Lecture 18

Question 1

Jumping genes

Answer 1

Aka transposable elements
Long repetitive sequences in the genome, they can be copied and pasted or moved

Question 2

What are the germline consequences of chromosomal inversions

Answer 2

Will lead to issues in meiosis, note that crossing over occurs in meiosis 1
In meiosis 1 we get homologous chromosomes lining up, theyre already duplicated, so 4 sister chromatids lined up, this is where we do crossing over
If we have an inversion, were making them uneven (in length) and the sequence is also not matching up, so its hard for them to actually pair, and it turns out for them to actually be able to pair we get an inversion loop (slide 7), and crossing over in this loop is a major issue, teh consequences depending on whether its a para or pericentric inversion

Question 3

What happens if you try and crossover with a chromosome with a paracentric inversion

Answer 3

Because you have the inversion loop, when you do the crossing over you’ll cross with teh wrong sister chromatid, so instead of getting a clean recombination you’ll accidentally have attached 2 of the chromosomes such that when theyre pulled apart, we have no choice but to get a break, an uneven distribution and a lost fragment
So in this case, net result: 2 normal gametes, one with a huge deletion and the other with a smaller but still significant deletion, and the lost fragment will segregate randomly but be degraded, without a centromere it doesn’t really fit anywhere
Refer to slide 10

Question 4

What happens if you try and crossover with a chromosome with a pericentric inversion

Answer 4

When the inversion includes the centromere
2 of the meiotic products wont be viable, theyll have a deletion and a duplication (so genes missing and extra), whats extra in one will be lacking in the other
One meiotic products will be perfectly normal
And the last will have all the right parts but will be completely inverted, still viable, not deffective

Question 5

What could be the clinical consequences for an individual with a chromosomal inversion

Answer 5

Potential infertility from the inability to produce proper gametes (50% of the time is not considered enough, that’s grounds for infertility)
Note: this would likely be a case where the inversion isn’t disrupting any genes and is therefore still viable, they may not even know they have an inversion

Question 6

If we see a high recombination frequency but very low amount of recombinant progeny, what does that suggest

Answer 6

Suggests that crossing over led to lethality, thus why they arent present in data, and the main reason for crossing over to lead to lethality would be that theres an inversion causing crossing over there to profoundly damage the chromosome
The viable crossing overs you do have are likely outside this loop and therefore dont lead to chromosomal damage

Question 7

Chromosomal translocations

Answer 7

Happens when we get a break in DNA, but were so unlucky that it doesnt get lost and deleted, it doesnt invert, no, it bind to a whole other chromosome
We are only responsible for balanced translocations, meaning no loss of genetic info, its just in the wrong place

Question 8

What are teh somatic consequences of chromosomal translocations

Answer 8

If a break doesn’t disrupt a gene and theres no loss of genetic info in the process then no harm done
Its when we dont get this perfect case that we have a problem

Question 9

What are the germline consequences of chromosomal translocation

Answer 9

They have pairing issues in meiosis, they have to line up homologous sequences with each other, even if on other chromosomes, so its a weird lining up the do rather than the normal side by side thing, refer to slide 20
There are 2 ways they can segregate, one way distributes them nicely, either by giving non translocated copies or by giving both translocated copies, making it a net of no loss no duplication
Could also segregate in such a way that there are duplicates and consequently deletions, and since this occurs randomly theres no way to know, just wont be viable

Question 10

Robertsonian translocation

Answer 10

A type of surprisingly balanced and viable translocation, refer to slide 21 for visual representation, but idea is that we get crossing over, it reorganizes, and then at the end we have uneven chromosomes and a small stubby chromosome, that one gets lost, net result it all of chromosome 14 and most of chromosome 21, half kind of fused to 14 (translocated) and the other standing alone, making it look like a monosomy
Has to happen with Acrocentric chromosome, meaning its centromere is very close to one end, and were losing some of those short ends

Question 11

What do robertsonian translocations tell us about acrocentric chromosomes, specifically the ends

Answer 11

Not essential, or at least not essential to have 2 copies, making the individual not exhibit any phenotype
This kind of thing is often only identified when the individual is diagnosed with infertility

Question 12

What are the germline consequences for robertsonian translocations

Answer 12

Don’t get the 50:50 ratio we got for literally everything else, here we get 6 possible gametes, because we only have 3 chromosomes
Refer to slide 23, but basically theres purple vs yellow and theyre pretty mixed now, but they will pair with their own color cuz thats the homologous region, and if 2 homologous regions pair, that third chromosome is segregated alone

Question 13

What could be a possible hereditary cause of Down syndrome

Answer 13

The robertsonian translocations between chromosomes 21 and 14, when crossing an individual that is a carrier for this translocation to an individual who does not carry it, one of the possibilities in the offspring is a trisomy where we get 2 copies of chromosome 21, Down syndrome not by non-disjunction but by inheritance