Determination Of Sex: Definition, Meaning, examples, Chromosomes, Genes, Hormones

Sex determination is a biological process by which the genetic makeup of an organism is determined, usually through genetic, environmental, or hormonal mechanisms. This process is controlled by specific chromosomes known as sex chromosomes. In human reproduction, X and Y chromosomes inherited from the parents decides the sex of the offspring. XX results in a female and XY in a male. It is important to understand sex determination to explain how traits related to sex are inherited and how sex development occurs.

This Story also Contains

1. What is Sex Determination?
2. Determining the Sex
3. Sex Determination in Other Organisms
4. Recommended Video on Determination of Sex
5. MCQs on Sex Determination

In genetics, this process helps in diagnosing sex-linked genetic disorders, like haemophilia. It is also useful in genetic counselling, and fertility treatments. Accurate knowledge of sex determination not only deepens our understanding of reproductive biology but also supports reproductive health and medical care.

What is Sex Determination?

Sex determination is a biological process that determines an organism to be either male or female. This process focuses on defining the rep structure and function of an organism. It comprises interaction of genetics, environment, and hormones that guide the process. In many species chromosomes also play an important role, including in humans.

Sex determination plays a key role in reproduction and ensures the genetic diversity of a species. It ensures both male and female organisms are present for sexual reproduction, where genetic material from two parents combines. This genetic recombination promotes variation, adaptability, and evolution.

The mechanisms of sex determination helps scientists understand how genes control development. These mechanisms allow researchers to study genetic expression and its regulation. This knowledge is critical in fields like genetic engineering, reproductive medicine, developmental biology, and biotechnology.

Sex determination has a major significance in evolutionary biology. Different species have different methods of sex determination, such as XY, ZW, or environmental sex determination. Such methods help trace evolutionary relationships and how selective pressures have shaped reproductive strategies over millions of years. This helps build evolutionary trees and identify relationships among species.

Determining the Sex

Sex determination is mainly controlled by the chromosomes inherited by the parents. In most organisms including humans, a particular combination of sex chromosomes plays a key role in determining whether the embryo/baby will be male or female. These chromosomes carry specific genes that influence the development of reproductive organs and sexual traits

One of the most common systems of sex determination is the XX/XY system. In this system, males have one X and one Y chromosome (XY), while females have two X chromosomes (XX). The Y chromosome carries a gene called SRY (Sex-determining Region Y) gene. It triggers the expression of male characteristics. In its absence, female characteristics develop in an organism.

Key Points In Genetic Determination of Sex:

• The Y chromosome contains the SRY gene responsible for male development.

• Without the SRY gene, the embryo develops as a female.

• Other genes on the X chromosome and autosomes also take part in sexual differentiation.

Sex Determination in Other Organisms

In most organisms, sex is determined biologically through genetic, environmental, or hormonal factors. In humans and most animals, it is mainly controlled by sex chromosomes. However, different organisms use different methods. The following are some important types of sex determination systems found in other organisms:

Chromosomal Sex Determination

Sex is determined by the presence of a specific combination of sex chromosomes. This genetic system plays a key role in the development of male or female characteristics. The chromosomal system shows diversity of genetic mechanisms that control sex determination across species. Some of the common examples include:

1. In humans and most mammals, the XX/XY system. Females have two X chromosomes (XX). While males have one X and one Y chromosome (XY).

2. In the roundworms (Ascaris), grasshoppers and some other insects, the XX/XO is seen. The females have two X chromosomes (XX), whereas the males have just one X chromosome (XO) with no corresponding Y.

3. Birds, reptiles, and some fishes have a ZW system. Females are ZW, and the males are ZZ.

4. In certain moths and butterflies, the ZO/ZZ system is observed. Females show Z0 (one Z chromosome) while males are ZZ.

Environmental Sex Determination

In some species, sex is determined by environmental conditions, such as temperature. It is common in reptiles, including turtles and crocodiles. The incubation temperature decides whether the hatchling will be a male or female. Higher or lower temperatures may also influence development to produce more of one sex than the other.

This type of sex determination allows the species to regulate their sex ratio based on environmental conditions This improves their chances of survival changing ecological situations.

Haplodiploid Sex Determination

This system is mainly found in honey bees, ants, and wasps. These organisms determine their sex based on the number of chromosome sets that they receive.

Fertilized eggs are diploid (two sets of chromosomes) and develop into females. While unfertilized eggs are haploid (single set of chromosomes) and develop into males. This system also reflects on the social behaviours and colony structure of insects. Females are often workers or queens and males have specific roles in reproduction.

Hermaphroditism and Sequential Sex Determination

Some organisms, particularly some species of fish and invertebrates, can change sex throughout their lifetimes based on social or environmental cues. This is known as sequential hermaphroditism.

For example, in clown fish, the largest fish becomes female and the smaller ones are male. If the dominant female is removed from the group or dies, then a male may change into a female to replace the female. This maintains reproductive balance and ensures survival.

Recommended Video on Determination of Sex

MCQs on Sex Determination

Q1. In birds, which sex chromosome determines the sex of the offspring?

1. Z chromosome

2. W chromosome

3. X chromosome

4. Y chromosome

Correct Answer: 1) Z chromosome

Explanation:

In birds, sex is determined by the presence of sex chromosomes, just like in mammals. However, the sex chromosomes in birds are different from those in mammals. In birds, females have two different sex chromosomes called Z and W. Males, on the other hand, have two identical Z chromosomes. The presence of the ZW chromosome pair in a bird determines its sex.

If an egg is fertilized by a sperm carrying a Z chromosome, the resulting offspring will develop as a male (ZZ), while if the egg is fertilized by a sperm carrying a W chromosome, the offspring will develop as a female (ZW). Therefore, it is the presence or absence of the W chromosome that determines the sex of the offspring in birds.

Hence, the correct answer is option 1) Z chromosome.

Q2. What is the most common method of sex determination in mammals?

1. The presence or absence of the Y chromosome

2. The presence or absence of the X chromosome

3. Combination of sex chromosomes

4. Environmental factors

Correct Answer: 3) Combination of sex chromosomes.

Explanation:

The most prevalent method of sex determination among mammals is the XY system. Which is the combination of sex chromosomes.

This mechanism operates through the presence of specific chromosomes:

• females typically possess a homologous pair of X chromosomes (XX)

• while males are characterized by a heterologous pair consisting of one X and one Y chromosome (XY). The critical factor initiating male development is the SRY gene, which is located on the Y chromosome. Known as the Sex-determining Region Y, this gene is instrumental in inducing the differentiation of male sex characteristics. In the absence of the SRY gene.

• As is the case in females, the organism follows a default pathway leading to the development of female traits.

Hence, the correct answer is option 3) Combination of sex chromosomes.

Q3. Which one of the following conditions of the zygotic cell would lead to the birth of a normal human female child?

1. Only one X chromosome

2. One X and one Y chromosome

3. Only one Y chromosome

4. Two X chromosome

Correct Answer: 4) Two X chromosome

Explanation:

At birth, females have two X chromosomes. (XX). One X and one Y chromosome each are present in males. Any condition wherein such chromosome number is altered would lead to abnormality.

Hence, the correct answer is option 3) Two X chromosomes.

Also Read:

• MCQs on Sex Determination

• Difference between Linkage and Crossing Over

• Molecular Basis on Inheritance