Sex Determination in Humans

Diagram for Sex Determination in Humans

Here, you can see the karyotypes of a male and a female human. A karyotype can be used to search for abnormalities in the number of chromosomes or to look for structural problems.

The figure also shows that the 22 pairs of autosomal chromosomes are the same in male and female humans. However, you can see a difference between the male and female sex chromosomes. The male has one X and one Y chromosome, while the female has two X chromosomes. You can also see that the Y chromosome is smaller than the X chromosome.

Sex Determination and Differentiation

The presence of the SRY gene on the Y chromosome determines biological maleness in most mammals, including humans. Because of this, individuals with abnormal sex chromosomal makeup, such as XXY, grow into males and develop male sexual characteristics.

This gene encodes a protein called the testis-determining factor (TDF) or the SRY protein. TDF is a transcription factor, and it binds to a specific region of the DNA and activates a cascade in the early embryo that initiates the development of testes from primary gonads. Without TDF, the primary sex organs develop into ovaries.

Specific cells in the male testes produce testosterone, the male sex hormone, which forms male primary sexual characteristics. These are biological sexual features present at birth, such as the penis. Without testosterone, the fetus develops female primary sexual characteristics, such as a vagina and a uterus.

Although, humans form secondary sexual characteristics in addition to the primary ones. This process occurs during puberty, including events such as elongation of the penis in males, growth of breast tissue in females, and growing pubic hair in both sexes.

Similar to the formation of primary sex traits, testosterone plays a crucial role in developing secondary sexual characteristics in males. In females, however, the absence of testosterone is not enough. The female sexual hormones, most importantly oestrogen, are needed for puberty in females.

Formation of Gametes in Males and Females

The formation of gametes (gametogenesis) occurs via meiosis of reproductive cells. Meiosis is a cell division that creates gamete cells while reducing the number of chromosomes in the parent cell by half. This mechanism is necessary to produce egg and sperm cells for sexual reproduction.

The male is the heterogametic sex in humans. This means that men produce two types of sperms, one that carries the X chromosome and another that carries the Y.

On the other hand, the female is the homogametic sex since all egg cells (aka ova) carry an X chromosome. The sex of the offspring is determined by whether the sperm that fertilises the egg has an X or a Y chromosome.

Flow Chart Summarizing Sex Determination in Humans

During fertilisation, the sperm from a male parent meets an egg cell from a female parent and forms a zygote. Since each gamete carries only half of the parent's genetic material, combining both restores the standard number of chromosomes, like in non-sex cells.

The zygote is the first cell of the offspring. If a sperm carrying the X chromosome fertilises with an egg cell, the zygote will have two X chromosomes, which will develop into a biological female. However, suppose the other sperm type that contains the Y chromosome fuses with an egg cell. In that case, the zygote will have an X and a Y chromosome, developing into a biologically male child. The flowchart below demonstrates this process.

Example of Sex Determination in Humans

Now that we have learned the principles of sex determination, we will go through an example using a Punnett square.

First, we need to write down the parents' genotype regarding their sex chromosomes:

Father: XY x Mother: XX

Now we have to write the possible genotype of gametes that the mother and father can generate:

Sperms: X or Y

Egg cells: X

In the next step, we put the gametes in a table wherein the columns are the gametes from one parent and the rows are the gametes from the other parent. It makes no difference which one of the parents is on the side or the top of the Punnett square. Here we place the mother's on the top:

In each box of the table, we can combine the gametes to get the genotype of the potential offspring:

As demonstrated by the Punnett square above, half of the potential children on male and the other half are female. We can use the equation below to calculate the probability of a child of this couple being male or female:

$Probability=\frac{Numberoftimestheoutcomeofinterestoccurs}{Totalnumberofpossibleoutcomes}$

So, the probability of a child being male is 2/4 or 50%. This means the probability of the child being male is equal to the probability of it being a female.