In this explainer, we will learn how to recall key facts about chromosomes and interpret karyotypes.
All living organisms contain genetic material in the form of DNA. For instance, your DNA will determine what eye color you have, your skin color, your blood type, and many other characteristics! In eukaryotic organisms (e.g., animals and plants), this genetic material is arranged into chromosomes. These chromosomes contain genes, which have the information needed to produce proteins, and these proteins will determine the traits of organisms. A simplified outline of the structure of DNA and chromosomes is shown in Figure 1.
A chromosome is a long molecule of DNA and associated proteins that contains the genetic information of an organism in the form of genes.
A gene is a section of DNA that contains the information needed to produce a functional unit (e.g., a protein). It is the functional unit of heredity.
In humans, each body cell (also called a somatic cell) contains 46 chromosomes. These 46 chromosomes are arranged into 23 homologous pairs. Homologous pairs are pairs of chromosomes that are of an almost identical size, have the same centromere position, and carry genes that control the same characteristics at the same location, or locus. They may carry different versions of these genes, which are known as alleles. For instance, a gene controlling eye color could have different alleles that produce brown, green, blue, or hazel eyes. An example of a homologous pair of chromosomes is shown in Figure 2.
Definition: Homologous Chromosomes
Homologous chromosomes are pairs of chromosomes that are similar lengths, with the same centromere positioning and the same genes at each location.
The only exception to this is the sex chromosomes, which are the X and Y chromosomes. In females, these chromosomes are arranged into homologous pairs because the female sex chromosomes are XX. However, in males, the sex chromosomes are XY. The Y chromosome is significantly shorter than the X chromosome, so only part of these chromosomes can be considered homologous.
Example 1: Defining the Term Homologous Pair
Chromosomes are arranged in homologous pairs in the nucleus of eukaryotic cells. What is meant by the term homologous pairs?
- Pairs of chromosomes that contain the same alleles
- Pairs of chromosomes that are of a similar length and that have a similar gene positioning
- Pairs of chromosomes that are inherited from one parent
- Pairs of chromosomes that code for the same characteristic in different organisms
Chromosomes are long, thread-like pieces of DNA, found in the nucleus of eukaryotic cells, that contain multiple genes. These genes control the expression of genetic characteristics like eye color, natural hair color, and blood type. Genes may have different variations, which are called alleles. For instance, a gene that controls eye color may have one allele that produces brown eyes and one that produces blue eyes.
We inherit chromosomes from our parents. In a homologous chromosome pair, one chromosome comes from our biological mother and the other from our biological father. This allows “mixing” of genetic information—we may find that the two chromosomes in a homologous pair produce different traits for each characteristic. On each chromosome in a homologous pair, the position of genes will be very similar. The diagram below indicates the position of the OCA2 gene that contributes to the expression of eye color.
We can see from the diagram that the position of the genes on each chromosome is the same, the position of the centromere is the same, and the two chromosomes are the same length.
Therefore, homologous chromosomes are pairs of chromosomes that are of a similar length and that have a similar gene positioning.
An autosome is any chromosome that is not a sex chromosome (X or Y).
The chromosomes that are not sex chromosomes are called autosomes. Each body cell contains 44 autosomes and 2 sex chromosomes.
Although each somatic cell in humans contains 46 chromosomes, the sex cells (also called gametes) only contain 23. This is because when gametes are produced from body cells, they undergo two rounds of division. This ensures that the 23 pairs of chromosomes are separated, so the sperm and egg cells produced have one copy of each chromosome rather than two. This is a very important process, as it ensures that when the gametes combine in fertilization, the resulting embryo will have the correct number of chromosomes in all of its cells. This process is briefly outlined in Figure 3.
In humans, a somatic cell that contains 46 chromosomes (the full amount) is referred to as diploid. A gamete, like a sperm or egg cell, that contains 23 chromosomes is referred to as haploid. An easy way to remember this is by the start of the words: “ha” is also the beginning of “half,” so a “haploid” cell has half the number of chromosomes of a diploid cell; “di” means “two,” so a “diploid” cell has twice the number of chromosomes of a haploid cell! This is true across some plants and most animal species; although different organisms contain different numbers of chromosomes in their cells, their gametes will be haploid and their somatic cells will be diploid.
A diploid cell is a cell that has two complete sets of chromosomes (2n), arranged into homologous pairs.
A haploid cell is a cell that only has a single set of chromosomes (n).
Example 2: Calculating the Number of Chromosomes in a Diploid Cell
A koala has 8 chromosomes in a haploid cell. How many chromosomes will a diploid body cell from this organism contain?
Molecules of DNA in a cell are coiled and condensed into chromosomes, which contain the genes that are required to determine all of an organism’s genetic traits. Most body cells are diploid; this means the cell has two complete sets of chromosomes that are arranged into pairs and numbered. For instance, a human body cell has 46 chromosomes that are arranged into 23 pairs.
Sex cells, or gametes, are haploid. The “ha” part of the word reminds us of the word “half.” A haploid cell will contain half the chromosomes of a diploid cell, so instead of having two sets of chromosomes, they will only have one set. In humans, the gametes have 23 chromosomes. This is very important for reproduction; when the haploid sex cells combine during fertilization, they will produce a diploid cell. This diploid cell can then go on to divide and multiply to eventually become a diploid organism, with each body cell having the correct number of chromosomes.
So, if a haploid cell of a koala has 8 chromosomes, the number of chromosomes in the diploid cell must be double this number ().
Therefore, the number of chromosomes in a diploid body cell of a koala should be 16.
The chromosomes within a cell can be visualized by producing an image called a karyotype. A karyotype is a visual display of all the chromosomes within a cell, which are arranged into their homologous pairs and then into descending size order. A brief outline of the method used to produce a karyotype is given below:
- A sample of blood, bone marrow, or skin is taken from an organism.
- The cells are cultured for a short period of time to allow growth and multiplication.
- The dividing cells are “arrested” (paused) when the chromosomes are most condensed and therefore most visible.
- The cells are treated with a solution that causes the nuclei to burst.
- The chromosomes are then stained and viewed with a microscope that has a digital camera attached.
- An image of the chromosomes is produced, and digital software can be used to arrange the chromosomes into a clear order.
A karyotype is a visual display of all the chromosomes in a cell, arranged in homologous pairs and in decreasing size order.
An example of a karyotype is shown in Figure 4.
From this karyotype, we can see that the homologous pairs of autosomes are arranged in size order, descending from the largest (chromosome 1) to the smallest (chromosome 22). We can also see that the person the cell was taken from was a female, as there are two X chromosomes. The sex chromosomes (X and Y) are the only chromosomes not arranged in size order, and in a karyotype, they always appear last. If they were arranged in size order, the X chromosome would be the 8th largest.
Example 3: Recalling the Number of Chromosomes in Human Body Cells
How many chromosomes should appear in a karyotype of a human diploid body cell?
To answer this question, we first need to understand some of the key terms. Chromosomes are molecules of DNA that have been coiled and condensed. Chromosomes contain all the genetic material that makes us who we are! A karyotype is a visual representation of all the chromosomes that are contained within a cell. A diploid cell is a cell that has two complete sets of chromosomes.
In a karyotype of a diploid body cell, the chromosomes are arranged into homologous pairs, with one chromosome from each pair being inherited from the father and one chromosome being inherited from the mother. An example karyotype of a human female is shown below.
As we can see from the karyotype, in a human diploid cell, there are 22 pairs of autosomes (non-sex chromosomes) and 1 pair of sex chromosomes. These are in the combination XY for males or XX for females. These chromosomes appear at the end of a karyotype and are not arranged in size order.
Therefore, a human diploid body cell has 23 pairs of chromosomes (22 pairs of autosomes and 1 pair of sex chromosomes), which gives a total number of 46 chromosomes.
Karyotypes are extremely useful for studying genetics. They can be analyzed to identify some genetic disorders—for instance, if a person has three copies of chromosome 21 rather than two, which leads to a condition called Down syndrome.
Let’s summarize what we have learned so far.
- The genetic material in cells is arranged into structures called chromosomes.
- A human body cell will contain 44 autosomes and 2 sex chromosomes.
- A somatic (body) cell contains homologous pairs of chromosomes, whereas a gamete (sex cell) will only contain one from each pair. These cells are called diploid and haploid respectively.
- Chromosomes within a cell can be visualized using a karyotype, which arranges them in descending size order.
- Karyotypes can be studied to identify chromosomal disorders.