Gene Linkage and Genetic Mapping 4 Gene Linkage and Genetic 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

Gene linkage was explained through fruit flies. Morgan found that linked traits are on the same chromosome. Chromosomes, not genes, assort independently during meiosis. Wild type Mutant

If two genes are on different chromosomes…

If two genes are on different chromosomes… Half look like they got a set of the parents’ chromosomes… And half look like they got a mix of both parents’ chromosomes…

Now suppose both gene A and B were next to each other on the same chromosome. Remember Prophase I of Meiosis? CROSSING OVER! What happens to the ratios in this diagram?

Homologous chromosomes exchange homologous genes during meiosis.

Gene Mapping Recombination results from crossing-over between linked alleles. Recombination changes how alleles are arranged on homologous chromosomes Fig. 4.4

KEY CONCEPT Genes can be mapped to specific locations on chromosomes.

Gene Mapping Gene mapping determines the order of genes and the relative distances between them in map units 1 map unit = 1 cM (centimorgan) 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)

Linkage maps estimate distances between genes. 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.

Fig. 4.6

Gene Mapping 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 Fig. 4.7

LINKED or NON-LINKED? A recombination frequency of 50% means that genes are unlinked. There are two ways in which genes maybe unlinked: They may be on separate chromosomes. They may be far apart on the same chromosome.

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.

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

Draw a linkage map based on the following cross over percentages: A – B = 8% B – C = 10% A – C = 2% 8.0 2.0 B A C 8.0 + 2.0 = 10.0

Mapping the distance between two genes Starting with pure breeding lines, Cross Parent 1(AA BB) with Parent 2(aa bb) So Parental chromosomes in the F1 have to be AB and ab Now cross (AB ab) F1 progeny with (ab ab) tester to look for recombination on these chromosomes. Suppose you Get…… AB ab 583 <parental ab ab 597 <parental Ab ab 134 <recombinant aB ab 134 <recombinant total= 1448 so…. 268 recombinants /1448 progeny = 0.185 recombinants/progeny= 18.5% recombinants= 18.5 mu