Question

Which of the following statements is false?

		The higher the coefficient of coincidence, the lower the degree of interference.
		Recombination mapping experiments become less accurate as the distance between genes increases.
		Double crossovers result in an overestimate of gene distances.
		Positive interference occurs when a crossover in one region of a chromosome interferes with crossovers in nearby regions
		A linkage map is usually shorter than the physical locations of genes on a chromosome.

Answer 1

The cofficient of coincidence is the measure of interference in the formation of crossovers during meiosis, whereas the phenomenon of reduction of frequency of a crossover in the vicinity of other crossover is called interference. Now coefficient of coincidence is obtained by dividing percentage actual double crossover by percentage expected double crossover. The sum of coefficient of coincidence and interference is one. So, higher the coefficient of coincidence lower will be the degree of interference.

Crossing over is directly proportional to the distance between two genes.The recombination frequency depends upon the distance between two genes, larger the distance between the genes higher is the recombination frequency. These recombination frequencies is utilized in predicting the sequence of genes on the chromosome which is known as genetic or recombination map or linkage map. Therefore the accuracy of recombination mapping experiments is directly related to recombination frequency which in turn is related to distance between genes, although the directly measured recombination frequency between two genes does not give the most accurate measure of map distance. So there is an ambiguity in the statement.

Double crossover is the two separate crossovers between two genes. This crossover actually put the original two genes back on the same chromosome. They are not counted in the directly measured recombination frequency, resulting a slight underestimate of the actual number of recombinants, so it does not result in overestimate of gene distances. The statement is false.

The reduction in the crossover of one region of a chromosome in the vicinity of a nearby crossover is called positive interference. The statement is true.

A linkage map is the mapping of genes on a chromosome based on linkage analysis. It does not show physical distance between genes but rather theur relative positions. Recombination frequency is not a direct measure of the physical distance between two genes on the chromosome. So a linkage map is usually shorter than the physical distance of genes on chromosome.