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Animation: X-Linked Genes

by Pearson
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Let's look at the inheritance of short legs and normal legs in MendAliens, a mythical alien species that has the same system of sex determination as humans. If we cross a short-legged female with a normal-legged male, all sons have the same phenotype as the mother and all daughters have the same phenotype as the father. The simplest hypothesis is that the short-legs trait is caused by a recessive allele on the X chromosome, an X-linked gene. Let's find out what happens when you cross two F1 individuals. We get 38 normal females to 33 short-legged females to 41 normal males to 35 short-legged males. Now let's add genotypes to the parental cross. We will use small s for the recessive short-legs allele and capital S for the dominant normal-legs allele. There is a recessive small s allele on each of the parental female's X chromosomes, while the parental male has the normal capital S allele on his X chromosome and no allele on his Y chromosome. The F1 female has the normal phenotype because she inherited the recessive allele small s on the X chromosome from her mother, but she inherited the normal allele capital S on the X chromosome from her father. The F1 male has the recessive phenotype because he inherited the recessive allele on the X chromosome from his mother, and there is no allele on the Y chromosome that he inherited from his father. A trait is inherited as an X-linked recessive trait if a cross between a female with the recessive trait and a true-breeding normal male results in all male offspring having the recessive trait and all female offspring having the normal phenotype, as shown here. Now let's add genotypes to the F1 cross. Assuming that our F1 individuals have the genotypes worked out in the previous step, we would predict that the F2 generation would have a ratio of one normal female to one short female to one normal male to one short male. These expected ratios are approximately matched by our observed F2 results of 38 normal females to 33 short females to 41 normal males to 35 short males. In conclusion, leg length can be explained by a single gene on the X chromosome with the short-legs allele recessive to the normal-legs allele. This cross illustrates the inheritance of an X-linked recessive allele. Now let us analyze a reciprocal cross, in which we switch the phenotypes of the parents. This is quite different from the result we got from the original cross. If we had looked at this first, we might have thought that short legs was caused by a simple recessive allele. This shows the importance of making the F1 cross, as shown in the next step. The F1 cross shows that short legs is not caused by a simple recessive allele. Looking at the F2 offspring of this cross, we see that there is a three to one ratio of normal to short offspring. But when we look at males and females separately, we see that this ratio is not the same for the two sexes. This indicates that the leg-length gene is linked to the X chromosome, with the short-legs allele recessive to the normal-legs allele. Had we not looked at the sexes separately, we might not have seen that X linkage was involved. Notice that for traits caused by an X-linked recessive allele, the F1 and F2 results are different for the parental cross between a female with the recessive trait and a normal male, as shown previously, and the parental cross between a normal female and a male with the recessive trait, as shown here. Thus, a reciprocal cross must always be made when the gene could be X-linked. Let's review how X-linked genes work by examining other scenarios. The pointed-ear trait in MendAliens is caused by an X-linked recessive allele. We will use small e for the pointed-ears, that is, the recessive allele, and capital E for the normal no-ear allele. A female MendAlien with no ears, which is the normal condition, and whose male parent had pointed ears, pairs with a male MendAlien with no ears. Let's make the cross. The proportions in the phenotypes are two females with no ears, one male with no ears, and one with ears. Overall, there will be three with no ears and one with ears. Here's another scenario. Let us now assume that the female MendAlien from the previous scene pairs with a male MendAlien with pointed ears. Let's make the cross. The proportions in the phenotypes are one female with no ears, one female with ears, one male with no ears, and one with ears. Overall, there will be two offspring with no ears and two with ears.
Let's look at the inheritance of short legs and normal legs in MendAliens, a mythical alien species that has the same system of sex determination as humans. If we cross a short-legged female with a normal-legged male, all sons have the same phenotype as the mother and all daughters have the same phenotype as the father. The simplest hypothesis is that the short-legs trait is caused by a recessive allele on the X chromosome, an X-linked gene. Let's find out what happens when you cross two F1 individuals. We get 38 normal females to 33 short-legged females to 41 normal males to 35 short-legged males. Now let's add genotypes to the parental cross. We will use small s for the recessive short-legs allele and capital S for the dominant normal-legs allele. There is a recessive small s allele on each of the parental female's X chromosomes, while the parental male has the normal capital S allele on his X chromosome and no allele on his Y chromosome. The F1 female has the normal phenotype because she inherited the recessive allele small s on the X chromosome from her mother, but she inherited the normal allele capital S on the X chromosome from her father. The F1 male has the recessive phenotype because he inherited the recessive allele on the X chromosome from his mother, and there is no allele on the Y chromosome that he inherited from his father. A trait is inherited as an X-linked recessive trait if a cross between a female with the recessive trait and a true-breeding normal male results in all male offspring having the recessive trait and all female offspring having the normal phenotype, as shown here. Now let's add genotypes to the F1 cross. Assuming that our F1 individuals have the genotypes worked out in the previous step, we would predict that the F2 generation would have a ratio of one normal female to one short female to one normal male to one short male. These expected ratios are approximately matched by our observed F2 results of 38 normal females to 33 short females to 41 normal males to 35 short males. In conclusion, leg length can be explained by a single gene on the X chromosome with the short-legs allele recessive to the normal-legs allele. This cross illustrates the inheritance of an X-linked recessive allele. Now let us analyze a reciprocal cross, in which we switch the phenotypes of the parents. This is quite different from the result we got from the original cross. If we had looked at this first, we might have thought that short legs was caused by a simple recessive allele. This shows the importance of making the F1 cross, as shown in the next step. The F1 cross shows that short legs is not caused by a simple recessive allele. Looking at the F2 offspring of this cross, we see that there is a three to one ratio of normal to short offspring. But when we look at males and females separately, we see that this ratio is not the same for the two sexes. This indicates that the leg-length gene is linked to the X chromosome, with the short-legs allele recessive to the normal-legs allele. Had we not looked at the sexes separately, we might not have seen that X linkage was involved. Notice that for traits caused by an X-linked recessive allele, the F1 and F2 results are different for the parental cross between a female with the recessive trait and a normal male, as shown previously, and the parental cross between a normal female and a male with the recessive trait, as shown here. Thus, a reciprocal cross must always be made when the gene could be X-linked. Let's review how X-linked genes work by examining other scenarios. The pointed-ear trait in MendAliens is caused by an X-linked recessive allele. We will use small e for the pointed-ears, that is, the recessive allele, and capital E for the normal no-ear allele. A female MendAlien with no ears, which is the normal condition, and whose male parent had pointed ears, pairs with a male MendAlien with no ears. Let's make the cross. The proportions in the phenotypes are two females with no ears, one male with no ears, and one with ears. Overall, there will be three with no ears and one with ears. Here's another scenario. Let us now assume that the female MendAlien from the previous scene pairs with a male MendAlien with pointed ears. Let's make the cross. The proportions in the phenotypes are one female with no ears, one female with ears, one male with no ears, and one with ears. Overall, there will be two offspring with no ears and two with ears.