GS-3 Section III Question 40

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[Chris777]

In Q40 of the GS 3 exam the question asks about organism 9 but the answer only refers to organism 6 and does not make any sense of how the answer relates to organism 9. If someone could help it would be appreciated.

[goldstanda3269]

The reason is that the clue given regarding organism 9 provides information that requires a reevaluation of the entire flow chart. Since the answer choices involve organisms 1, 2 and 3, those 3 organisms need to be reassessed given the new information. Of course, the 'new' information is that if organism 9 lacked any of the 3 traits then A is recessive. This is the only valuable information regarding organism 9.

[Chris777]

Thank you, for your help.

[RosieMcFadyen]

I'm confused with this one, if organism 9 lacks all of the traits, surely it does not mean A has to be recessive, it could just mean that 9 inherited the recessive allele a from both parents and 3 is heterozygous no?

[goldstanda3269]

The only thing similar to your question that we can see is question 22?

"22) If organism 9 were an open, solid-line organism (i.e. if it lacked any of the three traits), which of the following would necessarily be true?"

We will respond based on this assumption...

I'm confused with this one, if organism 9 lacks all of the traits, surely it does not mean A has to be recessive, it could just mean that 9 inherited the recessive allele a from both parents and 3 is heterozygous no?

Not really. If 9 inherited the recessive allele a from both parents then 9 would express Trait A. However, the question is stating "If organism 9 were an open, solid-line organism (i.e. if it lacked any of the three traits)" which means that in this hypothetical, 9 is NOT expressing Trait A.

Consider this: Organism 3 has Trait A. If it is homozygous dominant then all offspring will express Trait A which is not the case. If it is heterozygous then we expect that, in a large enough pool, 50% of offspring would express Trait A which may have been the original situation suggested by Figure 1.

However, the question stem for 22 changes Organism 9 such that none of the offspring expressed Trait A which suggests that Organism 3 was homozygous recessive for Trait A and produced offspring that are carriers but do not express Trait A.

Going a little further: Notice that Trait A carriers 8 and 9 can therefore produce offspring that are homozygous recessive (14 and 15).