an X (resulting in an XX female) or a Y (resulting in an XY male).
6. How is sex determination different from sex-linked inheritance?
These are two related but distinct concepts. Sex determination is the process that decides the sex of an organism (e.g., the XX-XY system). In contrast, sex-linked inheritance refers to the inheritance of traits controlled by genes located on the sex chromosomes (X or Y). For example, traits for colour blindness and haemophilia are located on the X chromosome. While sex determination establishes the chromosomal basis of sex, sex-linked inheritance describes how other non-sexual traits are passed down on these same chromosomes.
7. What is the role of the SRY gene, and could a person with XX chromosomes be male?
The SRY gene (Sex-determining Region Y) is a crucial gene located on the Y chromosome. It produces a protein called the Testis-Determining Factor (TDF), which triggers the development of embryonic gonads into testes. In a typical XY male, this gene initiates the male developmental pathway. It is theoretically possible for a person with XX chromosomes to be phenotypically male if the SRY gene is translocated (abnormally moved) from the Y chromosome to an X chromosome during sperm formation. This rare condition is known as XX male syndrome or de la Chapelle syndrome.
8. What is amniocentesis, and why is its use for sex determination banned in India?
Amniocentesis is a prenatal diagnostic technique where a small sample of amniotic fluid is withdrawn from the uterus. This fluid contains fetal cells, which can be karyotyped to detect chromosomal abnormalities like Down syndrome, Turner's syndrome, or Klinefelter's syndrome. Since the karyotype also reveals the sex chromosomes (XX or XY), it can be used to determine the sex of the fetus. In India, its use for sex determination is strictly banned under the Pre-Conception and Pre-Natal Diagnostic Techniques (PCPNDT) Act, 1994. This law was enacted to prevent female foeticide, a practice driven by a societal preference for male children, which has led to a skewed child sex ratio in the country.
