Question 27

[Young]

This question is asking about organism 9. How do we ended up with an answer relating to organism 2? I guess I don't see the relationship between them. Please explain. Thanks.

[admin]

OK, let's begin by examining the right side of Figure 1. If, as the question suggested, organism 9 lacks all 3 traits but yet the paternal organism (3) expressed Trait A, then that trait must be recessive. In other words, organisms 9, 10 and 11 are all heterozygous for the trait (carriers of "a") but none express Trait A.

Now let's examine the left side of Figure 1. Now that we know that Trait A is recessive, we know that organism 1 (just like #3) must be "aa." We know also that organism 6 must be "aa" which means that it received one "a" from organism 1 and one "a" from organism 2. Thus organism 2, which does not express Trait A, must be "Aa" (heterozygous at the trait A locus).

If you get through this passage, you'll never have a problem with family trees on the real MCAT!

[zeina_ghou6141]

I'm sorry, I'm still confused about the explanation for this question.
It was written "organism 9 lacks all 3 traits but yet the paternal organism (3) expressed Trait A, then that trait must be recessive"

Organism 9 does not have the traits, so why is this affecting paternal organism 3 since 9 is offspring of 3? I really want to understand this passage, and I think clearing this up may help. Thanks!

[admin]

If Organism 9 lacks all 3 traits that means that none of the offspring from Organisms 3 and 4 expressed Trait A. That means Trait A could not possibly be dominant otherwise it should have been expressed at least once. It is far more likely that Trait A was expressed in Organism 3 because it is aa (recessive) and therefore was not expressed in any of the offspring.

[yfangl097543]

The question asks for which choice would have to be true, which means the logic in deriving the answer has to be 100% true. However, I don't see how A HAS to be recessive. Let's say that A is dominant, which means 3 could exhibit Aa while 4 could have aa. Therefore, their children could very well all be aa and not exhibit the trait.

[admin]

Let's take your suggestion and do a Aa x aa (try a Punnett square) which would show a 50-50 probability of having offspring which express a dominant Trait A (50% would have Aa). But, according to the question, 0/3 offspring express trait A. Very unlikely.

Now the next step down will make A being dominant go from unlikely to impossible. If organism 8 does not express trait A and neither does organism 9, then how is it possible that 2/3 of their offspring express that trait? That cannot happen if A is dominant.

[yfangl097543]

I guess what confused me was the fact that the traits are mutually exclusive. I didn't know that if an organism expressed one trait, then it did not express any of the other traits.

[admin]

If ever it should be the case that an organism is capable of expressing more than one trait then it must be explicitly stated in the passage or, more likely, the Key to the Figure will present a specific shading/code to represent individuals who express 2 or more traits, or, the circle or square will be half shaded with the code for one trait and half shaded for the code of the other trait.

[bfjava8852]

Easy to get tripped up on this question...

[admin]

bump

[tltee192108]

Are traits A, B, C all for one phenotype like green, blue, or brown eyes, meaning all the traits are mutually exclusive? Or are the traits for three different phenotypes?