You discover a mutant eye color allele in Drosophila melanogaster that is recessive and causes purple eyes instead of the dominant wild-type eye color allele, red. You figure out that this is due to one gene on chromosome 3 (an autosome). You perform a cross in which the female parent is homozygous for the wild-type red allele and the male parent is homozygous for the purple allele. Then you breed the F1 offspring to each other and score the phenotypes of 1000 F2 offspring. Which of the following results is most likely?
a) 762 have purple eyes and 238 have red eyes.
b) 523 have red eyes and 476 have purple eyes.
c) 741 have red eyes and 259 have purple eyes.
d) 340 have red eyes, 325 have purple eyes, and 335 have one red and one purple eye.
e) All have eyes that are intermediate in color between red and purple.
want to guess B because it's not sex linked. Anyone??
a) 762 have purple eyes and 238 have red eyes.
b) 523 have red eyes and 476 have purple eyes.
c) 741 have red eyes and 259 have purple eyes.
d) 340 have red eyes, 325 have purple eyes, and 335 have one red and one purple eye.
e) All have eyes that are intermediate in color between red and purple.
want to guess B because it's not sex linked. Anyone??
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Don't guess, honey. Do the crosses. Let's work this out together. The answer is C, but I want you to know why.
First female is homozygous for red, so we'll give her RR as her genotype. Male is homozygous recessive, so we'll give him rr as his genotype.
There will be 100% heterozygous offspring with red eyes, Rr, but let's do the cross so you can see it.
R R
r Rr Rr
r Rr Rr
Now, we cross the offspring.
R r
R RR Rr
r Rr rr
As you can see, about 25% will be homozygous dominant, RR, and will have red eyes. 50% will be heterozygous, Rr, but will also have red eyes because you didn't say that this is a case of incomplete dominance where the eye colors would mix.
Then, 25% will be recessive, rr, and will have purple eyes. It means that the ratio of red eyes to purple is about 3:1.
Working this out for 1000 offspring and rounding the numbers off, it must be C. In a perfect system, 750 would have red eyes, 250 would have purple eyes, but this is just the mathematical probability. Genes between parents are mixed and shared randomly, so C has the closest pattern that fit the mathematical prediction.
The answer is C.
First female is homozygous for red, so we'll give her RR as her genotype. Male is homozygous recessive, so we'll give him rr as his genotype.
There will be 100% heterozygous offspring with red eyes, Rr, but let's do the cross so you can see it.
R R
r Rr Rr
r Rr Rr
Now, we cross the offspring.
R r
R RR Rr
r Rr rr
As you can see, about 25% will be homozygous dominant, RR, and will have red eyes. 50% will be heterozygous, Rr, but will also have red eyes because you didn't say that this is a case of incomplete dominance where the eye colors would mix.
Then, 25% will be recessive, rr, and will have purple eyes. It means that the ratio of red eyes to purple is about 3:1.
Working this out for 1000 offspring and rounding the numbers off, it must be C. In a perfect system, 750 would have red eyes, 250 would have purple eyes, but this is just the mathematical probability. Genes between parents are mixed and shared randomly, so C has the closest pattern that fit the mathematical prediction.
The answer is C.