Genetic Recombination

Question 1

How does the location of two genes affect genetic outcome

Answer 1

• recombinant gametes are the result of independent assortment of chromosomes during meiosis 1 and are a major source of genetic variation

-independent assortment predicts that 50% of gametes will be non-recombinant and 50% recombinant

• alleles are exchanged on non-sister chromatids
• independent assortment and chromatid exchange leads to genetic variation

Question 2

How does genetic outcome depend on physical location

Answer 2

• on the same chromosome (no change at any stage)
  linkage: two genes on a single pair of homologues, no genetic exchange occurs
  100% non-recombination
• on the same chromosome
  linkage: two genes on a single pair of homologues, exchange occurs between two non-sister chromatids
  maximum 50% recombinants, depending on physical distance between genes
• on different chromosomes
  independent assortment: two genes on two different homologous pairs of chromosomes
  50% recombinant and 50% non-recombinants
• genes on same chromosome segregate together

Question 3

What does F1 and F2 look like with genes on the same chromosome

Answer 3

F1: all F1 are heterozygous for both gene pairs and exhibit the dominant trait

top chromosomes on top line and bottom chromosomes on the bottom

F2: different to Mendelian dihybrid cross no 9:3:3:1 ration formed, 1:2:1 instead, one phenotype cannot be made die to physical location of two genes - no recombinant gametes

Question 4

What happens when non-sister chromatids are crossed over

Answer 4

crossing over of non-sister chromatids can generate recombinant chromosomes as long as crossing over takes place during prophase of meiosis 1, then the exchange between non-sister chromatids forms a recombinant allele that did not exist before

Question 5

How was recombination discovered

Answer 5

• Morgan introduced the notion of crossing over
• gene separation caused crossing over to occur at chiasmata, but Morgan could not explain results, why different frequencies of phenotypes occur
• then found meaning of different recombination frequencies, two genes that are very close together will have less chiasmata than genes very far apart

Question 6

How is the relationship between meiotic events and phenotypic classes explained

Answer 6

• test crosses between a double heterozygous parent and double hemizygous recessive, then if recombinant crosses occur its from one crossover in meiosis when the parental female formed gametes

Question 7

What are double cross-overs

Answer 7

double crossovers can only be identified when there are three genes linked on the same chromosome

• these are very infrequent as parents must be heterozygous for all three genes under consideration
• a triple heterozygous female is crossed with a male carrying recessive for all three genes

Question 8

What are the crossing over events in meiosis 1

Answer 8

• for linked genes crossing over in meiosis 11 produces new genetic combination in the gametes
• for two genes on the same chromosome
  no recombination is more likely than a single crossover and this is more likely than double crossovers

Question 9

Questions using mapping units

Answer 9

1 map unit (cM) = 1% recombination

eg. if the distance between genes is 20 cM, how many of the 200 chromosomes will be recombinant

20cM = 20% recombination
20% of 200 is 40 so 40 chromosomes undergo recombination