In rabbits, chocolate-colored fur (w⁺) is dominant to wh...

Question

In rabbits, chocolate-colored fur (w⁺) is dominant to white fur (w), straight fur (c⁺) is dominant to curly fur (c), and long ear (s⁺) is dominant to short ear (s). The cross of a trihybrid rabbit with straight, chocolate-colored fur and long ears to a rabbit that has white, curly fur and short ears produces the following results:

Phenotype Number
White, short, straight 13
Chocolate, long, straight 165
Chocolate, long, curly 13
White, long, straight 82
Chocolate, short, straight 436
Chocolate, short, curly 79
White, short, curly 162
White, long, curly 450
1400

Determine the order of the genes on the chromosome, and identify the alleles that are present on each of the homologous chromosomes in the trihybrid rabbits.

Accepted Answer

Hi, everyone. Welcome back. Let's look at our next problem. Consider the following genotype frequencies. And we have a list of different genotypes where we have three genes A B and C and different genotype possibilities are listed where A B and C are the mutant alleles and plus stands for the wild type, all of each of those genes and each genotype is listed with the number of offspring with that genotype. Then our question says, how would you determine which phenotype undergoes double crossovers? And our answer choices are a, the phenotypes with the highest frequencies. B, the phenotypes with frequencies near the mean C, the phenotypes with the lowest frequencies or d, the phenotypes that have a higher number of mutants. Well, let's keep in mind that we're presented with genotype frequencies. We asked about genotype frequencies and we're asked which phenotype undergoes double crossovers, but we don't always necessarily know which genotype represents which phenotype when we're presented with a problem like this. But we do know that double crossovers will be the least frequent event when there are three linked genes. The most frequent will be when there are no crossovers at all. So just the parental genotypes. So most frequent no crossovers and in parentheses, the parental and then of course, in the middle one crossover between two of the genes. So that when we look at our answer choices, we have choice c the phenotypes with the lowest frequencies and looking at our other answer choices, we can see that choice. A the phenotypes are the highest frequencies. As we said, that would be the parental phenotypes. Now, interestingly in the numbers were presented here. Although we don't need these numbers, we see that the highest number of offspring are for the genotypes plus B and plus and A plus and C. So this is just, I note this only because we can be tempted to look at something like this and assume that A B and C and then plus plus plus are the parental genotypes because often problems are presented like that where one parent is mutant for all the alley and one parent is wild type for all of the all. But that is not necessarily the case, which is why it's important to determine and look at these offspring numbers. In this particular problem that we presented. The parental genotypes are a mix of wild type and mutant. So just a good thing to note there if you had a different type of problem. But choice A is not our answer because the phenotypes with highest frequencies will be the parental phenotypes. And then choice B, the phenotypes with frequencies near the mean, not our correct answer. Because then we'd be talking about phenotypes with one crossover and joy D phenotypes that have a higher number of mutant all is not our correct answer. Because again, it depends on what the parental genotypes were as to whether crossing over results in more or fewer mutant alls. So that depends on parental genotypes. So our answer here how to determine which phenotype undergoes double crossovers. Choice C the phenotypes with the lowest frequencies. See you in the next video.

Key Concepts

Gene Dominance

Punnett Square and Genetic Crosses

Linkage and Gene Order

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