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Queen's University
BIOL 205
Ian D Chin- Sang

BIOL205:week 4, lecture 10 Genes on the same chromosome: Linkage Mapping by recombinant frequency Recombination • independent assortment of genes on different chromosomes at meiosis is one way to “shuffle” allele combinations. • How do we detect recombination? • compare the “inputs” into meiosis with the “outputs” • in humans the “inputs” are sperm and egg from mother and father and the “outputs” are the person’s eggs or sperm • any meiotic product that has a new combination of the alleles provided by the two input genotypes is by definition a recombinant In diploids, recombinants are best detected in a testcross Independent assortment produces 50 percent recombinants But wait a second… • chromosomal theory of inheritance suggested that genes are on chromosomes • remember Mendel did not know what genes were • for him, chromosomes and genes were the same thing: “factors”. • but what if Mendel decided to study more than 7 traits? • the pea plant has 7 pairs of chromosomes. • eventually he would have a situation where 2 genes would be on the same chromosome. Who was correct? Mendel? Chromosomal Theory? Footnote: • although all of Mendel’s 7 genes showed independent assortment, several were on similar chromosomes (three on chromosome 4 and two on chromosome 1). • however, the distance separating the genes were far apart so it behaved as if they were on separate chromosomes. Dihybrid (double heterozygous) test cross: If the two genes were on the same chromosome Was Mendel correct? • early 1900s: William Bateson + R.C. Punnett studying inheritance of two factors (genes) in peas • they carried out standard F1 Dihybrid self crosses but it did not show the 9::3:3:1 ratio as predicted by the principle of independent assortment • and suggested that the factors were coupled • confirmation of Bateson and Punnett’s hypothesis had to await the development of Drosophila as a genetic tool • Thomas Hunt Morgan found a similar deviation from Mendel's Second Law while studying two autosomal recessive genes in Drosophila • Morgan proposed linkage as a hypothesis to explain phenomenon of apparent allele association Arecombination-based map of one of the chromosomes of Drosophila → • recombination maps of chromosomes are usually assembled two or three genes at a time • when geneticists say 2 genes linked, they mean the loci of those genes are on same chromosome • hence, the alleles on any one homolog are physically joined (linked) by DNAbetween them • the way in which early geneticists deduced linkage is a useful means of introducing most of the key ideas and procedures in the analysis Why make genetic maps? • gene position is crucial information needed to build complex genotypes required for experimental purposes or for commercial applications • knowing position occupied by a gene provides a way of zeroing in on its structure and function • a gene's position can be used to define it at the DNAlevel • in turn, the DNAsequence of a wild-type gene or its mutant allele is a necessary part of deducing its underlying function • the gene arrangement on chromosomes are often slightly different in related species • for example, the rather long human chromosome number 2 is split into two shorter chromosomes in the great apes • hence, chromosome maps are useful in interpreting mechanisms of evolution • chimpanzees (closest living relative to humans) have 48 chromosomes, humans have 46 • either lost one, or they fused together to form one • more likely fused, don't lost → important Evolution and Human Chromosome 2 Iffused, would predict find telomere sequences
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