Presentation on theme: "April 2008 Mendelian Genetics Gene Linkage & Polyploidy April 9, 2008 Mr. Bromwell."— Presentation transcript:
April 2008 Mendelian Genetics Gene Linkage & Polyploidy April 9, 2008 Mr. Bromwell
Genetic Recombination new combination of genes produced by crossing over and independent assortment – possible combinations of genes in a gamete due to independent assortment is 2 n – In humans 2 n equals 2 23 – When male and female gametes fertilize, the possible offspring combinations are 2 23 x 2 23, or more than 70 trillion
Gene Linkage Question: What about genes on the same chromosome? Can they “assort independently?” Answer: Usually not. Genes that are linked together on the same chromosome usually travel together into the same gamete.
The Discovery of Gene Linkage Hypothesized by William Bateson & R.C. Punnett in the early 1990s. Studied the inheritance of flower color and pollen grain shape in the sweet pea. In the F 2 generation, the phenotypes obtained were very different from Mendel’s predictions. (see below) Bateson & Punnett’s Results PhenotypeObservedExpected based on Mendel’s 9:3:3:1 ratio Purple flower, long pollen48313911 Purple, round3901303 Red, long3931303 Red, round1338435 TOTAL6952
The Discovery of Gene Linkage In 1915, Thomas Hunt Morgan put forth an explanation of Bateson & Punnett’s observation. Worked with fruit flies, Drosophila melanogaster. Suggested that genes that do not assort independently are located on the same homologous chromosome and therefore travel together into gametes. Thus in the F 2 generation, the phenotypes obtained should be more similar to the parent’s genotypes than to follow Mendel’s predictions.
However, as Bateson & Punnett and Morgan both observed, linked genes do not always travel together during meiosis. They concluded that linked genes can separate during prophase I by crossing over. The Exception to the Exception!
Chromosome Mapping Alfred H. Sturtevant hypothesized that how often two linked genes recombined (separated from one another during crossing over) could be used to calculate the distance between them on a chromosome. – The more often they recombine, the farther apart they must be on the chromosome. – In other words, if genes are far apart on a chromosome, crossing over is more likely to occur between them, then if the genes were close together. – Permitted “maps” to be made of known genes in organisms. First done with fruit flies (Sturtevant was a student of Morgan’s – go figure!)
Chromosome Mapping Problem: Construct a chromosome map for the following genes: D and F crossover 15% of the time. G and D – 2% E and G – 8% E and D – 10% G and F – 17% E and F – 25% Place the genes in the proper order. Solution: 15% 2%8% 10% 17%
Challenge Problem Gene pair Crossover frequency Gene Pair Crossover frequency AB5.5BF4.3 AC6.4CD10.9 AD4.5CE2.6 AE9.0CF5.2 AF1.2DE13.5 BC0.9DF5.7 BD10.0EF7.8 BE3.5
occurrence of one or more extra sets of all chromosomes in an organism. – Rarely occurs in animals (always lethal in humans) – Many flowering plants are polyploid – usually imparts increased health and size Polyploidy Strawberries (8n) Coffee (4n)
References Biggs, Alton, et. al. Biology. New York: The McGraw Hill Companies, Inc., 2007. Griffiths, Anthony J.F., Miller, J.H., Suzuki, D.T., Lewontin, R. C., & Gelbart, W.M. "The discovery of linkage." An Introduction to Genetic Analysis. 2000. W. H. Freeman and Company. 8 Apr 2008. Nobel Lectures, Physiology or Medicine 1922-1941, Elsevier Publishing Company, Amsterdam, 1965. "Thomas H. Morgan: The Nobel prize in Physiology or Medicine 1933." Nobelprize.org. 2008. The Nobel Foundation. 8 Apr 2008.