9.2 - Meiosis and genetic variation

Question 1

What are the 2 ways that cell division occurs

Answer 1

• Mitosis (produces 2 daughter cells with the same number of chromosomes as the parent cells)
• Meiosis (produces 4 daughter cells with half the number of chromosomes as the parent cells)

Question 2

Why is meiosis important

Answer 2

• if each gamete (sperm/egg) had a diploid number (full set of chromosomes) then the cells that they produce would have double this number
• therefore meiosis is needed to half this diploid number (known as the haploid number) so that when the 2 haploid gametes fuse, the diploid number of chromosomes is restored (half from each parent)

Question 3

Describe the 2 nuclear divisions involved in Meiosis

Answer 3

1. FIRST DIVISION
   - homologous chromosomes pair up and their chromatids wrap around each other
   - Equivalent portions of these chromatids may be exchanged in a process called crossing over
   - By the end of this division: the 2 homologous pair are separated, with 1 chromosome from each pair going into one of the 2 daughter cells
2. SECOND MEIOTIC DIVISION
   - the pair of sister chromatids are separated (as their centromere divides)
   - 4 cells are now formed (in humans: each of these cells contains 23 chromosomes)

Question 4

As well as halving chromosomes, what else does meiosis do

Answer 4

• produces genetic variation amongst offspring (may lead to adaptations that improve survival chances)
• it does this in 2 ways:
  —> independent segregation of homologous chromosomes
  —> new combinations of maternal and paternal alleles by crossing over

Question 5

What is a gene

Answer 5

A length of DNA that codes for a polypeptide

Question 6

What is a locus

Answer 6

The position of a gene on a chromosome or DNA molecule

Question 7

What is an allele

Answer 7

One of the different forms of a particular gene

Question 8

What is a homologous chromosome

Answer 8

A pair of chromosomes, one maternal and one paternal, that have the same gene loci

Question 9

Describe how independent segregation of homologous chromosomes lead to genetic variation

Answer 9

• during the first division, each chromosome lines up alongside its homologous partner
• (in humans: it’d mean that there’s 23 homologous pairs of chromosomes lying side by side)
• when the pairs arrange themselves = randomly in line
• 1 of each pair will go into each daughter cell (determined by how they’re lined up in the parent cell)
• since they line up randomly = the combination of maternal/paternal chromosomes in each daughter = random
  —> This is called independent segregation

Question 10

Describe how variety from new genetic combinations leads to genetic variation

Answer 10

• each member of a homologous pair of chromosome has exactly the same genes and therefore determines the same characteristics (e.g. blood group)
• However the alleles may vary (may code for blood group A or B)
• The independent assortment of these chromosomes therefore produces new genetic combinations

Question 11

What happens after each chromosome lines up alongside its homologous partner, what’s this process called

Answer 11

GENETIC RECOMBINATION BY CROSS OVER
- the chromatids of each pair become twisted around each other
- during this twisting process, tensions are created and portions of the chromatids break off
- these broken portions might then rejoin with the chromatids of its homologous partner
- usually it’s the equivalent portions of homologous chromosomes that are exchanged
- in this way, new genetic combinations of maternal and paternal alleles are produced

Question 12

Why is it known as ‘Genetic recombination by crossing over’

Answer 12

• crossing over: the chromatids cross over one another many times so the process is known as crossing over
• recombination: the broken-off portions of chromatids recombine with another chromatid

Question 13

If there is no recombination by crossing over, what is the effect?

Answer 13

• only 2 different types of cell are produced
  —> if it does occur = 4 different cell types are produced
  —> Crossing over therefore increases genetic variety even further

Question 14

What is the mathematical formula for calculating the number of possible combinations of chromosomes of maternal and paternal origin in its daughter cells as a result of meiosis

Answer 14

2^n
—> where n = the number of pairs of homologous chromosomes

Question 15

Use the equation to work out how many possible combinations of chromosomes for each daughter cell can be produced by 4 homologous pairs

Answer 15

• 2^4
• or 16
  —> therefore there is 16 possible combinations

Question 16

How can we then calculate the different combinations of chromosomes in the offspring produced as a result of sexual reproduction

Answer 16

(2^n)^2
—> where n = the number of pair of homologous chromosomes

Question 17

What are the mathematical formulas based on

Answer 17

• based on chromosomes staying intact throughout meiosis
• in practise we known that crossing over between chromatids during meiosis exchanges sections of chromosomes between homologous pairs in the process called recombination
• As recombination occurs each time gametes are made, it will greatly increase the number of possible chromosome combinations in the gametes

Question 18

Describe the 3 stages in this diagram about variety from new genetic combinations leading to

Answer 18

STAGE 1
- 1 of the pair of chromosomes includes the gene for tongue rolling and carries 1 allele for roller and one for non-roller. The other chromosome includes the gene for blood group and carries the allele for blood group and the allele for blood group B
- There are 2 possible arrangements, P and Q, of the two chromosomes at the start of meiosis
- both are equally probable, but each produces a different outcome in terms of characteristic that may be passed on via gametes

STAGE 2
- At the end of meiosis 1, the homologous chromosomes have segregated into 2 separate cells

STAGE 3
- At the end of meiosis 2, the chromosomes have segregated into chromatids producing 4 gametes for each arrangement. the actual gametes are different, depending on the original arrangement (P or Q) of the chromosomes at stage 1

Question 19

what is a polyploidy

Answer 19

• changes in the whole set of chromosomes
• occurs when organisms have 3 or more sets of chromosomes rather than the usual 2 = polyploidy

Question 20

what is non-disjunction

Answer 20

• changes in the number of individual chromosomes can occur due to a failure in separation of chromosomes during meiosis is called non-disjunction
• this results in a gamete having 1 more or 1 fewer chromosome
• e.g. down’s syndrome individuals have an additional chromosome 21