Question

A diploid organism produces four gametes from one parent cell through the process of meiosis. Two gametes are found to have 4 chromosomes, one gamete is found to have 3 chromosomes and one gamete is found to have 5 chromosomes. A) Is this the expected number of chromosomes that would be found in each gamete following a normal cycle of meiosis? If yes, explain why. If no, explain why not and describe how the gamete situation described above occurred. B) Determine the number of homologous chromosome pairs that the original parent cell contained, before meiosis began. Explain how you determined this value.

Answer 1

A)

No, this is not the expected number of chromosomes.

The expected number of chromosomes in all the gametes is 4

The situation occured in the following way:

The diploid organism had 8 chromosomes in its dilpoid state. After the first meiotic division, each of the two daughter cell would have recieved 4 chromosomes each (each chromosome divided into two sister chromatids. So, each chromosome = 2 sister chromatids attached together at centromere). One of the two daughter cells would have underwent second meiotic division properly, leading to the two gametes with 4 chromosomes each. The other of the two daughter cells would have underwent an improper second meiotic divsion. The 4 chromosomes (8 sister chromatids) were distributed asymmetrically in them, leading to 3 chromosomes in one gamete and 5 in the other.

B)

The original parent had eight (8) dilpoid (2n) number of chromosomes.

Hence, before meiosis the number of homologous pairs = 8/2 = 4

Explanation: In diploid state, each type of chromosome exists as two homologous chromosomes. So, if we divide the total number of dilpoid chromosomes (here 8) by 2, we get the number of homologous pairs.

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