Most organisms, including humans, exist in two sexes. The factors determining whether an organism develops into a female or a male vary widely. In animals, sex determination is based on differences in chromosomal compositions. Researchers have extensively studied the process of sex determination and have discovered that it is regulated by a series of genes that come into effect in early embryonic development. In this article, we are going to consider sex determination in humans.
Chromosomal Sex Determination
Humans, in each of their cells, contain 23 pairs of chromosomes, adding up to 46 chromosomes. Of these, 22 pairs carry genes that only determine non-sex-related features. These 22 pairs are called autosomes and are the same in males and females. The last pair of chromosomes are the sex chromosomes that differ between males and females.
Autosomes are chromosomes that carry genes that determine non-sex-related characteristics only.
In the X-Y system of sex determination, there are two types of sex chromosomes, X and Y. 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.
It is essential to point out that the chromosomes are generally not aligned like this in a cell. Instead, they are entangled with each other and other proteins, forming chromatin.
A karyotype is a laboratory image of a person's whole set of chromosomes separated from an individual cell and placed in numerical order.
Chromatin is a mixture of DNA or RNA and proteins.
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.
To learn more about chromosomes check out our article on it!
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.
SRY gene contains instructions for making a protein called the sex-determining region Y (SRY) protein.
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.
The details of this pathway are complex and way beyond what is needed to know at this stage.
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.
Androgen insensitivity syndrome (AIS) is a disorder in which a male fetus with one X and one Y chromosome produces testosterone (an androgen hormone), but its cells resist this male hormone. As a result, testosterone does not affect this person. So, what is seen is a person with female physical characteristics but the chromosome composition of a man. AIS can be complete or partial;
- In complete AIS (CAIS), testosterone does not affect sex development, so the person forms female genitals.
- However, partial AIS (PAIS) is when testosterone has little influence on developing sexual characteristics. This results in ambiguous genitals that cannot precisely be said are for males or females.
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.
Those with CAIS have male gonads and so do not produce oestrogen. Therefore, they do not go through puberty and do not develop female sexual characteristics even though they have female genitalia.
The gonads are the primary internal reproductive organs; in human males, this is the testes, and in females, it is the ovaries.
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.
Dive deeper into meiosis and cell division by checking out our articles!
A gamete is an animal or plant reproductive cell.
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.
Remember that hetero = different, and homo = the same.
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.
To learn more about how chromosomes are passed down to us from our parents, check out our article on Genetic Inheritance!
The X-Y sex determination system is only one of several systems found in the animal kingdom. In reality, sex determination can vary significantly among organisms.
Insects such as crickets and butterflies use the X-O system, where sperm cells without an X chromosome (referred to as O) define maleness. Females in this species have two X chromosomes (XX) and exclusively generate gametes with X chromosomes. Conversely, males have only one X chromosome (XO) and create both X and no sex chromosomal gametes.
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.
The Punnett square is a table/diagram used to predict the genetic composition of the offspring in a cross or breeding experiment.
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:
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.
It is important to point out that probability is not the same as possibility. The probability of half does not mean that half of the children will definitely be male and the other half female. A couple can have four children, all of whom may be females!
Sex Determination in Humans - Key takeaways
Humans, in each of their cells, contain 23 pairs of chromosomes. Twenty-two pairs are autosomes, and the last pair are sex chromosomes.
In humans, the male has one X chromosome and one Y chromosome, whereas the female has two X chromosomes. This is called the X-Y sex determination system.
The presence of the SRY gene on the Y chromosome determines maleness in humans. This gene encodes the transcription factor TDF that initiates a cascade of events that result in the differentiation of the primary gonads to testes.
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. Meanwhile, females have only one type of egg cell that carries an X chromosome.
Fusion of a sperm carrying an X chromosome with an egg cell results in a female zygote. But if the sperm fertilises the egg cell and carries a Y chromosome, the zygote will have a male genotype.
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