IMAT 2014 Q37 [Meiosis Process]

The diagram below shows a pair of homologous chromosomes and the site of crossing over.

image

Which answer in the table is correct when meiosis is complete?

image

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

The answer here is choice E, row 4.

It will first be useful to be reminded of the main essence of meiosis, where it is a type of cell division that produces 4 gametes that have half the chromosome number as the parent cell. The explanation to this question is pretty straightforward, and will be best discussed if we tackle the table per column.

’Number of recombinant daughter chromosomes produced:'

  • As you can see in the diagram above that depicts crossing-over between 2 homologous chromosomes, you will be able to notice that the recombination or the crossing over actually happens among 2 chromatids only, 1 chromatid from each chromosome. Thus, when the crossing over is done, each chromosome will have both a recombinant and a non-recombinant chromatid. The question explicitly stated, “when meiosis is complete”, thus, you can imagine that these 2 homologous chromosomes will ultimately be split into 4 chromatids in total, which will be in each daughter cell. And as mentioned, crossing over only happens among 2 chromatids, which means that, among the 4 daughter cells of meiosis, only 2 will have the recombinant chromatids that underwent crossing over, which leaves only row 2 and row 4 in your considerations.

Number of daughter chromosomes that are genetically distinct from each other:

  • If you revisit the diagram again, you will be able to notice that, after the crossover, the recombinant and non-recombinant chromatids are all distinct. Thus, at the end of meiosis, when the 4 sister chromatids separate into individual daughter cells, there will be 4 daughter cells that are genetically distinct from each other, which can be attributed to the crossing over. Thus, this leaves only row 4 as your correct consideration, making choice E as the correct answer.