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3. Gene Linkage and Genetic Mapping.pdf

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McGill University
Biology (Sci)
BIOL 202
Tamara Western

Gene Linkage and Genetic Mapping: A locus is a small part of the genome which is responsible for a certain phenotype, or contributes to a phenotype  Within these loci can be multiple alleles, like we have seen before; however no matter which allele is there, the locus is always in the same place o Some alleles can be on both sides of the gene, since the DNA is double stranded, which improves efficiency  It is important to know the approximate physical location of the genes to figure out their function, and also how they are interacting with other nearby loci o These interactions can involve recombination, which in theory happens 50% of the time in independent assortment  Huntington’s disease is a mutation in which the individuals suffering have too many copies of a trinucleotide repeats 5’-CAG-3’ o This was determines through genetic mapping of the disease’s locus on chromosome 4 There are cases in which free recombination is not observed; in these cases, two different loci are often linked to one another because they are located on the same chromosome, probably close together, and normally segregate together as a block. Therefore, the frequency of recombination is usually much less than 50%  For example, take the following 2 loci on the fruit fly genome: o pr+ or pr code for red or purple eyes respectively o vg+ or vg codes normal of vestigial (short) wings respectively  These 2 genes tend to move as a block, and therefore the frequency of recombination is quite low o By crossing a completely recessive fly with a wild type fly will produce an F1 heterozygote, after a testcross, the resulting offspring will not be in a 1:1:1:1 ratio o The parental offspring populations were 1333 and 1195, while the recombinant populations will only be 151 and 154  From this information, we can tell that the genes are not segregating independently from one another o Crossing over is known to happen at the 4-chromosome stage; if it happened at the two chromosome stage, then all of the progeny would be recombinants, which never happens o Physical evidence has actually been seen microscopically of crossing over occurring during meiosis One important rule in genetic mapping is that longer regions between the genes of interest have a higher frequency of recombination  In fact, the distance between loci are measures in map units, which have been designed so that the number of map units between two loci is equal to the frequency of recombination  If looking at more than two loci at once, which is often the case, the map units are generally additive o Looking at the diagram on the right, if A and B are 5 m.u. apart, and A and C are 3 m.u. apart, then B and C are approximately 8 m.u apart  This is not completely accurate because of double recombinants, which we will look at later Again, linkage analysis is not only limited to two interacting loci; take for example the following three-point testcross involving three different loci  v gives vermillion eyes, cv gives cross-veinless wings and ct gives cut wing phenotypes in comparison with the wild-type  Two parents a crossed to produce a completely heterozygous F1 generation, and this F1 female is then testcrossed  The result can be seen in the table to the right; the recombinant frequencies are quite low, and their frequency, and therefore distance apart from each other, can be measured o There are the possible recombinant types, because there are three different loci; between v and cv, v and ct, and cv and ct  Each one is counted in the table o Therefore, for example, the v and cv recombinants by the total population will give the frequency of recombination of these alleles, approx. 18.5%  Likewise, v and ct = 13.2% and cv and ct = 6.4% o Note that the 13.2% and 6.4% roughly add together to produce the 18.5%, which means that ct is most likely located between
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