A biologist mixes sperm and eggs from an organism with a diploid number of 12. I

Question

A biologist mixes sperm and eggs from an organism with a diploid number of 12. In studying the resulting zygotes, he finds five with the following chromosome numbers: 13,16,19,20 and 23. He concludes that these cells resulted from multiple fusions of sperm and egg cells (polyspermy). Of course, you disagree. What could you say to him? How best could you criticize his conclusion? (choose and explain why one is correct and three is wrong)

A. You are forgetting that sperm carry the haploid number of chromosomes

B. Notice that the observed chromosome numbers, in your cells, would not result from multiple fusions of normal gametes.

C. Egg cells possess numerous mechanisms to try and limit the number of multiple sperm fusions that occur

D. Aneuploidies can result from nondisjunction of homologous pairs in meiosis.

Explanation / Answer

Ans. Correct option. A. You are forgetting----.

Both the male and female gametes are haploid (n). So, number of chromosomes in each gamete is equal to 6 (= 2n/ 2 = 12/ 2 = 6).

So, even several sperms (n) fuse with the egg (n), the total number of chromosomes in the zygote with always be a whole number integer divisible by n (n=6). That is-

If 3 sperms fuse with egg: Total number of chromosomes = 3 x 6 + 6 = 24

If 4 sperms fuse with egg: Total number of chromosomes = 4 x 6 + 6 = 30

If 5 sperms fuse with egg: Total number of chromosomes = 5 x 6 + 6 = 36

Note that the final chromosome number (24, 30, 36, and so on) is always divisible by the haploid number 6.

Since, 13, 16, 19, 20 and 23 are not divisible by 6, none of the cases represent polyspermy.

#B. Incorrect. The “organism” is NOT specified. So, it need not necessarily be a human cell.

#C. Incorrect. The statement is true for most of the sexually reproducing organisms. However, the statement des not clarify the unusual chromosome count- the theme of this question. That is, the statement has nothing to do with “polyspermy”.

#D. Incorrect. The statement is true and most possible reason for unusual chromosome count. However, it does not rule out the term “polyspermy” as cause of unusual chromosome count- what the biologist has suggested.

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