Gene pairs that are close together on the same chromosome are genetically linked because they are transmitted together more often than not. The hallmark of linkage is that among the progeny of double heterozygotes produced from testcrosses, the number of parental types is greater than the number of recombinant types.
The recombination frequencies of pairs of genes indicate how often two genes are transmitted together. For linked genes, the recombination frequency is less than 50%.
Gene pairs that assort independently exhibit a recombination frequency of 50%. This is because the number of parental types equals the number of recombinants. Genes may assort independently either because they are on different chromosomes or because they are far apart on the same chromosome.
Statistical analysis helps determine whether or not two genes assort independently. The probability value (p) calculated by the chi square test measures the likelihood that a particular set of data supports the null hypothesis of independent assortment, or no linkage. The lower the p value, the less likely is the null hypothesis, and the more likely the linkage. The chi square test can also be used to determine how well the outcomes of crosses fit other genetic hypotheses.
The greater the physical distance between linked genes, the higher the recombination frequency. However, recombination frequencies become more and more inaccurate as the distance between genes increases.
Recombination occurs because chromatids of homologous chromosomes exchange parts, that is, cross over, during the prophase of meiosis I, after the chromosomes have replicated.
Genetic maps are a visual representation of relative recombination frequencies. The greater the density of genes on the map (and thus the smaller the distance between the genes), the more accurate and useful the map becomes in predicting inheritance.
Organisms that retain all the products of one meiosis within an ascus reveal the relation between genetic recombination and the segregation of chromosomes during the two meiotic divisions. Organisms like Neurospora that produce ordered octads make it possible to locate a chromosome's centromere on the genetic map.
In diploid organisms heterozygous for two alleles of a gene, rare mitotic recombination between the gene and its centromere can produce genetic mosaics in which some cells are homozygous for one allele or the other.
