In this explainer, we will learn how to recall the sex chromosomes in humans and explain their role in determining the sex of a fetus.
Before the discovery of sex chromosomes, scientists believed that sex determination in humans was controlled by environmental factors, like temperature. Now, we know that sex is determined by the presence of the sex chromosomes.
In most human cells, there are 46 chromosomes. These chromosomes are passed down, or inherited, from one’s biological parents; half of one’s chromosomes come from their biological mother and half from their biological father. During reproduction, the chromosomes that are inherited from the parents form 23 chromosome pairs. A brief outline of this process is given in Figure 1.
The first 22 pairs of chromosomes are called autosomes and look similar in male and female cells. In the word autosome, the prefix auto- means “self” and the suffix -some means “body.”
All of the autosomes are arranged into homologous pairs. In homologous chromosome pairs, each chromosome carries the same genes in the same order and location. Autosomes can be referred to using their chromosomal pair number. The smaller the number used to name the chromosome pair, the larger the chromosome size. For example, chromosome 1 (meaning the first pair of chromosomes) is the largest of all autosome pairs. However, the only exception to this general rule is chromosome 22. It was believed it was the smallest of the autosomes, and that is why it was named 22, until it has been discovered that, in fact, chromosome 21 is a little bit smaller than chromosome 22.
Definition: Homologous Chromosomes
Homologous chromosomes are pairs of chromosomes that have similar lengths, the same centromere positioning, and the same genes at each location.
An autosome is any chromosome that is not a sex chromosome (X or Y).
A karyotype is a visual display—or a picture—of all the chromosomes inside a cell. It is obtained from taking a sample of tissue from an organism and stimulating cell division, before pausing the division when the chromosomes are most condensed and therefore most visible. We can see how autosomes are arranged in size order in Figure 2, which shows an example of a human karyotype.
The last “pair” of chromosomes are not really a pair since they are not homologous chromosomes like the autosomes. Instead, the last pair of chromosomes are the sex chromosomes. In humans (and most other mammals), there are two types of sex chromosomes, the X and the Y.
Definition: Sex Chromosomes
A sex chromosome is a chromosome that helps in determining sex in humans and can be one of two types, X or Y.
Example 1: Defining an Autosome
What is the meaning of the term autosome?
- A chromosome that is found in the gametes of a human
- Any chromosome that is not a sex chromosome (X or Y)
- A chromosome that has been automatically replicated, ready for cell division
- A chromosome that is automatically destroyed when damaged
- Any chromosome that has had a section removed
In human somatic cells, there are 46 chromosomes. These chromosomes contain all the genetic information that is to be passed down from biological parents to their biological offspring. The 46 chromosomes are organized into 23 pairs. The last pair (23) of chromosomes are the sex chromosomes, which are different in males and females and help determine sex in humans. There are two types of sex chromosomes, X and Y. The first twenty-two (22) pairs of chromosomes are called autosomes. Autosomes look the same in both male and female cells. An autosome is any chromosome that is not a sex chromosome (X or Y).
Therefore, an autosome is any chromosome that is not a sex chromosome (X or Y).
Unlike the autosomes, the X and Y sex chromosomes have dramatically different sizes and contain a different number of genes. The X chromosome is very large and contains about 900 genes. The Y chromosome, however, is very small and only contains around 55 genes. You can see a simple comparison of these sizes in Figure 3 below. Most female somatic cells have one identical pair of X chromosomes (XX) and 22 pairs of autosomes, while most male cells have one X and one Y chromosome (XY) and 22 pairs of autosomes.
Fact: Biological Sex
In mammals, like humans, biological sex is determined by sex chromosomes. Sex chromosomes are the only difference between male and female somatic cells. Female somatic cells have one identical pair of X chromosomes (XX) and 22 pairs of autosomes. Male somatic cells have one X and one Y chromosome (XY) and 22 pairs of autosomes. This is often simplified to female XX and male XY.
Even though the sex chromosomes help determine the sex-specific features in both male and female fetuses, the development of those characteristics does not begin until weeks after fertilization. This means that until the 6th week of pregnancy, both male and female fetuses develop as female fetuses! After the 6th week of pregnancy, the presence of the Y chromosome in the fetus begins to impact development. The Y chromosome contains a “male-determining gene” called the SRY gene that causes the development of male reproductive organs in the embryo and, later on, external male physical features. After the 12th week of pregnancy, in the absence of the Y chromosome, development and differentiation of the female reproductive organs will begin.
Example 2: Describing the Difference between the X and Y Chromosomes
How do the structures of the X and Y chromosomes in humans differ?
- The X chromosome contains less than of the genes found on the Y chromosome.
- The X chromosome is much shorter than the Y chromosome.
- The Y chromosome is much shorter than the X chromosome.
- The Y chromosome is much wider than the X chromosome.
- There is no difference in structure between the X and Y chromosomes.
In human somatic cells, there are 46 chromosomes. These chromosomes contain all the genetic information that is to be passed down from biological parents to their biological offspring. The 46 chromosomes are organized into 23 pairs. The last pair of chromosomes are the sex chromosomes. The pairing of these chromosomes is different in males and females and helps determine sex in humans.
There are two types of sex chromosomes, X and Y. The X and Y sex chromosomes have dramatically different sizes and contain a different number of genes. The X chromosome is very large and contains about 900 genes. The Y chromosome, however, is very small and only contains around 55 genes. A simple diagram outlining the different sizes of these chromosomes is given below.
One of the genes on the Y chromosome is the “male-determining gene,” the SRY gene. The SRY gene causes the development of male reproductive organs in the fetus after the 6th week of pregnancy. In the absence of the Y chromosome and the SRY gene, the fetus will develop female internal and external reproductive organs.
Therefore, looking back at our answer options, we now know the structures of the X and Y chromosomes in humans differ because the “Y chromosome is much shorter than the X chromosome.”
Sex cells, like sperm and egg cells in humans, are called gametes. A gamete is a cell that contains 23 chromosomes or one from each homologous pair (22 autosomes and 1 sex chromosome). The one sex chromosome found in sex cells helps to determine the final sex chromosome pair in the fetus.
Gametes are an organism’s reproductive cells that contain half the genetic material of a normal body cell.
The female sex cells of most mammals are eggs. Egg cells all contain an X sex chromosome. This means that the female egg cell only contains one X chromosome and can only contribute an X chromosome to the fetus. In human females, an egg cell is produced in the ovary and therefore referred to as ova (more than one egg cell) or ovum (just one egg cell). Therefore, a female human ovum only contains X chromosomes.
Key Term: Egg Cell (Ovum)
An egg, or ovum, is a female sex cell that contains an X chromosome.
Human male sex cells are sperm cells, which can contain an X or Y sex chromosome. When male sperm cells are formed, there is an equal chance that a sperm will contain an X or Y chromosome (see Figure 4). If the sperm cell carries an X chromosome and fuses with the female egg cell that carries one X chromosome, then the fetus will have two X chromosomes (XX). If the sperm cell carries a Y chromosome to the female egg cell with one X chromosome, then the fetus will have one Y sex chromosome and one X sex chromosome (XY). Because the of the ability of the sperm cell to carry either an X or a Y sex chromosome, the male sperm cell is responsible for determining the sex of the fetus. Since the Y chromosome is inherited by male fetuses from their biological fathers, all individuals carrying a Y chromosome are related to a single ancestor who also had an XY sex chromosome and lived around 300 000 years ago!
Key Term: Sperm
A sperm is a male sex cell that can contain either an X or Y chromosome and is responsible for determining the sex of the fetus.
Example 3: Examining the Gamete Responsible for Determining Sex
Which gamete is responsible for determining the sex of a fetus?
Gamete is the term used to describe sex cells that contain one set of chromosomes. Human gametes or sex cells are different in females and males. Human female sex cells are called eggs, or ova.
Each egg cell, or ovum, contains one X chromosome. This means that the female egg cell only contributes one X chromosome to the fetus. In males, the sex cell is called a sperm. Sperm can contain an X or Y sex chromosome. This means that sperm can contribute either an X or Y chromosome to the fetus.
If the sperm contributes an X chromosome to the female egg cell with one X chromosome, then the fetus will have two X chromosomes (XX). If the sperm cell contributes a Y chromosome to the female egg cell with one X chromosome, then the fetus will have one Y sex chromosome and one X sex chromosome (XY). Due to the ability of sperm to contribute either an X or a Y sex chromosome, the sperm is responsible for determining the sex of the fetus.
Therefore, the gamete responsible for determining the sex of a fetus is the sperm.
Due to the ability of the sperm to contribute either an X or a Y chromosome, the chance, or probability, of a fetus having an XX or an XY sex chromosome pair can be calculated using Punnett squares. This method is outlined below.
How To: Constructing a Punnett Square to Show How Sex Is Determined in a Fetus
A Punnett square is an incredibly useful tool to show the inheritance of genetic material by an organism from their parents. When organisms reproduce, their gametes (sex cells) combine in a process called fertilization. We can use the Punnett square to show how the combination of gametes determines the sex of the fetus.
Let’s start by putting the sex chromosomes from each parent in the row and column header of our Punnett square.
Shown in blue are the sex chromosomes carried by sperm cells. Sperm cells can have either a Y or X chromosome. Shown in green are the sex chromosomes carried by egg cells (ova). Ova will only have X chromosomes.
We then complete the Punnett square by filling in each of the cells. We take the letter that is in the column header and the letter that is in the row header and combine them to give a combination of two letters. This is outlined in the diagram below.
We should end up with a Punnett square that looks something like this! Here, we have also added what sex the fetus will be if they inherit this combination of chromosomes.
Using this Punnett square, we can determine that for each successful fertilization, the fetus has a chance of being male and a chance of being female.
The cells of females that will divide to produce gametes contain two X chromosomes. In this case, each ovum produced will have one X chromosome. The cells of males that will divide to produce gametes contain X and Y chromosomes. In this case, each sperm may have one X chromosome or one Y chromosome. Figure 5 outlines how we can demonstrate the inheritance of these sex chromosomes from the gametes using a Punnett square.
Figure 5 demonstrates that for each fetus produced, there is a chance they will be female and a chance they will be male!
Even though the presence of X and Y chromosomes determines the development of our reproductive anatomy (the internal and external reproductive organs), the classification of all human females as XX or all human males as XY is not necessarily correct. Sometimes the development of reproductive anatomy does not always happen as expected.
Humans who have unique variations in their reproductive anatomy fall under the umbrella of being intersex. Intersex is a general term used for a variety of situations where a person has reproductive anatomy that would not be typical of a person with XX or XY chromosomes. Being intersex is a naturally occurring variation in humans and not typically associated with medical health issues. An individual who is intersex may be an individual who has no change in number of sex chromosomes but does have a change in reproductive anatomy. Similarly, an individual who is intersex may have a different number of sex chromosomes that has led to changes in the development of their reproductive anatomy.
Let’s summarize with some key points what we have learned.
- In humans, sex is determined by the presence of the X and Y chromosomes.
- Any chromosome that is not X or Y is called an autosome.
- The Y chromosome is significantly smaller than the X chromosome and contains less genes.
- Punnett squares can be used to estimate the chance of a fetus having XY or XX sex chromosomes.
- Most human males have XY chromosomes and most human females have XX chromosomes.
- Intersex is a term used to describe humans who have unique variations in their reproductive anatomy and do not fit into the expected binary XX or XY categories.