Chapter 9

Question 1

What does meiosis produce and what does it involve

Answer 1

produces daughter cells that genetically Diff from each other.

involves two nuclear divisions and creates four haploid daughter cells from a single diploid parent cell

Question 2

What are 2 mechanisms in meiosis that introduce variation

Answer 2

Independent segregation of homologous chromosomes

Crossing over between homologous chromosomes

Both occur in meiosis I (first round of division)

Question 3

Independent segregation

Answer 3

In meiosis I, homologous pairs of chromosomes line up opposite each other at the equator of the cell.

It is random which side of the equator the paternal and maternal chromosomes from each homologous pair lie.

These pairs are separated, so one of each homologous pair ends up in the daughter cell.

Question 4

Independent segregation - calculation

Answer 4

This creates a large number of possible combinations of chromosomes in the daughter cells produced.

This can be calculated using 2^n

n = number of homologous pairs

Question 5

What is crossing over and how does it work

Answer 5

homologous pairs line up opp each other at equator in meiosis I, parts of chromatids twist around each other.

puts tension on chromatids, pairs of chromatid to break.

broken parts of chromatid recombine with another chromatid.

results in new combo of alleles.

Question 6

Meiosis compared to mitosis

Answer 6

Meiosis
- 2 nuclear divisions
- haploid cells (one set of chromosomes)
- introduces genetic variation

Mitosis
- 1 nuclear division
- diploid cells (2 sets of chromosomes)
- create genetically identical cells

Question 7

Identifying meiosis in a life cycle

Answer 7

Tip: Meiosis involves diploid (2n) parent cell dividing to become haploid (n) cell

Question 8

Final increase in genetic variation

Answer 8

Random fertilisation further increases genetic variation.

There 2n possible chromosome combo in gametes ( 2^23 in humans)

It is random which egg and sperm will fuse is fertilisation. Therefore, variation is increased.

There is altuaily (2n)2 possible combinations of chromosomes when you consider random fertilisation.

For a human this = (2^23)^2 = 7.04 x 10^13

This is before crossing over is considered.

Question 9

What’s non-disjunction

Answer 9

chromosomes or chromatids dont split equally during anaphase.

Question 10

Chromosome mutations

Answer 10

Mutations in number of chromosomes arise spontaneously by chromosome non-disjunction during meiosis.

Question 11

Chromosome non-disjunction

Answer 11

Changes in the number or structure of whole chromosomes can arise spontaneously due to chromosome non-disjunction during meiosis.

This can occur in two forms:
1. Changes in whole sets of chromosomes (polyploidy)
2. Changes in the number of individual chromosomes (aneuploidy)

Question 12

What is polyploidy

Answer 12

Changes in whole sets of chromosomes occur when organisms have three or more sets of chromosomes rather than the usual two. This condition is called polyploidy and mainly occurs in plants.

Changes in whole sets of chromosomes occur when organisms have three or more sets of chromosomes rather than the usual two

Question 13

What is aneuploidy

Answer 13

Changes in the number of individual chromosomes.

Sometimes individual homologous pairs of chromosomes fail to separate during meiosis.

This is non-disjunction and usually results in a gamete having one more or one fewer chromosome.

On fertilisation with a gamete that has the normal number of chromosomes, the resultant zygote will have more or fewer chromosomes that normal in all their body cells. E.g. Down’s syndrome is 3 copies of chromosome 21.

Question 14

Aneuploidy

Answer 14

Changes in the number of individual chromosomes.
On fertilisation with a gamete that has the normal number of chromosomes, the resultant zygote will have more or fewer chromosomes that normal in all their body cells.
E.g. Down’s syndrome is 3 copies of chromosome 21.