In cattle, an autosomal mutation called Dexter produces calves with short stature and short limbs. Embryos that are homozygous for the Dexter mutation have severely stunted development and either spontaneously abort or are stillborn. What progeny phenotypes do you expect from the cross of two Dexter cows? What are the expected proportions of the expected phenotypes?

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Accepted Answer

Hi, everyone. Welcome back. Let's look at our next problem. It says bulldog dwarfism is a lethal genetic defect in which fetuses, homesickness for the mutation in the A C A N gene have severe disproportionate dwarfism and are usually aborted around seven months of gestation when two cattle that are hetero cigars for the mutation are crossed. What is the probability that the offspring will be of the same genotype as the parents? So our problem is talking about a lethal genetic defect, lethal defect would be one where um having the defect causes a problem so severe that the offspring cannot survive to birth. In this case that occurs when the fetus is homesickness for the mutation. So we have two heterosexual parents for this trait. So we'll call the jean de, the parents would be big D little D cross with big D little D. And we want to know the probability that the offspring will be the same genotype as the parents. So hetero zegas, our answer choices are a one half, B, two thirds C, 3/4 or D, one. So let's look at Opponent Square. Put that up here. We've got the same genotype in each case. So at top and left, we'll both have big D, little D, Just a four square pundit square since we have just a single trait here, single gene. So fill in our punnett square, we have big D, big D, top left, big D, little D, bottom, left, big D, little D top, right, and little D, little D bottom. Right. So we're looking for how the proportion of offspring that are hetero sickness. And we see of our four boxes that two of them bottom, left and top right, our hetero sickness. So that would give us a proportion of one half. However, this problem speaks about lethal genetic defect in the case of homo Zegas being homesickness for the mutation. So in this case from our punnett square, the little D little D box represents offspring that do not survive to birth. So I'm gonna shade that in, in a gray color to show that lethal defect. So when we look at the proportion of the offspring that will be heterocyclic, we now have two boxes out of three surviving offspring. So our proportion here is actually 2/3 will be the same genotype as parents. So if it weren't a lethal genetic mutation, our answer would be one half. But in this case, our answer is choice B two thirds. See you in the next video.

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Key Concepts

Autosomal Mutation

Homozygosity and Phenotypic Expression

Punnett Square and Expected Ratios