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#crossover

2 public questions tagged with this topic.

Linkage and Mapping

Complete interference means

Complete interference describes extreme inhibition where one crossover totally prevents second crossover in adjacent interval, so observed double crossover count zero in genetic data. Consequently coefficient coincidence C = 0 / expected = 0 and interference I =1-0 =1 indicating 100 percent interference. Biologically ensures each bivalent obtains at least one obligate chiasma but not excessive nearby exchanges that might cause nondisjunction or chromosome entanglement. Phenomenon common over short intervals less than about 10 centimorgans in Drosophila, mammals, enhancing proper segregation fidelity.

Ref: Hartl & Ruvolo, Genetics, 6th ed., Chapter 5: Complete Interference Zero DCO Coefficient Zero

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Linkage and Mapping

Double crossovers are identified as

Double crossover requires two separate exchange events within short region between three genes. Probability product of two single crossover probabilities makes event relatively uncommon, further suppressed by positive interference that inhibits nearby second exchanges via chromosome axis signaling. Consequently progeny formed by double recombination represent smallest numeric class among testcross offspring, whereas parental nonrecombinant class most frequent and single crossovers intermediate frequency. Recognizing least frequent class as double crossover is key step in mapping because it identifies middle gene and allows interference measurement.

Ref: Klug et al., Concepts of Genetics, 12th ed., Chapter 5: Least Frequent Class Represents Double Crossovers

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