Question

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

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 the maternal and paternal chromosomes are passed on to each spermatozoon.

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

c. in some instances meiosis of a primary spermatocyte with six chromosomes results into spermatozoa that contain four chromosomes into spermatozoa that contains two chromosomes. Predict the most likely cause

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 the claim

Answer 1

a.

Synapsis and crossing over causes segregation and recombination may occur.

• During meiosis I-prophase I- homologous chromosomes pair up to form tetrad.
• Crossing over occurs during synapsis: Synapsis is alignment of homologous chromosomes, forming tetrad. Crossing over involves exchange of genetic material between non-sister chromatids.

b.

• Primary spermatocytes undergo meiosis I to generate secondary spermatocytes.
• In meiosis I, after cytokinesis, 2 genetically haploid cells are produced, which enter meiosis II.
• Meiosis II to generate spermatids, which form spermatozoa.
• In meiosis II, after cytokinesis, 4 genetically haploid cells are produced.
• Thus, in primary spermatocytes there are diploid number of chromosomes.
• While, after meiosis II, spermatozoa will have haploid number of chromosomes.

c.

• Non-disjunction may occur due to- unable to pair, synapse, or cross-over during prophase of meiosis, failing to align during metaphase, or improper or failure to separate or dis-join during anaphase and telophase.
• In meiosis-I, non-disjunction will be occurring in Anaphase-I, when pairs of homologous chromosomes fail to separate.
• For meiosis –II in anaphase-II, the sister chromatids may not separate.
• Thus, some cells will have more chromosomes (4) than normal and some less (2).

d.

• Mitochondrial genes are transmitted from mother to offspring.
• This is because, mitochondria are inherited from cytoplasm of egg cells.
• Since, only egg cells contribute to mitochondrial inheritance, females can pass mutations in mitochondrial DNA to offspring.
• The mitochondrial disorders may be passed to both males and females but cannot be passed from the father.
• Mitochondrial DNA have high substitution rate and is random,but is majorly related to maternal lineage.
• Thus, spermatozoa being male gamete will not pass mutation in mitochondrial DNA.