A three-gene system of additive genes (A, B, and C) controls plan...

Question

A three-gene system of additive genes (A, B, and C) controls plant height. Each gene has two alleles (A and a, B and b, and C and c). There is dominance among the alleles of each gene, with alleles A, B, and C dominant over a, b, and c. Under this scheme, the dominant genotype for a gene contributes 10 cm to height potential, and the recessive genotype contributes 4 cm.

What is the height potential of a plant that is homozygous for all three recessive alleles?

Accepted Answer

Hi, everyone. Let's take a look at this question together in a certain animal species, coat color is determined by a single gene with two alleles. The dominant allele produces black fur and the recessive allele produces brown fur. Another gene with two alleles determines the animal size with the dominant allele producing large animals and the recessive allele producing small animals. If a heterozygous, black and large animal is crossed with a homozygous brown and small animal, what is the expected phenotypic ratio of their offspring? And here we have four different answer choices containing different expected phenotypic ratios or the offspring of this cross. And we want to determine which of the following expected phenotypic ratios is correct. So, from the question, we are able to identify that we have a cross of a heterozygous black and large animal, which means that our cross involves our first parent, which is heterozygous for black fur. And we know that coat color is determined by those two alleles, the dominant bee and the recessive bee. So we have our heterozygous or black coat color. And since the animal is large, we note that we have the two alleles which are that dominant and recessive s for the size. So we have that heterozygous for large animal size and we are crossing them with a homozygous brown and small animal. So we have that homo, a recessive for brown and homozygous recessive for small and to determine that expected phenotypic ratio of their offspring, we have to set up a punt square or the cross between two different genes. So we have our pundit square for the B gene and we have to set up a second punt square or the cross between the S gene and from the pont square. For both the B gene and the S gene, we are able to determine that we have 50% of the offspring expected to have that heterozygous phenotype and 50% to have that homozygous recessive phenotype for both the B gene and the S gene, which means that we have a 1 to 1 ratio of black fur coat to brown fur coat. Since we have that 50% our heterozygous which produces black fur color since the black fur is dominant over the brown fur. And we have 50% our homozygous recessive, which leads to that brown fur color. And we have that 1 to 1 ratio of the large animal size to the small animal size. Since we have 50% of the expected offspring to have that large phenotype since they are heterozygous and large is dominant over small and 50% are homozygous recessive which produces that small phenotype, which means that the expected phenotypic ratio of the offspring of this parent cross is answer choice B which is one with black fur and large, one with brown fur and large one with black fur and small and one with brown fur and small. Since we have that 1 to 1 ratio for both black and brown fur coat as well as large and small. So answer choice B is the correct answer. I hope you found this video to be helpful. Thank you and goodbye.

Key Concepts

Additive Gene Action

Dominance and Alleles

Genotype and Phenotype Relationship

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