Question

Read each question carefully. Write your response in the space provided for each part of each question. Answers must be written out in paragraph form. Outlines, bulleted lists, or diagrams alone are not acceptable and will not be scored.

Primary spermatocytes are diploid (2n) cells with all of the organelles typically found in eukaryotic animal cells. A representation of spermatogenesis from a primary spermatocyte with six chromosomes is shown in Figure 1.

(a)  Describe the process in meiosis that ensures that both maternal and paternal chromosomes are passed on to each spermatozoon.

Question 2

(b)  Explain why the genetic content of individual chromosomes in a spermatozoon most likely differs from the genetic content of individual chromosomes in a primary spermatocyte.

Question 3

(c) In some instances, meiosis of a primary spermatocyte with six chromosomes results in two spermatozoa that contain four chromosomes and two spermatozoa that contain two chromosomes.  Predict the most likely cause.

Question 4

(d) A student claims that if the animal producing the spermatozoa has a mutation in a mitochondrial gene, the probability that any offspring will inherit the mutation is zero. Provide evidence to support this claim.

Answer 1

(a) The first meiotic division of the primary spermatocyte(2n) is reductional and results in two secondary spermatocytes(n) which then undergoes the second meiotic division which is equational and results in the formation of four haploid spermatozoa. The maternal chromosome (X) and the paternal chromosome (Y) separate in the first/reductional meiotic division and therefore the secondary spermatocytes have either the X chromosome or Y chromosome. During the second/equational division the secondary spermatocytes further divide to produce four spermatozoon with two carrying the X chromosome while the other two carry the Y chromosome. Thus, the process in meiosis ensures that both paternal and maternal chromosomes are passed on to each spermatozoon.

Question 2

(b) The genetic content of individual chromosomes in the primary spermatocyte is diploid(2n) as they arise from mitotic divisions of spermatogonia cells(2n), whereas the genetic content of individual chromosomes in a spermatozoon is haploid(n) as they arise from meiotic divisions of primary spermatocytes(2n) and hence the difference.

Question 3

(c) Non-disjunction of chromosomes is the failure of paired chromosomes to separate from one another in the event of transitioning from the metaphase to anaphase stage in cell division, which results in an unequal number of chromosomes in the divided cells. Here, the division of the primary spermatocyte having six chromosomes results in the formation of two spermatozoa having four chromosomes while the other two spermatozoa have two chromosomes, which is due to non-disjunction of chromosomes during the meiotic division.

Question 4

(d) Spermatozoa carrying a mitochondrial mutation will not affect the offspring as for the fertilization event to be successful leading to the formation of the zygote, the nuclear fusion of the spermatozoon with the nucleus of the ovum is necessary. The rest of the cytosolic components including the mitochondria in the spermatozoon do not add to the cytoplasm of the zygote and hence do not play any role in the fertilization process. However, the mitochondrial DNA does have an impact on the zygote from the maternal side, if the ovum carries any mitochondria having mutations. This is because the whole cytoplasm which is available to the fertilized egg is of the ovum.