Linkage and Gene Mapping

Mendel’s Laws: Chromosomes Locus = physical location of a gene on a chromosome Homologous pairs of chromosomes often contain alternative forms of a given gene = alleles Different alleles of the same gene segregate at meiosis I Alleles of different genes assort independently in gametes Genes on the same chromosome exhibit linkage: inherited together Linked genes are not always inherited together

Gene Mapping Gene mapping methods use recombination frequencies between alleles in order to determine the relative distances between them Recombination frequencies between genes are inversely proportional to their distance apart Distance measurement: 1 map unit = 1 percent recombination (true for short distances)

Genes with recombination frequencies less than 50 percent are on the same chromosome = linked) Linkage group = all known genes on a chromosome Two genes that undergo independent assortment have recombination frequency of 50 percent and are located on nonhomologous chromosomes or far apart on the same chromosome = unlinked

The closer together two genes are, the more likely they will be inherited together. Cross-over frequencies are related to distances between genes. The higher the frequency, the further the genes are apart Linkage maps show the relative locations of genes.

Recombination Recombination frequency is specific for a particular pair of genes Recombination frequency increases with increasing distances between genes No matter how far apart two genes may be, the maximum frequency of recombination between any two genes is 50 percent.

Gene Mapping Recombination results from crossing-over between linked alleles. Recombination changes the allelic arrangement on homologous chromosomes

Gene Mapping The map distance (cM) between two genes equals one half the average number of crossovers in that region per meiotic cell The recombination frequency between two genes indicates how much recombination is actually observed in a particular experiment; it is a measure of recombination Over an interval so short that multiple crossovers are precluded (~ 10 percent recombination or less), the map distance equals the recombination frequency because all crossovers result in recombinant gametes. Genetic map = linkage map = chromosome map

Cross-over frequencies can be converted into map units. Ex: A 5% cross-over frequency equals 5 map units. gene A and gene B cross over 6.0 percent of the time gene B and gene C cross over 12.5 percent of the time gene A and gene C cross over 18.5 percent of the time

Example Draw a linkage map based on the following cross over percentages: A – B = 8% B – C = 10% A – C = 2%