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. Please note: a value without a written explanation will not be graded.

Please clearly label your responses as A) and B) or it will not be possible to grade.

Answer 1

A)

No, this is clearly not the expected number of chromosomes.

The actual expected number of chromosomes in all the gametes formed must be 4

The situation would have occured in the following fashion due to improper meiosis:

The diploid organism would have had a total of 8 chromosomes in its dilpoid (2n) state. After the first meiotic division which leads to the formation of 2 daughter cells, each two daughter cell would have recieved 4 chromosomes each (remember that each chromosome is divided into two sister chromatids which are attached at the centromere. 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 (remember that the sister chromatids for each chromosome are still together attached at centromere). The other of the two daughter cells formed after first meiotic division would have underwent an incorrect second meiotic divsion. As a result, the 8 sister chromatids (of the four chromosomes) were distributed asymmetrically in them, thereby leading to 3 chromosomes being assembled in in one gamete and 5 chromosomes being assembled in in the other.

B)

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

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

(Each pair has two homologous counter parts which make a total of 8 chromosomes)

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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