In Drosophila, a cross was made between females—all expressing th...

Question

In Drosophila, a cross was made between females—all expressing the three X-linked recessive traits scute bristles (sc), sable body (s), and vermilion eyes (v)—and wild-type males. In the F₁, all females were wild type, while all males expressed all three mutant traits. The cross was carried to the F₂ generation, and 1000 offspring were counted, with the results shown in the following table.

Phenotype Offspring
sc s v 314
+ + + 280
+ s v 150
sc + + 156
sc + v 46
+ s + 30
sc s + 10
+ + v 14

No determination of sex was made in the data.

Using proper nomenclature, determine the genotypes of the P₁ and F₁ parents.

Accepted Answer

Everyone. Let's look at our next problem. It says the purple gene in Drosophila specifies the normal and red eyes for the dominant pr plus a little and purple eyes for the recessive? Pr Another gene called vestigial gene specifies normal long wings for the dominant V. G. Plus a little and short wings. The recessive V. G. Allele a double header as I guess female mates with a tester homesickness recesses for both genes and yields the following offspring. So let's think about our cross here we have a female hetero zegas for both traits. So she's pr plus P. R. V. G. Plus V. G. And she's crossed with the homos I guess recessive male. So he's just pr pr V. G. V. G. And um we know that crossing over only occurs in females which is why that's important there. So when we think about our different gametes of course this male can only give gametes that our P. R. V. G. That's all he has. He's homos I guess um recessive. But this female Can give four types of gametes. Put that down here. Female gametes. There's the parental type that would be if there was no crossing over. So that would be P. R. Plus V. G. Plus or P R. V. G. But then there could be some recombination occurring. So the recombination type gametes would be P. R. Plus V. G. Or P. R. V. G. Plus. And now we have a formula for recombination frequency. Because our question down here asked us what is the recombination frequency between the purple and vestigial genes given the offspring that are given to us with their different dream types. So we have formula for recombination frequency which equals number of recombination offspring divided by total number of offspring Times 100%. So let's look up here. And first we'll look at the number of recombination offspring. So we know this is these are recombination combinations. So up here pr plus Fiji gives us offspring and P. R. V. G. Plus give this 139 offspring. You'll note that um the other alley all they have is P. R. V. G. And all of these cases. Because that test cross was a homogeneous recessive male. So when we combine our recombinant offspring we get a total number of 273 recombination offspring. Now if we add up the total number offspring so 14 20 to 10 30 to 1 34 and 1 We obtained 2,722. So now we can plug these numbers into our formula here. So we will have 273 recumbent offspring over 2722 times 100%. And if we're on a test for instance we don't even need to pull out our calculators, we can see that this here is about 1/10 times 100% gives us 10% recombination frequency In this case. So our answer choices D 10%. So again we've got our heterocyclic female for both things. Made it in a test. Cross um were given the number of offspring with their Jenna types. And we calculated the recombination frequency based on our formula here, number of competent offspring over total offspring times 100%. And our answer is choice d. See you in the next video.

Key Concepts

X-linked Inheritance

Genotype and Phenotype

Punnett Square and Genetic Ratios

Watch next