Video Transcript
In this video, we will learn about the inheritance of genes through chromosomes. We will discuss how some genes can be linked to each other and learn how to describe sex-linked and autosomal linked genes. As you know, genetic material is passed on from parents to offspring. This genetic material controls the functions of all the cells in the body. We all have certain traits that we inherit from members of our family. Have you ever noticed that some members of a family may have freckles, while others may not? What about baldness or maybe certain health conditions like those related to eyesight? How are these traits passed on from one generation to the next? We’ll answer some of these questions in this video.
Nearly every cell in our body contains chromosomes, which are linear structures that carry DNA. Human cells contain 23 pairs of chromosomes, or 46 chromosomes in total. Of these, 22 pairs of chromosomes are autosomes. These autosome chromosomal pairs are numbered here from one to 22, where chromosome one is the largest and chromosome 22 is the smallest. The 23rd pair of chromosomes are the sex chromosomes. Sex chromosomes can either be X chromosomes or Y chromosomes, and the 23rd pair is typically either XX or XY. The pairing of chromosomes in this 23rd set determines the biological sex of an individual. Biological females will typically have an XX 23rd pair, while biological males will typically have an XY 23rd pair.
When humans reproduce, each parent’s gamete, or sex cell, contains half the total chromosome number. That is, they contain just 23 chromosomes each. The egg cell from the biological mother carries 22 single autosomes and also one sex chromosome, which is always an X chromosome. The sperm cell from the biological father carries 22 single autosomes as well and also carries one sex chromosome, which may either be an X chromosome or a Y chromosome.
After fertilization, a zygote is formed when the egg cell and sperm cell fuse together. This zygote will have 22 pairs of autosomes. Each pair of chromosomes consists of one chromosome from the biological mother and one from the biological father. These pairs are called homologous chromosomes, and they’re not really different colors. But we’re using pink and blue here to distinguish between the chromosomes inherited from the mother and from the father.
The zygote also has a pair of sex chromosomes. If the sperm carried an X chromosome, the zygote will have two X chromosomes and will be biologically female. On the other hand, if the sperm carried a Y chromosome, the zygote will have one X and one Y chromosome and will be biologically male. Let’s take a closer look at a pair of homologous chromosomes. As you can see, the two chromosomes that make up a homologous pair are similar in length. They also both have their centromeres in the same position. We know that chromosomes carry stretches of DNA called genes, which determine the characteristics of an individual. Each chromosome can carry hundreds of genes depending on their size. Let’s say that these chromosomes carry three genes, represented here in orange, green, and red.
You might notice that both chromosomes in a homologous pair carry the same genes at the same locations. However, each chromosome might carry a different version of each gene, that is, a different allele for that gene. An allele is an alternative version of a gene. Let’s look at an example of different alleles for a gene.
Say the gene represented here in orange is the gene that determines whether or not a person will have freckles. The chromosome on the left might carry a dominant allele for freckles, which we represent with a capital F. The chromosome on the right, however, might carry a recessive allele for no freckles, represented here by a lowercase f. The genotype of the person with these chromosomes will therefore be capital F lowercase f, and the combination of these two alleles will determine whether or not the person will have freckles. In other words, the genotype helps determine the phenotype, which is the observable outcome. In this case, because the genotype contains the dominant allele for freckles, that’s what will be expressed as the phenotype of the individual.
So we’ve learned that our chromosomes are passed down to us from our parents. But what determines the combination of alleles of each gene that we will receive? To understand this, we need to take a look back at what happens to chromosomes when gametes are formed. Gametes, or sex cells, are produced through a special type of cell division called meiosis. Meiosis produces four gametes, each with half the organism’s total chromosome number. Remember our homologous chromosomes? Just before meiosis begins, each chromosome duplicates itself so that there will be a copy for each daughter cell after the division. When we’re discussing meiosis, we’ll be seeing this replicated somewhat X-shaped representation of a chromosome.
During the preparation for the first division in meiosis, a cell has 23 pairs of replicated chromosomes. At this stage, an interesting process takes place, which is called crossing-over. Crossing-over is an exchange of portions of a chromosome within a homologous pair. Let’s look at an example of how this would happen in one pair of homologous chromosomes that have been duplicated.
Let’s say that these chromosomes carry the genes A, B, and C in the areas indicated. We can see that within this pair, one of the chromosomes carries the dominant alleles, capital A, capital B, and capital C, while the other homologous chromosome in the pair carries the recessive alleles, lowercase a, lowercase b, and lowercase c. And we can also see that both sides of each replicated chromosome carry the same alleles or essentially that they are identical copies of each other.
During the crossing-over process, the homologous chromosomes actually physically exchange portions of the chromosome so that afterward the two replicated sides of each chromosome are no longer identical. We can see that on the first chromosome, on the side where crossing-over took place, the chromosome still carries the dominant alleles for genes A and B, together with a recessive allele for gene C. And for the second chromosome in the set, on the side where crossing-over took place, the chromosome still carries the recessive alleles for genes a and b, together with a dominant allele for gene C.
We now have combinations of alleles different from those seen on the original chromosomes. This means that when meiosis finishes, each of the four gametes produced will have a different combination of alleles on this chromosome. We’ve only shown crossing-over occurring on one chromosomal pair here. But remember that crossing-over can occur on all chromosomal pairs in a cell. As we learned earlier, each gamete produced in humans will typically have 22 autosomes in a single sex chromosome. One such gamete from the biological mother and one from the biological father will fuse together during fertilization as we saw earlier. And this is how a zygote receives its alleles. So if we say that these are the alleles received from each of the parental gametes, then our zygote would end up with a genotype like this.
As we mentioned earlier, chromosomes contain hundreds, sometimes thousands, of genes. If two genes are located far apart on a chromosome, like, for example, genes A and C here, it is more likely that the alleles may be separated during crossing over. In fact, in this example, gene C was crossed over, but A was not. On the other hand, if two genes are located very close together on a chromosome, such as genes A and B here, their alleles are less likely to be separated during crossing over. This means that these alleles will likely stay together even in a rearranged chromosome and therefore are very likely to be inherited together by the offspring.
Such genes that are located very close to one another are called linked genes. Chromosomes are microscopic structures that carry hundreds of genes, so linked genes are actually far closer together than they have been drawn here. Autosomal linkage is when linkage occurs on autosomes rather than on one of the sex chromosomes.
To understand this better, let’s think of some example traits that might be determined by these genes. For example, instead of gene A, we might have gene G, where the dominant allele, or capital G, is coding for green eyes and the recessive allele, or lowercase g, is coding for blue eyes. And gene B might determine hair color, where the dominant allele, or capital B, codes for brown hair and the recessive allele, or lowercase b, codes for red hair. And then gene C might determine hair texture, where the dominant allele, or capital C, codes for curly hair and the recessive allele, or lowercase c, codes for straight hair.
Since genes G and B are located more closely together than, for example, genes G and C, G and B are less likely to be separated during a crossing over event. And in fact, in our example, you can see that they weren’t. In this case, only gene C was separated from the other two. Since G and B are more closely linked, that means that in this case, we would be more likely to see offspring with green eyes and brown hair or blue eyes and red hair and less likely to see offspring with green eyes and red hair or blue eyes and brown hair.
On the other hand, gene C is located further away on the chromosome and is less closely linked. So in this case, where the original chromosomes carried either all dominant alleles or all recessive alleles for these traits so that these would be the most common combination of traits in the offspring, we would be more likely to see offspring with straight brown hair and green eyes or curly red hair and blue eyes than we would be to see different combinations of eye color and hair color. Of course, this example is only hypothetical. Linkages among eye color, hair color, and hair texture are not this simple in reality.
Linked genes also occur in sex chromosomes, and this is called sex linkage. Let’s learn a little bit more about these sex chromosomes before we begin to understand sex linkage. As you can see here, the X and Y chromosomes are very different in size. The X chromosome is about three times as large as the Y chromosome, with the X chromosome carrying about 900 genes, while the Y chromosome only carries about 55 genes.
Aside from determining the biological sex of an individual, the sex chromosomes also carry other genes. While some of these genes are common to both X and Y chromosomes, most of the genes are different. These genes that are not common to both X and Y chromosomes have a unique inheritance pattern, which is called sex-linked inheritance. For example, the Y chromosome carries the SRY gene. This gene is responsible for the development of male reproductive anatomy. The X chromosome does not carry this gene, which is why an XY genotype always results in a biological male. This SRY gene is said to be a sex-linked gene since it is linked to one of the sex chromosomes. Any trait determined by such genes that are carried on only one sex chromosome is called a sex-linked trait.
To understand sex-linked inheritance better, we’ll look at the experiments performed by a scientist named Dr. Thomas Hunt Morgan. Dr. Morgan studied inheritance patterns in fruit flies. Most fruit flies have red eyes, but Dr. Morgan noticed a single male fruit fly that had white eyes. In genetics, we call the version of a trait found in almost all individuals of a species the wild type. In this case, red eyes would be the wild type. And we use the term mutant type to refer to a variant of the characteristic rarely found in the species. So in this case, white eyes would be the mutant type.
Dr. Morgan decided to try and figure out how this single male fly had ended up with white eyes and how this trait had been inherited. So he crossed this white-eyed male with a red-eyed female. We typically call the first cross in a genetic experiment the parental, or P, generation. The offspring from this cross, or the F one generation, all had red eyes. This confirmed that the red-eyed version of the trait was the dominant one. He then crossed the males in this generation with the females. Among the offspring from this cross, or the F two generation, 75 percent had red eyes and 25 percent had white eyes, which is what he had expected based on a dominant recessive trait.
However, he was surprised to note that once again all the white-eyed flies were male and there were no female white-eyed flies. This led him to believe that the gene for eye color was carried on the X chromosome. But how did he reach this conclusion? Because by using a Punnett square for an X-linked trait, he was able to accurately predict the proportions of different types of offspring.
When we are using Punnett squares for sex-linked traits, we need to first keep track of which sex chromosome the gene appears on. And then we place the allele as a superscript on that chromosome. So in this Punnett square of the P generation, we place the genotype of the red-eyed wild-type female across the top and the genotype of the white-eyed mutant-type male down the side. Using a Punnett square of genotypes in the F one generation, we’re also able to accurately predict the outcomes of the F two generation. All X-linked recessive traits are most likely to affect males just as we see here.
So far, we’ve discussed sex-linked traits that are located on sex chromosomes. However, there are some traits for which expression depends on the biological sex of an individual, but these traits are actually located on autosomes. There are sex- influenced traits, which are located on autosomes, but which are influenced by hormones so that they are more likely to appear in one biological sex or the other. One example of this is pattern baldness, which is influenced by testosterone. So we’re generally more likely to see this in biological males. And there are sex-limited traits, which are located on autosomes, but their expression is limited to only one biological sex. Examples are lactation, which we generally only see in biological females, and beard growth, which we generally only see in biological males.
Let’s review some of the key points that we’ve learned in this video. Genes that are found near one another on an autosome are often inherited together. This is called autosomal linkage. Sex-linked traits are carried by the sex chromosomes. Dr. TH Morgan studied sex-linked inheritance in fruit flies. Sex-influenced traits are traits that are located on an autosome and whose expression is influenced by sex hormones. Sex-limited traits are traits that are located on an autosome and whose expression is limited to one biological sex.
