Pedigree

[Snorlax]

I'm quite confused while doing an exam question on a past VCAA paper.

http://www.vcaa.vic.edu.au/Documents/exams/biology/2010biology2-w.pdf

Question 9 of the Multiple choice.
Answer is written as C, but why?

NVM. I did NOT read the question and answers properly -_-;;

[vox nihili]

I'm quite confused while doing an exam question on a past VCAA paper.

http://www.vcaa.vic.edu.au/Documents/exams/biology/2010biology2-w.pdf

Question 9 of the Multiple choice.
Answer is written as C, but why?

NVM. I did NOT read the question and answers properly -_-;;

For the benefit of anybody else who may have seen this, I'll explain anyway.

First of all, a useful mnemonic with pedigrees is that females have curves, and thus are a circle and males have square shoulders and thus are a square. Some people think it's silly, but it always worked for me!

Ok, so we know it's an X linked-dominant trait, so that actually makes this a pretty straightforward question. We can do some trial and error.

A. I1 is a female (circle therefore curves therefore female) and does not carry the trait. It is suggested that it is reasonable to assume that her mother had the trait. Based on this evidence (that she does not have the trait and she is a she) we cannot reasonably assume that this is the case. It is possible, but there is no evidence to support this contention either. So bugger off A
B. We've already established that I1 is a female. The contention now is that the father had the trait. Given that there is a 100% probability that I1 received her X chromosome with her father (barring any hanky-panky her mum may have had with the milkman) and that I1 does not carry the trait, it is completely unreasonable and a biological impossibility that her father has the trait.
C. I2 is a male (square, square shoulders, male). He has the trait. We know with certainty that his X-chromosome, the one to carry the dominant defect, was supplied by his mother. Therefore, it is reasonable, and indeed almost certain, that his mother carries the trait. Woohoo for C!
D. The father of I2 could have had the trait, that being said, the only remnants of the father of I2 surviving in our pedigree is his Y chromosome (the rest of his genetic material isn't included for obvious reasons). Given the trait is X linked, we do not have the necessary information to say whether or not he carries the trait.

The answer is C.