9.2 Meiosis and genetic variation

Question 1

How is the number of chromosomes maintained through generations?

Answer 1

• If gametes contained the diploid number of chromosomes (46) they would end up with double the amount of their parents
• so, the number halves during meiosis, to form the haploid number (23)
• when gametes meet at fertilization, the diploid number is restored.

Question 2

Describe the two nuclear divisions which make up meiosis.

Answer 2

DIVISION 1: homologous chromosomes pair up and their chromatids wrap around each other. equivalent portions are swapped in the ‘crossing over’ process. then, they separate into the two daughter cells.
DIVISION 2: chromatids move apart and their are four daughter cells containing 23 chromasomes each.

Question 3

How does meiosis bring about genetic variation among the offspring?

Answer 3

• independent segregation of homologous chromosomes

- New combinations of maternal and paternal alleles by crossing over

Question 4

What is a gene?

Answer 4

A length of DNA that codes for a polypeptide

Question 5

What is a locus?

Answer 5

the position of a gene on a chromosome or DNA molecule

Question 6

What is an allele?

Answer 6

one of the different forms of a particular gene

Question 7

What are homologous chromosomes?

Answer 7

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

Question 8

Describe the independent segregation of homologous chromosomes.

Answer 8

• during M1, each chromosome lines up alongside a homologous partner.
• When the pairs line up, they do it randomly
• one of each pair will pass to the daughter cell, depending on how they are lined up
• since this is random, the combination of chromosomes of maternal and paternal origin in the daughter cell at M1 is random.

Question 9

Describe how variety comes from new genetic combinations.

Answer 9

• each member of a homologous pair has exactly the same genes which determine the same characteristics.
• however, the alleles of these genes may vary.
• so, the independent assortment of these chromosomes produces new genetic combinations.

Question 10

Describe the process of genetic recombination by crossing over.

Answer 10

• chromosomes line up alongside their partner
• the chromatids become twisted around each other
• during twisting tensions are created and portions break off
• these rejoin with the chromatids of the homologous partner
• new genetic combinations of maternal and paternal alleles are produced

Question 11

How do you calculate the number of possible combinations of chromosomes for each daughter cell in mitosis?

Answer 11

2n

where n=the number of pairs of homologous chromosomes

Question 12

How do you calculate the number of possible combinations of chromosomes for each daughter cell in sexual reproduction?

Answer 12

(2n)2