In this video, we're going to begin our introduction to homologous chromosomes. Now, before we define homologous chromosomes, it's first helpful to point out that a karyotype is defined as an ordered display of all of the chromosomes in a cell. And so if we take a look at our image down below, over here on the left-hand side, notice that we're showing you a human karyotype, and you can see that it's an ordered display of all of the chromosomes in one single cell. Now it's important to note that human somatic cells or human body cells contain 23 pairs of chromosomes. Now within each pair of chromosome, there are actually 2 chromosomes. Just like within every pair of shoes, there are actually 2 shoes. And so there are 23 pairs of chromosomes, but that's actually a total of 46 chromosomes. Now, this is really important because sometimes your textbooks and your professors will use 23 pairs of chromosomes and, other times, they'll use a total of 46 chromosomes to define the total number of chromosomes in human somatic cells.
Now, of these 23 pairs of chromosomes found in human somatic cells, 22 of those 23 pairs are known as autosomes. And autosomes is really just the fancy scientific name for non-sex chromosomes or chromosomes that are not related to the sex of the organism, whether the organism is male or female. And so these 22 pairs of autosomes, because they are non-sex chromosomes, they're going to be found in both males and females. Now of course, if 22 pairs out of the 23 total pairs are autosomes, then that only leaves one pair left. And so the one pair that's left is going to include the X and the Y chromosomes which are going to be sex chromosomes. And of course, sex chromosomes, as their name implies, are going to determine the sex of the organism, whether the organism is female or male. Now females, which are symbolized with this symbol here, which is basically a circle with a plus sign right below it, contain 2 X chromosomes. Whereas males, on the other hand, whose symbol looks like this, a circle with an arrow coming off of it, are going to have only one X chromosome and a Y chromosome instead of an X chromosome.
Now, if we take a look at our image down below at this human karyotype, notice that we can see these pairs of chromosomes, and these pairs of chromosomes notice have numbers below them to keep track of them, and notice that we're showing you with a green background that there are 22 pairs of chromosomes that are known as autosomes. And so here with the greenish background, we're saying that there are 22 pairs of autosomes or non-sex chromosomes. And those down below, here in pink, we're saying that that 23rd pair is going to be the sex chromosome. So there's one pair of sex chromosomes. And of course, it could either be XX if it's a female or it could be XY if it's a male, and so you could see the XX here if it's a female, or it could be XY if it's a male.
Now each of these pairs of chromosomes that we see here are really defined as homologous chromosomes. And so homologous chromosomes are going to be pairs of chromosomes that are very very similar in size, shape, and carry the same genes, but can have different versions of those genes or different alleles. And so it's important to note that homologous chromosomes are going to be very similar, but they are not necessarily going to be identical to each other. And so again, they're going to be similar in size and shape and they're going to carry the same sets of genes, but again they can have different versions of those genes or different alleles. Now, also what's important to note is that within each homologous chromosome pair, one of those chromosomes is going to be paternally inherited or inherited from the father's sperm, and the other chromosome is going to be maternally inherited or inherited from the mother's egg.
If we take a look at our image down below, again all of these pairs that we see here would represent homologous chromosomes because once again they are similar in size and shape, they carry the same genes, but can have different alleles. And so if we were to zoom into just one of these pairs of homologous chromosomes, as we're doing over here in this box, zooming into this one pair of homologous chromosome over here. Again, this would be an example of homologous chromosomes. And homologous chromosomes, which you'll notice is that they are similar in size and shape.
So, the blue and the pink chromosomes are homologous chromosomes. And so what you'll notice is that some of the genes on these homologous chromosomes might be identical to each other, such as this gene A is identical in both homologous chromosomes. However, other genes may not be identical. And so, notice that the blue chromosome over here is going to have the capital B version of the gene or the capital B allele. Whereas, this pink one over here is going to have the lowercase B version of the gene or the lowercase B allele. And so these here are representing these alleles, and sometimes again these alleles can be identical between homologous chromosomes but, other times the alleles will be non-identical between the homologous chromosomes.
Now, here as we're showing you the chromosomes, these are replicated chromosomes with sister chromatids. So over here, this represents 1 sister chromatid, this represents the other sister chromatid, and the sister chromatids are called sister chromatids because these are identical to each other. And notice over here, this chromosome also has sister chromatids, that are identical to each other, but notice that the sister chromatids of homologous chromosomes do not necessarily have to be identical to each other. So this chromatid and this chromatid of the other homologous chromosome, they are identical at this A position but they are non-identical at the B position because this one has a capital B, and this one has a lowercase B.
And so it's important to note that homologous chromosomes are not necessarily going to be identical to each other and, in fact, in most cases, they will not be identical to each other. And so this here concludes our introduction to homologous chromosomes, and we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.
