## AAMC SB: B/B question 30 (discrete)

lotus0618
Posts: 182
Joined: Sun Dec 29, 2019 5:47 pm

### AAMC SB: B/B question 30 (discrete)

So I found this excellent explanation written by a random person on the internet (I attached the pictures with this post). I have a couple of questions regarding this problem and this person's explanation. Please make it simple for me, I suck at genetics.

1) I understand that to get both brown and red F2 offsprings, the genotypes of F1 have to be heterozygous Rr * Rr. Now what I'm confused is why are we looking for the probability of the parents F1? Also, how is this different from this type of question "What are the probability of the offsprings that are red and brown," which asks for the Probability of the offsprings, not the parents?

2) Fooballa then asked his/her own question "What are the odds of producing F2 red beetles only from F1 beetles?." So out of 3 different red genotypes, only Rr*RR (1/3 * 1/3= 1/9) and RR*RR (1/3 *2/3= 2/9) would yield only red F2 offsprings. Then I added 1/9 + 2/9 = 1/3 as the answer. However, this is wrong. I have a couple of questions regarding his/her explanation:
a) Why did she/he find the probability of Rr*Rr when these genotypes will give red and brown? I excluded the heterozygous genotype.
b) In this question, why did he/she also include the probability of the offspring having the genotype in the multiplication, when the original AAMC problem doesn't include the genotype of the offspring in the multiplication?
Attachments
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NS_Tutor_Yuqi
Posts: 53
Joined: Fri May 29, 2020 11:43 pm

### Re: AAMC SB: B/B question 30 (discrete)

1. This is important because we know that for the F2 generation to contain brown beetles, the mating pair from F1 must both be Rr. Therefore, you need to work back from there in order to find the total probability.
From the split of the F1 generation, we know that the original mating pair must have both been Rr since there were brown beetles present. That means that of the 31 red beetles, 2/3 of them are Rr. Since we just established that Rr beetles must mate to produce brown F2 beetles, you can get the probability from calculating (2/3) x (2/3), which yields 4/9. We use Rr x Rr because that's the only combo that will give us brown beetles.
lotus0618
Posts: 182
Joined: Sun Dec 29, 2019 5:47 pm

### Re: AAMC SB: B/B question 30 (discrete)

how would the question be phrased so the answer only includes the probability of the offspring's genotype only?