# Genetics Question

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#### 5words

##### Full Member
5+ Year Member
So i am having problems solving the following question:

What is the probability that offspring produced by a mating between Individuals III.j and III.m would be
afflicted with either Disease 1 or Disease 2?
A. 1/4
B. 1/2
C. 3/4
D. 1

So i actually chose C due to the fact that the probability of either of tow events happneing is equal to P1 + P2 - P1*P2

So P of disease ! happening here is = 1/2 becuase only IIIm is carrying the allele for disease I which is dominant. And that of P2 = 1/2 that IIIj will pass its allele * 1/2 that IIIm is a carrier * 1/2 The probability that IIIm will pass its disease two allele. Because the disease 2's allele is recessive. = 1/8

Hence P 1 or 2 = 1/2 + 1/8 - 1/16 = 3/4.

But the answer is B, can someone explain why?

@Next Step Tutor @aldol16 @NextStepTutor_2 @NextStepTutor_3

I'm not so good at genetics and so there might be nuances I'm missing, but say A is the dominant allele for disease 1 and b is the recessive allele for disease 2. So you already know that IIIj must have both copies of the disease 2 recessive gene and none of the disease 1 alleles. Only one of IIIm's parents has the disease, so IIIm is heterozygous for the disease I gene. However, I don't see information that would tell me whether IIIm is heterozygous for the recessive disease 2 gene or homozygous dominant. So I have aabb for IIIj and AaXX for IIIm, where I can't assign XX.

1 user
I'm not so good at genetics and so there might be nuances I'm missing, but say A is the dominant allele for disease 1 and b is the recessive allele for disease 2. So you already know that IIIj must have both copies of the disease 2 recessive gene and none of the disease 1 alleles. Only one of IIIm's parents has the disease, so IIIm is heterozygous for the disease I gene. However, I don't see information that would tell me whether IIIm is heterozygous for the recessive disease 2 gene or homozygous dominant. So I have aabb for IIIj and AaXX for IIIm, where I can't assign XX.
Thanks for catching that, my head is hurting so bad, so probably going to sleep... i ll think about it more tomorrow.. but you are right, there is no way of knowing whether of not IIIm is heterozygous recessive for disease 2,

I'm not so good at genetics and so there might be nuances I'm missing, but say A is the dominant allele for disease 1 and b is the recessive allele for disease 2. So you already know that IIIj must have both copies of the disease 2 recessive gene and none of the disease 1 alleles. Only one of IIIm's parents has the disease, so IIIm is heterozygous for the disease I gene. However, I don't see information that would tell me whether IIIm is heterozygous for the recessive disease 2 gene or homozygous dominant. So I have aabb for IIIj and AaXX for IIIm, where I can't assign XX.
Figure it out, disease 2 is X recessive, So because male only get their X from mom. IIf has normal version of the X, or he would have been being IIh.

So there is noway IIIm is herozygous for disease 2 , she is def X+X+ like you have noted above. Hence PA = 1/2 PB = 0 that IIIm would pass her recessive X2 allele * 1 that IIIj will pass its allele = 0 and the probabilty of both happening become zero as well.

Thanks again, the help pal.. Would have never figured this out alone.

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Wait, the fact that disease 2 is sex-linked does change things, but I'm not sure it makes it clearer. IIf has the normal X chromosome. But IIg, the mother, could still be heterozygous for disease 2 (she could have one normal X and one bad X). Since the son, IIIn, can get either of his mother's X chromosomes, it's possible that the mother, IIg, is heterozygous and he happened to get the good copy. IIIm, the daughter, definitely has one good X but could have gotten her mother's bad X, making her heterozygous. Or she could have gotten her mother's good X and be homozygous dominant.

In any case, no use worrying about this problem because the new MCAT doesn't make you do pedigree analysis.

Wait, the fact that disease 2 is sex-linked does change things, but I'm not sure it makes it clearer. IIf has the normal X chromosome. But IIg, the mother, could still be heterozygous for disease 2 (she could have one normal X and one bad X). Since the son, IIIn, can get either of his mother's X chromosomes, it's possible that the mother, IIg, is heterozygous and he happened to get the good copy. IIIm, the daughter, definitely has one good X but could have gotten her mother's bad X, making her heterozygous. Or she could have gotten her mother's good X and be homozygous dominant.

In any case, no use worrying about this problem because the new MCAT doesn't make you do pedigree analysis.
I apologize for not posting the entire passage but " Assume that individuals not blood related to Individuals I.a and I.b (“in-laws”) do not possess either disease-producing allele. " .

So true

In any case, no use worrying about this problem because the new MCAT doesn't make you do pedigree analysis.
not like this question but there was a question that required people to know about dom/rec/auto/x-linked on my exam to solve the question.

However, it was like 2 questions on whole section/exam so... invariably, you could just get down to 2 potential answers, pick one and move on.

But this kind of pedigree analysis is not on exam.

I apologize for not posting the entire passage but " Assume that individuals not blood related to Individuals I.a and I.b (“in-laws”) do not possess either disease-producing allele. " .

Ah, I see. Yes, in that case, IIIj is aaX(bad)Y and IIIm is AaXX. So all the possible combinations are: 4 AaX(bad)X, 4 AaXY, 4 aaX(bad)X, and 4 aaXY. So the chances of being afflicted with Disease 1 or Disease 2 but not both is the same as the chances of getting the AaX(bad)X and AaXY combinations, which is 8/16 or 1/2.

1 user
not like this question but there was a question that required people to know about dom/rec/auto/x-linked on my exam to solve the question.

There will be no pedigree analysis on the exam. It was very obviously removed from the outline. Yes, you will need to know basic inheritance patterns. But that's not the same at all as pedigree analysis.

1 users
There will be no pedigree analysis on the exam. It was very obviously removed from the outline. Yes, you will need to know basic inheritance patterns. But that's not the same at all as pedigree analysis.
let's not quibble over semantics here.

Probability calculations require knowledge of pedigree's and how to analyze them. I had that on my exam. It was only 3-generations and very minor with a well known disease.

To answer the question, I had to draw the pedigree on my scratch paper, write in the probabilities for each individual in the generation and do the math. thankfully, it was only 3-gens with like 3 people in each subsequent generation... nothing like that mess above.

Not hard, not as hard as the one above but pedigree was on my exam (Aug 2016)

To answer the question, I had to draw the pedigree on my scratch paper, write in the probabilities for each individual in the generation and do the math. thankfully, it was only 3-gens with like 3 people in each subsequent generation... nothing like that mess above.

I'm not sure you had to draw a pedigree for that question as you described it because it could have been answered probabilistically.

1 user
I'm not sure you had to draw a pedigree for that question as you described it because it could have been answered probabilistically.