Genetics often entails “detective work.” Detectives spend a lot of effort locating suspects, and geneticists spend a lot of effort locating genes. Finding a gene and putting it on a map has several applications:
To determine that a genetic factor maps to a point on a chromosome (Did one character do the deed, or is somebody in cahoots?)
To aid in further study of a gene
To help diagnose a human genetic disorder
This chapter tells about genetic, chromosomal, and physical maps. Genetic maps are based on genetic linkage. Chromosome maps place genes relative to chromosome landmarks. Physical maps are based on DNA sequencing.
Recombination and Chromosome Exchanges
General recombination occurs when paired homologous chromosomes undergo exchanges, or crossovers. Homologous chromosomes recombine in pachynema, the third stage of prophase I of meiosis. General recombination rarely happens in cells undergoing mitotic cycles, but that is not taken up here.
At any chromosome locus (location; a gene's address), a crossover is a random event – random in the sense that it is unpredictable and not predetermined. The farther apart two loci, the greater the chance for an exchange between them. Therefore, the probability of an exchange is positively correlated with distance. The probability of recovering a recombinant chromosome, r, measures genetic distance; express r as a percentage. One map unit separates two linked genes if 1% of the recovered chromosomes are recombinant for these two genes.