Lecture 10: Linkage, Recombination and Mapping

Question 1

What is required for Mendel’s law of independent assortment to be true?

Answer 1

Genes must be unlinked from each other

Question 2

What are two ways that allows genes to behave as if unlinked?

Answer 2

1. The two genes are on different chromosomes
2. The two genes are far away from each other if on the same chromosome
   
   • allows for crossing over to occur - 50% chance of gene loci ending up on either homologous chromosome

Question 3

What is the relationship between the distance of gene loci and the frequency of crossing over?

Answer 3

If gene loci are close, cross overs will be infrequent (less likely)

If gene loci are far apart, more cross-overs are seen (more likely)

ie: the probability of crossing over between two genes increases with greater distance between gene loci

Question 4

Provide the general definition for: Recombination?

Answer 4

Any process in a diploid cell that generates new allele or chromosomal combinations

Question 5

What are 3 Recombination Processes?

Answer 5

1. Random Fertilization
2. Independent Assortment
3. Crossing Over

Question 6

How is Random Fertilization an example of recombination?

Answer 6

Random fusion of gametes creates organisms with new genotypes

Question 7

How is independent assortment an example of recombination?

Answer 7

• Each synapsed chromosome pair orients itself independently from other chromosome pairs in Meiosis I
• Each sister chromatid orients itself independently in Meiosis II (they may no longer be identical due to crossing over)
• Key driver of diversity = random alignment = unique combinations

Question 8

How is Crossing over an example of Recombination?

Answer 8

Exchanges alleles between homologuous chromosomes

Question 9

How is the recombination frequency calculated?

Answer 9

Sum of all recombinants divided by the total number of progeny

(eg 300 recombinants out of 1500 = 20%)

Question 10

Why cant the recombination frequency ever exceed 50%

Answer 10

1. if genes on one chromosome behave as unlinked = two equally likely locations to be in
   
   • 50/50 chance of being on chromosome I or II
   • osmosis example: start with 100% of all molecules on one side, you will not have more than 50% on either side at equilibrium
2. If you compare the gamete distribution of unlinked genes you will see it is not possible to achieve more than 50% (whether on the same or different chromosomes)

Question 11

What kinds of gametes are produced if TWO gene loci are on homologous chromosomes?

What proportions of these gametes are seen?

“Crossover vs no cross-over”

Answer 11

• No crossing-over:
  
  • produce parental gametes
• Cross-over:
  
  • Produce both parental and recombinant gametes
  • 1:1 ratio

Question 12

What is the original definition of synteny?

Answer 12

• two or more genes located on the same chromosome
  
  • therefore physically linked (syntenic)

Question 13

What is the modern usage of synteny

Answer 13

Syntenic blocks = evolutionary conserved regions of chromosomes that display the same order of genes in dif. species

• image shows mouse chromosome 11 and human chromosome 17 and ancestral chromosome.
• SHARED SYNTENY or “CONSERVED SYNTENY”

Question 14

What is the relationship between map units and %recombination?

Answer 14

1:1

1 map unit = 1% recombination

Question 15

If genes are less than 50 map units apart, what type of assortment do they exhibit?

Answer 15

The do not assort independently

• more parental combination, fewer recombinants

Question 16

If genes are more than 50 map units apart what type of assortment would they show?

Answer 16

They assort independently (even though they are on the same chromosome)

• 1:1 number of parental and recombinants

Question 17

What are three reasons why recombination frequency might not reflect true distances?

Answer 17

1. Cross overs are reduced near the centromere
2. once a cross over occurs in one area, it tends to interfere with additional cross-overs in that area
3. Genes that are far apart show a max of 50%

Question 18

What does a three point testcross allow?

Answer 18

Allows the order and distance between three potentially linked genes to be determined in a single experiment

Question 19

How would you do a three point testcross?

Answer 19

Take a triple heterozygote (trihybrid) and testcross it using a completely recessive individual

• calculate RF by looking at #of Recombination (cross over) events between each pair of genes
  
  • rarest progeny types = two crossover events occurred