D2.1 - meiosis as a source of genetic variation (5f)

Question 1

what are 2 ways genetic variation occurs within meiosis?

Answer 1

• random orientation
• crossing over

Question 2

what is random orientation?

Answer 2

the process by which 2 genes located on different chromosomes are inherited independently of one another

Question 3

what is the impact of random orientation?

Answer 3

• the 2 traits caused by those genes are also inherited independently; there is independent assortment of genes and traits
• due to the frequency of crossing over, this pattern of inheritance is also seen for genes located far apart on the same chromosome

Question 4

how does random orientation lead to genetic variation?

Answer 4

random orientation occurs due to homologous chromosomes aligning independently and randomly on the equator during metaphase I, and then the homologues are pulled to seperate poles during anaphase I
- this means that a random combination of chromosomes (1 from each pair) will end up at each pole.

Question 5

what is crossing over?

Answer 5

exchange of alleles between non-sister chromatids, causing a new combination of alleles to occur in chromosomes, gametes and therefore offspring

Question 6

when does crossing over occur?

Answer 6

during prophase I, when synapsis brings homolgous chromosomes together to form bivalents

Question 7

how does crossing-over lead to genetic variation?

Answer 7

chiasmata form, which are points of attachment between non-sister chromatids (chromatids of homologous chromosomes), which increases the stability of bivalents
- the exchange of DNA and alleles between the non-sister chromatids leads to 4 genetically distinct chromatids within the bivalent
- at anaphase 1, the homologues are separated to each pole

Question 8

what is the impact of crossing-over?

Answer 8

1. at the end of meiosis I, the chromosomes have 2 different chromatids; one has the parental combination of alleles and the other has the recombinant mix of alleles.
2. in meiosis II, these genetically distinct sister chromatids are separated, leading to 4 genetically different chromosomes and 4 genetically different gametes, which have a unique combination of alleles.