IMAT 2011 Q52 [Dihybrid Cross]

In a dihybrid cross between two heterozygous individuals, which is the most likely combination in their offspring?

A. AaBB

B. AaBb

C. aaBB

D. aaBb

E. AABB

Before you start answering this question, you must first be familiar with a few concepts:

  • Basically, a dihybrid cross is a cross between two organisms with 2 different traits. In other words, this cross is concerned with 2 genes, where each gene controls 1 trait.
  • As you already know, a gene is made up of 2 alleles, and each allele can be either a dominant or a recessive allele. Thus, a “heterozygous” gene is made up of 1 dominant and 1 recessive allele.

What is this question looking for?

  • The question is asking for the most “likely” genotype in the offspring of parents that are heterozygous. Which means that you are to find the most probable combination that will be seen in the offspring.
  • Thus, it is safe to assume that each parent has a genotype of AaBb.

How do we start answering this question?

  1. First figure out all the possible combinations of alleles in the gamete of each parent given their genotype. For a genotype of AaBb, there are 4 possible combinations: AB, Ab, aB, and ab. This applies to both parents since they are both heterozygous.

  2. Since each parent can give 4 different combinations of alleles, you must draw a 4x4 square, with the different combinations along the edges of this square. Fill up the boxes accordingly.
    image

  3. The next step is to now find the combination that happens the most number of times, since this will be the most likely combination that the offsprings will get from the dihybrid cross. Please remember: Do not be confused about the order of the letters! aA and Aa are the same since it contributes to the same phenotype! In other words, it does not matter from which parent the allele came from.
    image

  4. Keeping in mind the reminder stated in #3, in the image above, you will get to see that the combination of the alleles A, a, B and b happens 4 times. This combination of 4 alleles appear the most compared to the other combinations.

Thus, the answer to this question is B, the most likely combination in the offspring is AaBb.

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