Questions: Meiosis: Generating Genetic Diversity

Question 1

A diploid cell (2n = 4) completes Meiosis I. What does each resulting daughter cell contain?

• A. 4 chromosomes, each a single chromatid — homologs and sister chromatids have all separated
• B. 2 chromosomes, each consisting of 2 joined sister chromatids — homologs have separated but sister chromatids remain together
• C. 4 chromosomes, each consisting of 2 sister chromatids — exactly as in the original cell
• D. 2 chromosomes, each a single chromatid — identical to what mitosis would produce

Answer: (B)

Meiosis I separates homologous chromosomes — the paternal and maternal copies of each chromosome — not sister chromatids. In a 2n=4 cell (2 pairs of homologs), each daughter cell after Meiosis I has 2 chromosomes, but each chromosome still consists of 2 sister chromatids joined at the centromere. Sister chromatid separation happens in Meiosis II. The most common confusion is treating Meiosis I like mitosis, where sister chromatids separate. Meiosis I is unique: it separates whole homologs, reducing the cell from diploid to haploid chromosome number while DNA remains in a 'two-chromatid' form.

Question 2

A nondisjunction event during Meiosis I causes both copies of chromosome 21 to end up in the same secondary oocyte. If this gamete is fertilized by a normal haploid sperm (carrying 1 copy of chromosome 21), what is the result?

• A. A normal diploid offspring, because the extra chromosome will be eliminated during fertilization
• B. Trisomy 21 (three copies of chromosome 21) — the gamete contributed 2 copies, plus 1 from sperm = 3 total
• C. Monosomy 21 (one copy of chromosome 21) — the nondisjunction egg lost a chromosome in the process
• D. Tetrasomy 21 (four copies), because the sperm also carries a duplicated chromosome 21

Answer: (B)

Nondisjunction in Meiosis I means both homologs of chromosome 21 fail to separate and end up in the same gamete — that gamete now has 2 copies instead of the normal 1. Fertilization by a normal sperm (1 copy) produces a zygote with 3 copies = trisomy 21 (Down syndrome). Option C (monosomy) describes the OTHER gamete from that meiotic event, which received zero copies of chromosome 21. The distinction between Meiosis I and Meiosis II nondisjunction matters clinically: Meiosis I errors affect all chromatids of both homologs; Meiosis II errors affect only two of the four chromatids.

Question 3

Crossing over during prophase I creates chromosomes that carry allele combinations not present in either parent's original chromosomes — it generates genetic variation beyond what independent assortment alone provides.

• T. True
• F. False

Answer: True

This is correct and important. Independent assortment shuffles whole chromosomes randomly between daughter cells (2²³ combinations in humans). Crossing over does something different: it reshuffles alleles within chromosomes, producing new chromosome mosaics that are partial maternal, partial paternal. The result is allele combinations that never existed in either parent's genome. Both mechanisms contribute to gametic diversity, but crossing over is the only one that actually breaks up the linear arrangement of alleles along a chromosome.

Question 4

Meiosis II is genetically equivalent to mitosis because both processes separate sister chromatids and produce identical daughter cells.

• T. True
• F. False

Answer: False

Meiosis II resembles mitosis in its mechanism (sister chromatids separate), but the resulting cells are NOT genetically identical to each other or to the parent cell. Crossing over during prophase I ensured that the sister chromatids entering Meiosis II already carry recombined allele combinations — they are not perfect copies of each other. 'Like mitosis' accurately describes the mechanical steps (spindle formation, chromatid separation), but it does not describe the genetic outcome. The products of Meiosis II are four genetically distinct haploid cells.

Question 5

Why does meiosis require two rounds of cell division but only one round of DNA replication, and what does this arrangement achieve?

Model answer: DNA is replicated once before meiosis begins, producing a 4n DNA content. Meiosis I then separates homologous chromosomes (each still consisting of two sister chromatids), reducing chromosome number from 2n to n while each cell still has two-chromatid chromosomes. Meiosis II separates those sister chromatids, producing four haploid (n) cells with single-chromatid chromosomes. The two divisions without a second replication halves the chromosome number so that fertilization (fusion of two haploid gametes) restores the diploid count. If cells replicated again between divisions, the chromosome number would not be reduced.

This 'one replication, two divisions' logic is the core of how meiosis solves the ploidy problem of sexual reproduction. Mitosis maintains ploidy: replicate once, divide once, same chromosome number. Meiosis reduces ploidy: replicate once, divide twice, half the chromosome number. Every generation of sexually reproducing organisms depends on this arithmetic being exactly right.