IMAT 2016 Q24 [Mitotic Division]

An organism is heterozygous for two genes. These two genes make up part of the same DNA molecule.

For one gene, E represents the dominant allele, e represents the recessive allele.

For the other gene, R represents the dominant allele, r represents the recessive allele.

Assuming there is no mutation, at the end of a mitotic division producing two cells which row(s) is/are possible?


A. row 6 only
B. rows 4 and 5 only
C. row 1 only
D. rows 2 and 3 only
E. rows 6 and 7 only

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The correct answer is E.

What is mitosis ?

To answer this question, we need to know that mitosis is.

It is a type of cell division that begins with the duplication of the DNA.

Furthermore, the daughter cells are genetically similar to the mother cell : a gene A with the allele y is transmitted to the daughter cells.

Its purpose is to ensure cell multiplication for tissue repair or growth.

In other words, from a single diploid mother cell (2n), 2 diploid daughter cells (2n) are obtained.

A diploid cell has 2 complete sets of chromosomes, usually written as being a 2n cell, as opposed to a haploid cell, that only has 1 set of each chromosome, and written as a n cell.

What is heterozygous ?

To understand this concept, let’s take the same random gene A :

  • Because humans are diploid, they will have 2 alleles (versions) of the same gene

  • A is a gene that has 2 alleles : Ay and Az

  • Heterozygous is when the 2 alleles of a gene are different, which can be represented by (Ay/Az)

  • On the other hand, homozygous is when when the 2 alleles of a gene are similar, which can be represented by (Ay/Ay)

Now, let’s answer the question :

  • It is said that the organism is heterozygous for both genes, and that it goes into mitosis

  • There are 2 genes :

    • one that has the alleles E and e
    • on that has the alleles R and r
  • All mitotic progeny are genetically similar

From what we just said, we can say that the daughter cells will have 4 different allele combinations possible :

  1. E and R on one molecule
  2. e and r on the other


  1. E and r on one molecule
  2. e and R on one molecule

This question is driving me nuts! Can someone plz map it out for me on a piece of paper?

ok this has to be a typo in the question? it doesn’t make any sense that both genes are located on the same dna molecule right??? even one gene for that matter. because each gene has two alleles, in diploid organisms at least, so there has to be at least two dna molecules in play here, one to hold the e and r and one for the E and R, (or e and R & E and r) because it is heterozygous



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No no no, you can have multiple genes on the same DNA molecule - remember there are 46 chromosomes that code for thousands of genes, its the alleles (versions of genes) that are on the different DNA molecules

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ok, so at least the alleles are on different DNA molecules. But the genes, how could they say there is a gene or two or any number of them on a single DNA molecule?? since each gene is comprised of two alleles (which again, can be found on two DNA molecules) in diploid organisms. So basically, wouldn’t saying that only refer to one of the two alleles that make up a gene? Can you please explain that part? I think I don’t understand what a gene is exactly.

Remember we have homologous ‘pairs of chromosomes’ - there are alleles for a particular gene on both chromosomes in the pair (on the two DNA molecules as you said), these code for a particular trait (eg Eye Colour) - genes are found in particular locus (position) on the chromosome

Each chromosome has multiple genes that code for different functions, imagine books on a bookshelf - and that each book is a gene and the bookshelf is a chromosome.
Now for pairs of chromosomes this would be like having 2 of these bookshelfs with the same books (but sometimes slightly different versions - alleles)

When it says 2 genes are on the same DNA molecule its like talking about ‘2 different books on the same bookshelf’ in my example. - so in this case Gene E and Gene R (which each have their own alleles on both chromosomes in the pair)

We know in this question, that the organism is heterozygous for both genes, so the alleles are different from each on both chromosomes in the pair - and that as Ari says in the solution in mitotic division - daughter cells are genetically similar to the mother cell - so we get this same genetic makeup in the answer as we do from he start of the question.

Idk if this helps, hope it does though!


I think I got it now. Thanks!

Edit: I just ran into another problem; In row 7, there is really no way to get that combination unless the mother cell itself had that combination: E and r on one molecule and e and R on the other. Because mitosis doesn’t have crossing over.
So either the mother cell had the combination: E and R on one molecule and e and r on the other, and the resulting two cells of that division BOTH had that same combination. Or the mother cell had the combination: E and r on one molecule and e and R on the other, and again, the resulting cells of that division will BOTH have that combination.
So really the question is just “what are the ways a cell can be heterozygous for two genes”, since mitosis isn’t really changing anything here, right?

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Yup! Mitosis is just passing down the genetic info from mother to daughter cell, and the question rules out mutation which would alter things, glad you got it!

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I got it but at what cost!

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