1. Explain how single and double crossover events influence genetic distance calculations.
Added by Patrick Q.
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It quantifies how different the genetic makeup of two individuals or groups is, often used in phylogenetics and population genetics. Show more…
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The smaller the map distance, the more accurate the distance estimate tends to be. Usually, but not always, when the two shortest map distances are added up, the total doesn't always equal the longest distance when assessing the distance between 3 genes. The reason is due to interference. Interference is calculated as 1 - coefficient of coincidence (C). C = observed number of double crossovers / expected number of double crossovers. We will start with getting the value of expected number of double crossovers for a new problem: Suppose the percent of single crossovers between a gene for eye color (v+ and v) and wing shape (m+ and m) is 3.2%. The percent of single crossovers between wing shape and a gene for body color (s+ and s) is 6.6%. What is the expected frequency of double crossovers? Report your answer as a percent with one number after the decimal.
Jenny W.
1) Using the same testcross above, you generate 1000 progeny. There were 250 of each of the most frequent phenotypes among the progeny (ABc and abC) and 50 of each of the least frequent phenotypes (abc and ABC). What are the map distances between A and B, B and C, and A and C? A – B: 25 m.u.; B – C: 25 m.u.; A – C: 50 m.u. A – B: 30 m.u.; B – C: 30 m.u.; A – C: 60 m.u. A – B: 50 m.u.; B – C: 25 m.u.; A – C: 25 m.u. A – B: 60 m.u.; B – C: 30 m.u.; A – C: 30 m.u. 2) The map distances for genes that are close to each other are more accurate than map distances for genes that are quite far apart because: double crossovers often lead to lethal recombinants that reduce the number of recombinant progeny. interference causes more double crossovers events to occur than would be expected and thus result in a higher number of recombinant progeny. double crossovers between genes that are far apart often lead to nonrecombinant phenotypes and thus underestimate the true map distance. double crossovers occur more often when genes are close to each other and can be readily detected, so these map distances are more accurate. when genes are far apart, single-crossover recombinant classes are more difficult to detect than when genes are close together.
Katlin K.
What aspects separate single-crossover events from double-crossover events? a) Single-crossover events result in one-way displacement of chromosomal content from one chromosome to another, while double-crossover events always reverse this one-way displacement, resulting in chromosomes identical to the pre-crossover chromosomes. b) Single-crossover events occur during mitosis when a cell splits into two cells, while double-crossover events can only occur during meiosis when a cell splits into four cells. c) Single-crossover events affect only the ends of chromosome arms, while double-crossover events can affect segments in the middle of chromosome arms. d) Single-crossover events only affect one arm of each chromosome, while double-crossover events affect two arms of each chromosome.
Adi S.
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