Lesson 9 Video 8 Sex-Linked Genes Question

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vgp1993
Posts: 18
Joined: Wed Dec 20, 2017 12:14 pm

Lesson 9 Video 8 Sex-Linked Genes Question

Post by vgp1993 » Thu Mar 01, 2018 7:14 pm

On slide 8, Dr. Anthony says that each gender appears to be affected equally. However the affected female in the second generation is not part of that same line. That same female has a an affected daughter with an affected male, but no other daughter in the tree is affected. Doesn't this suggest that the trait could very well be sex linked?

Thanks!
NS_Tutor_Andrew
Posts: 520
Joined: Mon May 23, 2016 1:47 pm

Re: Lesson 9 Video 8 Sex-Linked Genes Question

Post by NS_Tutor_Andrew » Fri Mar 02, 2018 3:57 pm

Hi vgp1993,

That's a really good question. Probably, the simplest way to look at it is that a recessive sex-linked trait on the MCAT would show a sharper sex asymmetry than the 67%:33% male-female ratio we see if we exclude the affected female in generation 2 -- in other words, it's still close enough to even for MCAT purposes.

As some further background info, X-linked recessive pedigrees on the MCAT usually have pretty unambiguous sex ratios because the sex asymmetry has to be pretty sharp to be able to rule out chance. After all, the odds of flipping a fair coin 6 times and getting 2 heads and 4 tails isn't that low -- about 25%. Plus, most sex-linked recessive mutations are not super common in the population, so females are WAY less likely to be affected than males, because the odds of an affected male mating with a carrier female is not very high. According to our good buddy Wikipedia, red-green color blindness is present in 8% of males and 0.5% of females in northern European populations, for a male:female ratio of 16:1.

Nonetheless, your point is taken. We're always looking for ways to make our materials as clear as possible, so I'll make a note of this pedigree as one to potentially revise. Thanks!
Andrew D.
Content Manager, Next Step Test Prep.
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