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%