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 crossing- over The rate of at which linked genes are separated can be used to produce a “map” of distances between genes Alfred Sturtevant

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

M N M N Linked GenesUnlinked Genes