Chapter 5.3: Mapping: Locating Genes Along a Chromosome

Question 1

What are the two limitations of two-point crosses?

Answer 1

 It may be difficult to determine gene order if some gene pairs lie very close together
 Actual distances in Sturtevant’s map do not always add up

Question 2

What do three-point crosses account for that two-point crosses don’t?

Answer 2

double crossovers

Question 3

What is chromosomal interference?

Answer 3

 The occurrence of one crossover reduces the likelihood that another crossover will occur in an adjacent part of the chromosome
• This phenomenon of crossover not occurring independently- is called chromosomal interference

Question 4

What is the coefficient of coincidence?

Answer 4

 Investigators can obtain a quantitative measure of the amount of interference in different chromosomal intervals by first calculating a coefficient of coincidence, defined as the ratio between the actual frequency of double crossovers observed in an experiment and the number of double crossovers expected on the basis on the basis of independent probabilities
• Interference = 1 – coefficient of coincidence

Question 5

What are recombination hotspots?

Answer 5

 Most crossovers take place in so-called recombination hotspots- small regions of DNA where the frequency of recombination is much higher than average

Question 6

What is a linkage group?

Answer 6

o Genes chained together by linkage relationships are known collectively as a linkage group