IMAT 2012 Q49 [Genetics]

Which one of the following would be different in a pair of non­-identical twins?

A. alleles
B. amount of nuclear DNA
C. the total of adenine plus guanine
D. genes
E. chromosome number

The answer to this question is A.

Firstly, what are non-identical twins? Also called dizygotic/fraternal twins, are twins that have been conceived by 2 separate sperm that penetrated 2 separate eggs. Thus, they may not share the same sex or appearance, similar to siblings that were not conceived in the same pregnancy. And like any other siblings, these twins share 50% of their genome.

With the definition of non-identical twins in mind, why is choice A the answer?

  • The clue is in the definition of non-identical twins itself. Since these twins are dizygotic, they will only share 50% of their DNA, which means that their phenotype will be different from each other. As a reminder, phenotypes are the traits that we can observe such as height and eye color. Which then makes sense why these kinds of twins are called “non-identical”.
  • Your job now, is to discern which among the choices are responsible for dictating this phenotype. Which you will then arrive at the answer A, which are alleles. An as you already know, alleles are variants of a gene, that gives rise to different genotypes, and whether these alleles are dominant or recessive, their combinations within these genotypes dictates how our phenotypes are expressed. It would now make sense therefore, that these twins will have different alleles, since they have different phenotypes.

What’s common among choices B to E that makes them not the correct answer for this question?

  • It is safe to assume that there is no pathological error that could lead to any chromosomal abnormality, since there was no mention of such in this question. Thus, there is 1 central theme that is common among all the other choices, which is that they are essential properties/characteristics of human DNA! For example, we humans have 46 chromosomes located in our nucleus, which eliminates choices B and E. In addition, we have genes for different traits, but HOW they are expressed is determined by the specific alleles that make up the gene in the first place. For example, both twins in this question has the gene for eye color as a general trait, but the ALLELES will determine what color exactly each twin will get, thus eliminating choice D in your considerations. In terms of base pairing, the relative proportions of adenine and guanine are constant within species. Thus, this should not be in your consideration.

Therefore, the only correct answer would be choice A.