IMAT 2015 Q28 [Alleles]

In the nerve cells of a person, one gene has two alleles, ‘A’ and ‘a’, present. For this person, which row correctly shows the alleles present in all three different situations given in the table?


A. row 1
B. row 2
C. row 5
D. row 4
E. row 3

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The answer to this question is choice A, row 1.

How do we begin answering this question? This is a bit of a tricky question, that you will need to approach in a systematic manner in order to arrive at the correct answer. Firstly, you will need to figure out the main distinguishing factor that will help you discern the correct arrangement of alleles given the specific situations. For questions like these, it would be useful to keep it simple to avoid confusion. Look at the column headings, and you will realize 1 central concept: The situations detail whether or not they are in the process of cell division. As simple as that, you know have the foundation that you need as long as you keep this central concept in mind.

With that being said, let’s rationalize the answer:

  • Now that you know the central distinguishing factor among the columns, it would be wise to eliminate choices/considerations to make it more efficient for yourself. Let’s look at the first column entitled “Stem cell starting to specialise into a liver cell”. You might argue, “but stem cells divide in order to give rise to more stem cells or to more specialized stem cells.” Yes, but that is not the situation given in the column. If you read closely, it said “starting to specialise”, meaning it is NOT in cell division YET, but instead is in the process of differentiation. This process includes changing cell morphology, metabolic processes, and even responsiveness to different signals, which changes the whole function of the cell as a whole. So how is this then relevant to the question? Since this cell is NOT in cell division, logically, the gene needs to be in its natural state, wherein both alleles are present, and NOT in duplication, or in other words, the gene with the alleles “Aa”. Why not in duplication? because duplicated genes are a function of cell division, which will be discussed later. Thus, you will need to eliminate choices that do not involve “Aa”, leaving only row 1 and 4 in your considerations.
  • Let’s move on to the next 2 columns. If you read the next 2 columns, they will mean yield the same situation. Why? Because the columns said “at the start”, and what comes before BOTH mitosis and meiosis is interphase. As you recall, a phase in interphase occurs called the S phase, which duplicates the genetic material, in preparation to yield daughter cells. Thus, you would expect that the alleles in both of these situations would be duplicated, or AAaa, because BOTH situations has just come from interphase.

Thus, the only consideration remaining is choice A, or row 1.


so first, a stem cell differentiates and morphs into a particular type of cell and then it starts dividing through mitosis?

That is true for some stem cells if they can re-enter the cell cycle…
But here, Stem cells are specializing into liver cell. Liver cell is a terminally differentiated cell just like neurons, cardiac cells, adipocytes in which after the stem cells acquire their specialized function, they withdraw from the cell cycle thus entering G0 phase.

Stem cells about to specialize are in G0 phase and are quiescent. If we recall the cell cycle, G0 is before S phase, so the DNA is unreplicated and will be in Aa not Aaaa. Stem cells can only have AAaa if they re-enter cell cycle or if they have not entered G0 at all.

DNA replication is highly coordinated with cell proliferation and is uniquely regulated in different cell types and organs. The regulation of differentiation requires cross-talk between DNA replication and differentiation.

Liver cells are in G0 phase but they can regenerate. Liver can regenerate after partial hepatectomy, although there are irreversible damage like cirrhosis where regeneration is very limited.
Liver cells are differentiated cells, but they are also quiescent cells which can re-enter the cell cycle.


I see… thank you so much for the elaborate response.

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