Chromosome 4

Bent wing /Cubitus veins Vx~Shaven hairs

Grooveless scutellum "Eyeless

~~ Minute bristles

I 7.15 Drosophila melanogaster has four linkage groups corresponding to its four pairs of chromosomes. Distances between genes within a linkage group are in map distances.

together is the multiplication of their independent probabilities. Applying this principle, we should find that the proportion (probability) of gametes with double crossovers between st and e is equal to the probability of recombination between st and ss, multiplied by the probability of recombination between ss and e, or 0.146 X 0.122 = 0.0178. Multiplying this probability by the total number of progeny gives us the expected number of double-crossover progeny from the cross: 0.0178 X 755 = 13.4. Only 8 double crossovers — considerably fewer than the 13 expected — were observed in the progeny of the cross (see Figure 7.13).

This phenomenon is common in eukaryotic organisms. The calculation assumes that each crossover event is independent and that the occurrence of one crossover does not influence the occurrence of another. But crossovers are frequently not independent events: the occurrence of one tends to inhibit additional crossovers in the same region of the chromosome, and so double crossovers are less frequent than expected.

The degree to which one crossover interferes with additional crossovers in the same region is termed the interference. To calculate the interference, we first determine the coefficient of coincidence, which is the ratio of observed double crossovers to expected double crossovers:

coefficient of coincidence =

number of observed double crossovers number of expected double crossovers

For the loci that we mapped on the third chromosome of D. melanogaster (see Figure 7.14), we find that:

coefficient of coincidence

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