# 2021 James Ruse Q20 (1 Viewer)

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#### Eagle Mum

##### Well-Known Member
I think the answer should be A (2/14) but I'm wondering if you have posted this question because either you or whoever set the exam considers B (3/16) to be the appropriate answer.

#### zizi2003_

##### Well-Known Member
I think the answer should be A (2/14) but I'm wondering if you have posted this question because either you or whoever set the exam considers B (3/16) to be the appropriate answer.
Wait, could you please explain this question?

##### Member
I think the answer should be A (2/14) but I'm wondering if you have posted this question because either you or whoever set the exam considers B (3/16) to be the appropriate answer.
I got A as well

#### Eagle Mum

##### Well-Known Member
Wait, could you please explain this question?
They stated that there are only four genotypes and showed that anyone who is dominant for both traits has both dominant genes on the same chromosome (type 1). Therefore the male of the original couple must be type 1. His partner only carries recessive alleles and every partner that has joined this family also only carries recessive alleles. Therefore anyone in the pedigree who only has one dominant trait, carries a recombinant chromosome that is a result of a crossover.

The denominator should be all the offspring of any coupling between a parent with both dominant traits and their partner with only recessive alleles. This includes individuals #2, #3 & #5 in the first generation of offspring and individuals #1 to #11 in the second generation of offspring, a total of 3+11 = 14. For the numerator, #5 in the first offspring generation and #2 in the second offspring generation express only a single dominant trait each, which must have resulted from crossover. Therefore, with these two individuals, from 14 offsprings of ‘double dominant’ & ‘double recessive’ pairings, we calculate the estimate as 2/14 = 14.3% (option A).

I was speculating that the question might have been posted because OP does not agree with the answer and the most obvious red herring I can see is that individuals #12 & #13 may have been included in the calculations, with individual #12 being included in the numerator and both included in the denominator - hence (2+1) / (14+2) = 3/16 = 18.8% (option B). However, this would be erroneous because individual #5 of the first offspring generation only expresses one dominant trait, so her offspring should be excluded entirely from calculations.