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?

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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

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

OR

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