B Inheritance 1 (Meiosis and Genetic Variation)

Question 1

When does meiosis occur and what does it produce?

Answer 1

• After interphase, a reproductive cell undergoes meiosis.
• This results in 4 genetically different, haploid daughter cells (half the number of chromosome as the diploid parent cell).
• During meiosis 1, homologous chromosomes separate into two daughter cells.
• During meiosis 2 the sister chromatids move apart from each other to form a new pair of daughter cells.

Question 2

Describe meiosis I

Answer 2

1. Prophase I: homologous chromosomes pair up and the nuclear envelope breaks down.
2. Metaphase I: these homologous pairs line up along the equator of the cell.
3. Anaphase I: homologous chromosomes are pulled to opposite poles.
4. Telophase I: chromosomes uncoil and nuclear envelopes form around the two groups.
5. The cell then undergoes cytokinesis, forming 2 daughter cells.

Question 3

Describe meiosis II

Answer 3

Meiosis II is the same as mitosis, since there are no longer any homologous chromosomes.
The cell then undergoes cytokinesis, resulting in 4 daughter cells.

Question 4

Compare mitosis and meiosis:

Answer 4

• Mitosis involves one division, whereas meiosis involves two.
• Mitosis produces two genetically identical daughter cells, whereas meiosis produces 4 genetically different cells.
• Mitosis produces cells with the same number of chromosomes as the parent, whereas meiosis produces cells with only half the number of chromosomes as the parent.

Question 5

How does genetic variation occur during meiosis?

Answer 5

1. Crossing over during prophase I
2. Independent assortment during metaphase I
3. Spontaneous mutations affecting the number of chromosomes.

Question 6

How does crossing over in prophase I result in genetic variation?

Answer 6

• Homologous chromosomes are close enough for chromosomes of each pair to twist around each other.
• Portions of each chromatid then exchange with the homologous partner, resulting in new allele combinations (but doesn’t always).

Question 7

How does independent assortment occur during metaphase I?

Answer 7

• Homologous chromosomes pair up at random.
• This randomises the chromosomes that end up in each daughter cell.

Question 8

How can mutations create genetic variation?

Answer 8

• During meiosis I, a homologous pair of chromosomes may not separate, resulting in two gametes with one extra chromosome, and two gametes missing one chromosome. This is called non-disjunction.
• During meiosis II, non-disjunction prevents two sister chromatids from separating, resulting in one gamete with an extra chromosome, and one gamete missing a chromosome.