IMAT 2012 Q54 [Genotypes]

Which of the following crosses is most likely to produce offspring of genotype GgNn?

A. ggNN x GGNn
B. GGNn x GgNn
C. ggNn x GGNN
D. GGNN x ggnn
E. GgNn x GgNn

The answer to this question is choice D.

This question seems confusing at first, but it would be wise to figure out what the question is asking:

  1. "Genotype GgNn" : the fact that there are 2 genes with 2 alleles each, should prompt you to think that this is indeed a dihybrid cross.
  2. "…most likely to produce" : Now that you know that this is a dihybrid cross, you would now need to combine this with the fact that you need to have a punnett square that has the most combinations that include GgNn, which is a heterozygous genotype, among all the choices given.

Before we rationalize the choices, let us first establish, that you should not be tempted to answer choice E.

  • As mentioned earlier, it will be useful to be aware of what the question is asking from you. Thus, you are to find the cross that yields GgNn the MOST out of ALL THE CHOICES.
  • As you know with dihybrid crosses, 2 heterozygous parents, GgNn for example, will yield characteristic phenotypic ratio of 9:3:3:1, as well as the genotype GgNn happening the most within this specific dihybrid cross. Yes, GgNn occurs the most WITHIN this dihybrid cross BUT it does not happen the most compared to the other choices.

If you plot all the punnett square for the choices above, they would look like this:

Let us then tally how often the genotype “GgNn” occurs within each dihybrid cross:
A. ggNN x GGNn = 8 times
B. GGNn x GgNn = 4 times
C. ggNn x GGNN = 8 times
D. GGNN x ggnn = 16 times (the whole punnett square)
E. GgNn x GgNn = 4 times

Thus, among all the choices involved, choice D yields the most number of GgNn genotypes.

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