Question

Explain the mechanism of the genetic determination of sex in humans.

Short answer

The genetic determination of sex in humans is based on the presence or absence of the Y chromosome, which carries the SRY gene responsible for masculinization. During sexual reproduction, sperm cells carrying either an X or Y chromosome fertilize an egg cell that always carries an X chromosome. The resulting zygote's sex is determined by the sperm cell's sex chromosome: XX for female and XY for male.

Step-by-step solution

1. Understand the role of sex chromosomes in humans

In humans, there are 46 chromosomes in each cell, with 23 derived from the mother and 23 derived from the father. Among these chromosomes, there are 22 pairs that are the same between males and females, called autosomes. The 23rd pair is called the sex chromosomes, which are different between the two sexes. Females have two X chromosomes (XX), while males have one X chromosome and one Y chromosome (XY). The presence of the Y chromosome is the key factor in determining the sex of an individual.

2. Explain the role of the SRY gene in sex determination

On the Y chromosome, there is a specific gene called the SRY (Sex-determining Region Y) gene. This gene is responsible for the masculinization of the developing embryo, leading to the formation of testes and the production of male hormones. The absence of the Y chromosome and, therefore, the SRY gene, allows for the development of ovaries and female characteristics.

3. Describe the process of gamete formation during sexual reproduction

During sexual reproduction, sperm cells and egg cells (gametes) are produced through a process called meiosis. Meiosis occurs in the gonads (testes in males, ovaries in females) and results in the reduction of chromosome number by half, with each gamete having 23 chromosomes. During sperm cell formation, there are two possible outcomes for the sex chromosomes – either an X-bearing sperm or a Y-bearing sperm. In egg cells, however, they always carry an X chromosome.

4. Explain the process of fertilization and the resulting zygote

Fertilization is the process where a sperm cell and an egg cell combine to form a zygote, which then develops into an embryo. Since the egg cell always carries an X chromosome, the sex of the zygote (and therefore the offspring) is determined by the sperm cell's sex chromosome. If an X-bearing sperm fertilizes the egg, the zygote will have two X chromosomes (XX) and will develop into a female. If a Y-bearing sperm fertilizes the egg, the zygote will have one X and one Y chromosome (XY) and will develop into a male.

Key concepts

Sex Chromosomes

In humans, sex is determined by a special pair of chromosomes known as sex chromosomes. These form the 23rd pair in the 46 chromosomes found in each cell.

• Females possess two identical X chromosomes (XX).
• Males have one X and one Y chromosome (XY).

While the X chromosome is present in both sexes, the Y chromosome is what differentiates males from females. The presence of the Y chromosome is crucial because it carries genes responsible for male characteristics. Hence, sex chromosomes play a key role in determining an individual's sex. Every person inherits one sex chromosome from each parent, which influences their development as male or female.

SRY Gene

The SRY gene, short for Sex-determining Region Y, is a specific gene located on the Y chromosome. This gene holds the blueprint for male development.
When present, it triggers the formation of testes in the developing embryo. These testes then produce male hormones like testosterone. Here’s how this process goes:

• The SRY gene is activated early in embryonic development.
• It initiates a cascade of genetic activity that leads to the development of male reproductive structures.

Without the SRY gene, as in XX females, the embryo develops ovaries by default and becomes female. This makes the SRY gene pivotal in sexual differentiation.

Meiosis

Meiosis is a specialized form of cell division crucial for the formation of gametes—sperm in males and eggs in females.
This process reduces the chromosome number by half, producing cells with 23 chromosomes instead of the usual 46. Meiosis has two main stages and results in four genetically unique cells. During meiosis:

• Each parent's chromosomes are mixed during crossover events, creating genetic diversity.
• In males, meiosis results in sperm cells that carry either an X or a Y chromosome.
• In females, every egg cell formed carries an X chromosome.

This ensures that when a sperm cell fertilizes an egg, the resulting zygote will have the correct number of chromosomes to develop into a healthy individual.

Fertilization

Fertilization is the process that marks the union of a sperm cell with an egg cell, creating a zygote. This step is crucial in sexual reproduction as it determines the genetic makeup of the offspring.

• The egg cell, always carrying an X chromosome, waits for fertilization in the female reproductive tract.
• The sperm cell, carrying either an X or a Y chromosome, reaches the egg.

If an X chromosome from the sperm fertilizes the egg, the resulting zygote will have two X chromosomes (XX), leading to a female.
If a Y chromosome fertilizes the egg, the zygote will have one X and one Y chromosome (XY), resulting in a male.
This genetic combination will now guide the development of the embryo, ultimately determining the sex of the new individual.