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Animation: Incomplete Dominance in "MendAliens"

by Pearson
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Incomplete Dominance Let's examine how the horn trait is inherited in MendAliens, a mythical alien species. If we cross an alien with two horns and an alien with no horns, what do you think the offspring will look like? Let's find out. The F1 offspring all have one horn. Note that the phenotype of the F1 generation falls in between the phenotypes of the two parents. If we cross two F1 individuals, what do you think the offspring will look like? Let's make the F1 cross. For the F2 generation, we observe 24 MendAliens with two horns, 47 MendAliens with one horn, and 22 MendAliens with no horn. The F2 generation exhibits a phenotypic ratio of 1 to 2 to 1. This is different from the ratio of 3 to 1 in Mendel's pea plants. In Mendel's peas, the dominant allele had the same effect whether an individual had one copy of the dominant allele or two copies. What do you think is going on with the horn trait in MendAliens? The simplest hypothesis is that the horn trait exhibits incomplete dominance, which means that neither the two-horn allele nor the no-horn allele is dominant. To understand this trait, let's assign genotypes to the parental cross. Since neither allele is dominant, we can't use capital and lowercase letters to indicate the alleles. Instead, we'll use H for the horn trait with a superscript T for the two-horn allele. Thus, the homozygous MendAlien with two horns has the alleles H superscript T and H superscript T, and produces gametes with H superscript T. We'll use H with a superscript N for the no-horn allele. The homozygous MendAlien with no horn has the alleles H superscript N and H superscript N, and produces gametes with H superscript N. The F1 generation inherits the alleles H superscript T and H superscript N. This heterozygous genotype gives the intermediate phenotype of one horn. In general, if two homozygotes are crossed that differ in a trait and the offspring all have a phenotype intermediate between those of the two parents, then the trait exhibits incomplete dominance. Now let's assign genotypes to the F1 cross. The F1 MendAliens with one horn produce the gametes H superscript T and H superscript N. We see the results of the F1 cross in the Punnett square for the F2 generation: the H superscript T, H superscript T genotype produces the two-horn phenotype; the H superscript T, H superscript N genotype produces the one-horn phenotype; and the H superscript N, H superscript N genotype produces the no-horn phenotype. In incomplete dominance, heterozygotes have their own unique phenotype, so the phenotypic ratio and genotypic ratio for the F2 generation are the same, 1 to 2 to 1. Now let's investigate the genetics of head shape in MendAliens. We begin with a cross between two MendAliens with round heads. The resulting offspring consist of 8 MendAliens with tall heads, 15 MendAliens with round heads, and 7 MendAliens with wide heads. What is the simplest explanation for the inheritance of these head shapes? The offspring are in a 1 to 2 to 1 phenotypic ratio, which is the characteristic ratio expected for a cross between two heterozygotes if incomplete dominance is involved. Thus, the simplest explanation is that these head shapes involve incompletely dominant alleles, with one type of homozygote having the tall head shape, the other homozygote having the wide head shape, and the heterozygote having the round head shape. If we look at the genotypes of the parents and offspring, that will help us understand the inheritance of this trait. Let's use S for the head shape trait with superscript T for tall head and superscript W for wide head. The round-headed parents are heterozygotes, S superscript T, S superscript W, so half the gametes are S superscript T and half are S superscript W. In the Punnett square of the offspring, we see a ratio of 1 tall head (S superscript T, S superscript T) to 2 round heads (S superscript T, S superscript W) to 1 wide head (S superscript W, S superscript W). What offspring would you expect from a cross between a MendAlien with a round head and a MendAlien with a wide head? A MendAlien with a round head has the genotype S superscript T, S superscript W, so the gametes are S superscript T and S superscript W. A MendAlien with a wide head has the genotype S superscript W, S superscript W, so it has only one type of gamete, S superscript W. The offspring are therefore in a ratio of 1 round head (S superscript T, S superscript W) to 1 wide head (S superscript W, S superscript W).
Incomplete Dominance Let's examine how the horn trait is inherited in MendAliens, a mythical alien species. If we cross an alien with two horns and an alien with no horns, what do you think the offspring will look like? Let's find out. The F1 offspring all have one horn. Note that the phenotype of the F1 generation falls in between the phenotypes of the two parents. If we cross two F1 individuals, what do you think the offspring will look like? Let's make the F1 cross. For the F2 generation, we observe 24 MendAliens with two horns, 47 MendAliens with one horn, and 22 MendAliens with no horn. The F2 generation exhibits a phenotypic ratio of 1 to 2 to 1. This is different from the ratio of 3 to 1 in Mendel's pea plants. In Mendel's peas, the dominant allele had the same effect whether an individual had one copy of the dominant allele or two copies. What do you think is going on with the horn trait in MendAliens? The simplest hypothesis is that the horn trait exhibits incomplete dominance, which means that neither the two-horn allele nor the no-horn allele is dominant. To understand this trait, let's assign genotypes to the parental cross. Since neither allele is dominant, we can't use capital and lowercase letters to indicate the alleles. Instead, we'll use H for the horn trait with a superscript T for the two-horn allele. Thus, the homozygous MendAlien with two horns has the alleles H superscript T and H superscript T, and produces gametes with H superscript T. We'll use H with a superscript N for the no-horn allele. The homozygous MendAlien with no horn has the alleles H superscript N and H superscript N, and produces gametes with H superscript N. The F1 generation inherits the alleles H superscript T and H superscript N. This heterozygous genotype gives the intermediate phenotype of one horn. In general, if two homozygotes are crossed that differ in a trait and the offspring all have a phenotype intermediate between those of the two parents, then the trait exhibits incomplete dominance. Now let's assign genotypes to the F1 cross. The F1 MendAliens with one horn produce the gametes H superscript T and H superscript N. We see the results of the F1 cross in the Punnett square for the F2 generation: the H superscript T, H superscript T genotype produces the two-horn phenotype; the H superscript T, H superscript N genotype produces the one-horn phenotype; and the H superscript N, H superscript N genotype produces the no-horn phenotype. In incomplete dominance, heterozygotes have their own unique phenotype, so the phenotypic ratio and genotypic ratio for the F2 generation are the same, 1 to 2 to 1. Now let's investigate the genetics of head shape in MendAliens. We begin with a cross between two MendAliens with round heads. The resulting offspring consist of 8 MendAliens with tall heads, 15 MendAliens with round heads, and 7 MendAliens with wide heads. What is the simplest explanation for the inheritance of these head shapes? The offspring are in a 1 to 2 to 1 phenotypic ratio, which is the characteristic ratio expected for a cross between two heterozygotes if incomplete dominance is involved. Thus, the simplest explanation is that these head shapes involve incompletely dominant alleles, with one type of homozygote having the tall head shape, the other homozygote having the wide head shape, and the heterozygote having the round head shape. If we look at the genotypes of the parents and offspring, that will help us understand the inheritance of this trait. Let's use S for the head shape trait with superscript T for tall head and superscript W for wide head. The round-headed parents are heterozygotes, S superscript T, S superscript W, so half the gametes are S superscript T and half are S superscript W. In the Punnett square of the offspring, we see a ratio of 1 tall head (S superscript T, S superscript T) to 2 round heads (S superscript T, S superscript W) to 1 wide head (S superscript W, S superscript W). What offspring would you expect from a cross between a MendAlien with a round head and a MendAlien with a wide head? A MendAlien with a round head has the genotype S superscript T, S superscript W, so the gametes are S superscript T and S superscript W. A MendAlien with a wide head has the genotype S superscript W, S superscript W, so it has only one type of gamete, S superscript W. The offspring are therefore in a ratio of 1 round head (S superscript T, S superscript W) to 1 wide head (S superscript W, S superscript W).