Conclusion

Three conditions should ideally be met for a successful mapping cross of three linked genes:

I. The genotype of the organism producing the crossover gametes must be heterozygous at all loci under consideration.

II. The cross must be constructed such that the genotypes of all gametes can be determined directly from observation of the offspring phenotypes.

III. Enough offspring should be produced in the mapping experiment to recover a representative sample of all crossover classes.

Gametes derived from the heterozygous female parent can be determined by examining the phenotypes of the progeny because gametes from the male parent contain all recessive alleles. When a crossover occurs in the female parent, it creates reciprocal crossover classes (both crossover chromosomes appear in the progeny).

In a three-point mapping cross, generally the events corresponding to noncrossover are most numerous and the double crossover events are least numerous. This allows these categories to be identified from the data.

The order of the three genes in a three-point mapping is generally unknown. It may be determined by locating the reciprocal noncrossover phenotypes and considering the double crossover phenotypes that would result from the three independent possibilities for the gene order. Only one order will yield the double crossover phenotypes that are actually observed.

The map distance between any two gene loci is equal to the percentage of all detectable exchanges occurring between them. This will include all appropriate single crossovers as well as double crossovers.

If crossovers occur independently, the probability of a double crossover is equal to the product of the probabilities for corresponding single crossovers. The coefficient of coincidence is the ratio of the observed to expected double crossovers. Interference is one minus the coefficient of coincidence. Thus, these quantities measure the degree to which a crossover inhibits the chance of a second crossover in its vicinity.

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