Lecture 10: chromosomal mappint

Question 1

1. Define genetic linkage (2 components of the definition). Define syntenic.

Answer 1

Genetic linkage involves being syntenic(genes located on the same chromosome) and said genes not being independently assorted after meiosis.

We see less gamete combinations than we would see if genes were independently assorted

Question 2

2. How does the genetic content of parental and recombinant chromosomes differ?

Answer 2

recombinant chromosomes have alleles combinations that differ from the parents

Question 3

3. Crossovers are more likely in longer intervals of chromosomes than shorter intervals. How does
   this property of crossovers allow for the mapping of genes?

Answer 3

If cross over rate is higher for a set of genes it will indicate that they are further apart from one another.

Question 4

4. If two genes are on the same chromosome, does that automatically mean they are genetically
   linked and do not assort independently?

Answer 4

No, all syntenic genes are not linked

Question 5

How does genetic linkage distort independent assortment? (what is there an excess of?)

Answer 5

There is an excess of parental type gametes relative to what we would expect in independent assortment

There is no 1:1 assortment of gametes

Question 8

Distinguish between complete genetic linkage and incomplete genetic linkage.

Answer 8

Complete genetic linkage: rare event when no crossing over occurs between linked genes.

Incomplete genetic linkage:
there is a recombination between linked genes sometimes, but parental type gametes are still produced in excess

Question 9

Explain Morgan’s crossing-over hypothesis to explain recombinant phenotypes

Answer 9

two-point test cross =
crossed female homozygous for two recessive x-linked mutations with wildtype male

results: all female were wild type (getting wildtype x from father)
all males (had both recessive phenotypes)

excess of red and normal wings and white eyes and mini wings( excess of parental phenotypes)

Question 10

10. In an alternate universe, you are an undergraduate in a fly lab in 1911. You are given the following
    data concerning two-point test crosses for 5 genes (ABCDE):

Question 11

11. In a three-point test cross, what distinguishes the parental allele combinations? The double
    crossover allele combinations?

Answer 11

Parental allele combinations:
two crossovers possible, 6 gametes produced in total
(highest frequency)

Double crossover allele combination:
middle chromosome is flipped from recessive to dominant (AbC/ aBc)

(lowest frequency)

single crossover types:
two phenotypes with similar frequencies correspond to one crossover

Question 12

Why do genetic maps underestimate recombination frequency for genes that are far apart?

Answer 12

when there is no marker intervening crossovers double crossovers are not detected leading to underestimation of r

Question 13

What determines recombinational hotspots in mammals? How can it be true that there is a
general relationship between distance and recombination rate if recombination rate is variable across
the genome?

Answer 13

hotspots are driven by specific sequences binding prdm9

because hotspots and cold spots are dispersed throughout the genome

Question 14

Give two reasons why recombination can be evolutionarily advantageous.

Answer 14

Recombination brings beneficial genes together on one chromosome without having a mutational event. It also can concentrate deleterious mutations on one gene.

Question 15

Explain how natural selection and migration can induce genetic linkage.

Answer 15

lowers LD over time through mating and recombination which reduces the p