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VOCABULARY: •Gene map KEY CONCEPT: •What structures actually assort independently? Remember • Genes are located on chromosomes • Independent assortment—genes for different traits can segregate independently during the formation of gametes The Big Question… • It may be easy to see that genes located on different chromosomes assort independently, but what about genes located on the same chromosome? Thomas Morgan’s Research • Morgan identified more than 50 genes on Drosophila’s 4 pairs of chromosomes. • He discovered that many seemed to be “linked” together – They are almost always inherited together & only rarely become separated • Grouped genes into 4 linkage groups Morgan’s Conclusion • Each chromosome is actually a group of linked genes • BUT Mendel’s principle of independent assortment still holds true • It is the chromosome that assorts independently!! – Mendel missed this because 6 of the 7 traits he studied were on different chromosomes. Gene Linkage • Linked genes: genes located on the same chromosome that tend to be inherited together – Chromosomes assort independently, NOT individual genes • BUT…If 2 genes are found on the same chromosome are they linked forever? – NO!! Crossing Over • Crossing over is random • Crossing over during Meiosis (Prophase I) can separate genes • Crossing over sometimes separates linked genes and therefore there is a new combination of alleles! • CROSSING OVER increases genetic diversity by producing new combinations of alleles Gene Maps • Alfred Sturtevant was a graduate student working in Morgan’s lab part-time in 1911 • He hypothesized: the farther apart 2 genes are on a chromosome the more likely they are to be separated by crossingover • The rate of at which linked genes are separated can be used to produce a “map” of distances between genes Alfred Sturtevant 1891-1970 Gene Maps…in other words • Knowing how often crossing over occurs between genes allows us to map positions of genes on chromosomes • Yes, crossing over is random…BUT the distance between two genes determines how often crossing over occurs • Frequency of crossing over: how often crossing over occurs between 2 genes. – If two genes are close together, they have a LOW cross over frequency – Knowing this helps estimate the positions of genes on chromosomes. Gene Maps • This map shows the relative locations of each known gene on a chromosome – We have used this technique to help construct maps of the Human Genome Review…look in your notes… • If 2 genes are on the same chromosome but usually assort independently, what does that tell you about how close together they are? And…try this • Draw M and N as unlinked genes • Draw M and N as linked genes Linked Genes M N Unlinked Genes M N