Determination of Sex – Role of X and Y Chromosomes

Determination of sex encompasses a multifaceted investigation into chromosomes, genes, hormones, and developmental processes.

Have you ever wondered how we determine whether someone is male or female?

It all comes down to a set of special chromosomes inherited from our parents.

Join me on a fascinating journey to explore the science behind sex determination, including the role of genes, hormones, and developmental biology.

We'll discover the differences between male and female biology and the rare variations that can result in intersex conditions.

Sex Determination

The word "sex determination" in biology refers to figuring out an organism's sex. We use a sex-determination system that aids in predicting how an organism will develop its sexual characteristics. The majority of organisms that produce children through sexual reproduction have two sexes. For instance, two sexes, one male and one female, determine the sex of humans.

The technique that aids in separating one gender from another within a species is sex determination. When male and female gametes unite during fertilisation, an individual's sex is established. Environmental conditions in some organisms determine it.

Meiosis and the fusing of gametes facilitate the genetic material mixing that constitutes sex.

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Types of Sex Determination

The different methods of sex determination are as follows:

1. Genetic or Allosomic Sex Determination

Genetic or allosomic sex determination is the process by which the gender of an organism is decided by its sex chromosome.

In most organisms, females have  similar types of chromosomes(XX); on the other hand, males have dissimilar types of chromosomes (XY).

It makes clear that this sort of fertilisation exhibits male heterogamy because male organisms are in charge of deciding the sex of the offspring.

On the other hand, in organisms where the male has chromosomes of the same kind (ZZ) and the female has chromosomes of a different type (ZW).

The fact that female organisms determine the sex of the progeny in this sort of fertilisation demonstrates female heterogamy.

2. Non- Allosomic Sex Determination

The autosomes, which carry the genes necessary to determine an organism's sex, are used in non-allomic sex determination.

The f-factor-containing plasmid in bacteria is crucial for determining the gender of the bacteria.

Chlamydomonas autosomes, which contain strain-specific genes, are used to categorise the strains of this bacterium.

One type of gene in the maize plant's autosomes determines male inflorescence, while the other type of gene determines female inflorescence.

3. Environmental Sex Determination

Environmental factors are used to determine sex in a completely non-genetic manner.

As the larva of the bonelli worm swims alone in brackish water, it is converted into a female creature; however, when the larva comes into contact with a female bonelia, it is modified into a male organism.

Male mosquitoes often develop from eggs at high temperatures. In contrast, female mosquitoes typically develop at low temperatures.

4. Haplodiploidy

Some creatures, such as mites, wasps, honey bees, etc., do not contain allosomes. Therefore the sex of the individual is determined by the set of the genome with a significant function. The first gender is haploid, while the second sex is diploid.

Male organisms are haploid and develop from eggs without being fertilised. Parthenogenesis is the process through which an organism develops without being fertilised.

Conversely, females have grown from fertilised eggs, so they are diploid.

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Genetic Sex Determination

Genetic sex determination refers to the process by which the sex of an organism is determined by its genetic makeup. In most species, including humans, sex is determined by the presence or absence of specific sex chromosomes. An organism's sex is typically determined genetically by particular chromosomes termed sex chromosomes or allosomes.

There are five genetic ways to determine sex:

1. XX-XY Method

XX is the female, and XY is the male, as in mammals and some insects like Drosophila. In this type, the male is heterogametic or heteromorphic and possesses a shorter and morphologically distinct Y-chromosome (humans). At the same time, the female is homogametic (isomorphic), holding two identical sex chromosomes (XX). Morphology may vary across various organisms.

2. XX-X0 Method

XX-female, X0-male, for example insects, roundworms. In this type, the male has one chromosome and creates two types of sperms (heterogametic): (XX) gymnosperms with X and androsperms without X. The female has two homomorphic sex chromosomes and produces similar eggs (homogametic).

Male                             Female

Parents  (AA-X0)                        ( AA+XX)

Gamete  (A+X)(A+0)                  (A+X)(A+X)

Offspring  (AA+ XX), (AA +XX), (AA+ X0), (AA+ X0)

3. ZW-ZZ Method

ZW is female and ZZ is male, like in fish, birds, and reptiles. In this type, the male has two similar sex chromosomes (ZZ) and is homogametic, and the female has two sex chromosomes (Z, W) and is heterogametic.

Male                                Female

Parents (AA+ZZ)                         ( AA+ZW)

Gamete (A+Z)(A+Z)                    (A+Z)(A+W)

Offspring (AA+ZZ), (AA +ZZ), (AA+ ZW), (AA+ ZW)

4. Z0-ZZ Method

Z0-female, ZZ-male, such as butterflies and moths. The male is homogametic in this kind, while the female is heterogametic.

Male.                         Female

Parents (AA+ZZ)                      ( AA+Z0)

Gamete (A+Z)(A+Z)                  (A+Z)(A+0)

Offspring (AA+ZZ), (AA +ZZ), (AA+ Z0), (AA+ Z0)

5. Haploid Diploid Method

The haploid-diploid method of sex determination is a system in which the sex of an organism is determined by the number of sets of chromosomes it has. In this system, females have two sets of chromosomes (diploid), while males have only one set of chromosomes (haploid).

This method is found in some species of insects, such as ants and bees, and is known as haplodiploidy. In these species, unfertilised eggs develop into males with only one set of chromosomes, while fertilised eggs develop into females with two sets of chromosomes.

The haploid-diploid method of sex determination is also found in some species of plants, such as ferns, mosses, and liverworts. In these plants, the gametophyte stage of the life cycle is haploid, while the sporophyte stage is diploid. Therefore, the sex of the plant is determined by whether the gametophyte is male or female.

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Sex Determination in Humans

People in this kind has a sex-determining mechanism of the XX-XY variety. 22 pairs of the total 23 pairs of chromosomes—present in both males and females—are the autosomes.

Females have a pair of X chromosomes, whereas male characteristics are determined by the presence of an X and Y chromosome. The sperm's genetic composition affects the sex of the offspring.

Henking discovered the X chromosome (1891).

Stevens discovered the Y chromosome (1902).

Sex determination in humans occurs very early in fetal development. The presence of the SRY gene on the Y chromosome triggers the development of male gonads, which will produce testosterone and lead to the development of male genitalia. Without the SRY gene, female gonads will develop, and female genitalia will form.

It is important to note that while sex determination is based on genetic factors, biological sex is not always binary. Some individuals may be born intersex, meaning they have ambiguous genitalia or reproductive organs. Intersex conditions can be caused by various genetic, hormonal, or environmental factors and may require medical intervention.