Lecture 5

Question 1

What is neurospora

Answer 1

A mold that grows as a haploid, 1 copy of genetic info
Note that they can mate if they have opposite mating types, under certain conditions, and this would make them diploid

Question 2

Can neurospora undergo meiosis as a haploid organism

Answer 2

Yes, but only if it mates and fuses with its mate to become a diploid organism

Question 3

Ascus and why is it useful

Answer 3

Ascus is a sac of the 4 resulting haploid spores produced from the meiosis of a diploid neurospora
Useful because it gave scientists a way to evaluate the results of on meiotic cycle, to see the actual changes caused by crossing over

Question 4

What is in the Ascus by the end of the mating

Answer 4

The neurospora of different mating types fuse and undergo nuclear fusion, officially diploid, undergoes meiosis 1, then meiosis 2, finishing with 4 spores, but then each of these undergoes a round of mitosis, so we end up with 8 spores total, so an Ascus contains 8 cells

Question 5

At what point in meiosis does the crossing over occur

Answer 5

It occurs at the 4-chromosome stage, specifically at the point where each homologous chromosome (2 in a pair, obvi) duplicate, so each sister chromatid has 2 copies. When they line up at the cell line during meiosis 1, this is where crossing over occurs
Basically: after the first replication

Question 6

Why is crossing over at the 2 chromosome stage not a viable theory

Answer 6

If they crossed over at the 2 chromosome stage, between non-homologous sister chromatids, and THEN replicated, that would result in all 4 haploid cells being recombinant, which we saw in Morgans drosophila experiment is not likely, its actually a minority that was able to cross over, so its just not consistent

Question 7

Is there a limit to crossovers

Answer 7

No there is not, not in amount per chromosome and sort of for the amount of chromatids involved, you can involve all copies of the homologous chromosome, so 4 max, and it’ll be a giant crossing
Refer to slide 8

Question 8

Does recombination frequency of two genes have a cap, if so why

Answer 8

Yes, capped at 50%, for any number of genes, because even if theyre so far apart that theyre basically on different chromosomes (behaviorally), there is still only a 50/50 chance of inheriting a parental vs recombinant allele
We dont know how many crossovers there are, the gene might have jumped 10 times for all we know, so its gets to the point that theres a 50% chance it will be net: no cross over therefore parental or a 50% chance it will be net: crossed over therefore recombinant
Note: applies to multiply genes too
Note 2: if genes reach 50% then theyre basically behaving as if they are on different chromosomes, in terms of probability theres no linkage, like the numbers dont show it

Question 9

Recombination frequency

Answer 9

The frequency with which individuals produce recombinant gametes

Question 10

What’s the relation between RF and distance between genes

Answer 10

The greater the distance between two genes, the higher the recombination frequency because there will be more chances/opportunities for recombination between them