In this video, we will learn how biological sex is determined in humans by the sex chromosomes called X and Y. We will learn how these chromosomes are inherited and about the role that the human gametes, the sperm and egg cell, play in this process.
Biological sex in humans is largely determined by the sex chromosomes. We call these chromosomes X and Y. Human chromosomes come in pairs, one inherited from each parent. If an individual inherits XX, they’re likely to be biologically female, and if they inherit XY, they’re likely to be biologically male. You can see in this drawing that the Y chromosome in humans is much smaller than the X. Let’s take a closer look at these chromosomes and where they come from.
This drawing represents a human karyotype. A karyotype is a picture of all of the chromosomes in one cell. This karyotype came from a typical somatic cell, which is a diploid cell found in the human body. The first 22 pairs of chromosomes in a somatic cell are called autosomes. These are chromosomes that do not determine the biological sex of an organism. The last pair, or the 23rd pair, represent the sex chromosomes. Based on what we’ve learned so far, do you think that this karyotype shows the chromosomes in the cell of a male or of a female? Since we can see that these two chromosomes are different, that one is large and one is small, we know that they are X and Y and that this person is most likely biologically male.
These drawings show the karyotypes of two gametes produced by the same individual. A gamete is a reproductive cell, sperm in males and eggs, or ova, in females. Since our example human is male, the gametes that they produce will be sperm.
What differences do you notice between the chromosomes of the somatic cell and the chromosomes of the gametes? Well, we can see that the gamete has half of the number of chromosomes that the somatic cell has. The somatic cell has 46 chromosomes arranged in homologous pairs. One chromosome in each of these pairs was inherited from this person’s mother, and the other chromosome in each pair they inherited from their father. The gametes each have 23 chromosomes. Each gamete only carries one of the chromosomes that we see in the pairs in the somatic cell.
What about the sex chromosomes? Since each gamete inherits one chromosome from each pair, the sperm can carry either an X or a Y chromosome to pass on to their offspring. How about an egg cell? Well, since the somatic cell of a female would have two X chromosomes and no Y chromosomes, all of the egg cells produced by a female will carry an X sex chromosome. Let’s simplify our representation a little bit and just focus on the sex chromosomes.
A Punnett square is a tool that scientists use to predict the inheritance of certain traits. So let’s try using one here. So we’ve learned that all of the egg cells produced by a female will contain an X sex chromosome, while the sperm cells produced by a male can contain either an X or a Y chromosome. This tells us that all offspring invariably inherit one X chromosome from their mother, whereas there’s an equal chance of inheriting either an X chromosome or a Y chromosome from their father.
So, we can see that there’s a 50 percent chance of the offspring of any human couple inheriting two X chromosomes and being biologically female, as well as a 50 percent chance of inheriting X and Y and being biologically male. And since egg cells can only contribute an X chromosome to their offspring, the sex of the offspring is determined by the male gamete, or the sperm, since the sperm cell can either contribute an X or a Y sex chromosome.
We have already noted that the Y is much smaller than the X chromosome. Everyone inherits at least one X chromosome, and the X chromosome contains many different genes that carry out many different functions. The Y chromosome contains very few genes, and these genes are almost all associated with male characteristics. Whether an offspring inherits the XX sex chromosomes or X and Y, the development of the fetus will be the same until about six weeks and will be by default female. At around six weeks, a gene on the Y chromosome called the SRY gene becomes active. This gene triggers the development of male characteristics, which is why offspring that inherit a Y chromosome will be born biologically male.
In some rare instances, an SRY gene is transferred to the X chromosome, which can lead to the birth of offspring who possess two X chromosomes but have male characteristics. In other rare instances, offspring can inherit more than two sex chromosomes. In these cases, because of the presence of a Y chromosome, these offspring will also have male characteristics.
Now that we’ve learned about the X and Y chromosome, their characteristics, inheritance, and how they determine biological sex, let’s try a practice question.
Which of the following correctly describes the chromosomes contained within a human female somatic cell? (A) 46 autosomes plus XY, (B) 23 autosomes plus XY, (C) 44 autosomes plus XX, (D) 46 autosomes plus XX, or (E) 44 autosomes plus XY.
This question is asking us to describe the chromosomes, including autosomes and sex chromosomes, that we would expect to find within a typical female somatic cell. First, we’ll need to recall the chromosomes found within a typical body or somatic cell, as well as the sex chromosomes that we would expect a human female to possess. This is a basic diagram of a typical somatic cell. Somatic cells are the cells that you find anywhere in the human body. These are different than the gametes or reproductive cells that you will only find in the reproductive system.
Somatic cells are called diploid because they possess chromosomes in pairs. Gametes or reproductive cells are considered haploid because they possess half of the typical number of chromosomes, which is essential for reproduction. Here, I’ve drawn a karyotype, which is a photograph of all of the chromosomes within a cell as they appear during cell division. Human cells possess 23 pairs of chromosomes in diploid cells. 22 of these pairs of chromosomes do not influence biological sex, and they are called autosomes. In humans, the 23rd pair of chromosomes is responsible for determining biological sex. So, we call them sex chromosomes.
A human somatic cell will possess either an XX or an XY pair of chromosomes as this last pair. Humans who possess XX sex chromosomes are generally biologically female, while humans that possess XY sex chromosomes are generally biologically male. So now, we have all of the information that we need to answer our question. Human female somatic cells possess 22 pair or 44 individual autosomes plus the 23rd pair of sex chromosomes are XX.
Let’s try another practice question.
Which gamete is responsible for determining the sex of a fetus?
This question is asking us about the gametes, which are sex cells used in reproduction. It is asking us which type of sex cell is responsible for determining the biological sex, male or female, of a fetus or offspring. Every cell in the human body that is not a reproductive cell is called a somatic cell. Within the nucleus of each somatic cell, you’ll find 44 autosomes that do not determine the sex of the organism. You will also find two sex chromosomes, which do determine the sex of the organism.
If a human possesses two of what we call the X chromosome, they are likely to be biologically female. If a human possesses an X and what we call a Y chromosome in each of their somatic cells, they’re biologically what we consider to be male. Gametes, or sex cells, are the cells used for reproduction, and in humans they’re only found in the reproductive organs. The ova, or egg cells, are found in the ovaries of females. Sperm cells are found in the testes of males.
Somatic cells are diploid, meaning that they possess chromosomes in homologous pairs, one from each parent. Gametes are haploid, meaning that they only possess one chromosome from each pair to be passed on to their offspring.
That means that if each parent can only pass on one of each pair of chromosomes to their offspring and a male somatic cell possesses the sex chromosomes X and Y, then each of their sperm cells can possess either an X or a Y sex chromosome. But since the sex chromosomes in a female somatic cell are both X, then every ovum produced by a female will have an X sex chromosome. That means that each offspring can only inherit an X chromosome from their mother. Offspring can either inherit an X chromosome from their father or a Y chromosome. This shows us that the gamete responsible for determining the sex of a fetus is the sperm.
Let’s wrap up our lesson by taking a moment to review what we’ve learned. In this video, we learned how sex is determined by the sex chromosomes X and Y. And we learned how sex is inherited by the chromosomes found in gametes.