Presentation on theme: "KEY CONCEPT Genes can be mapped to specific locations on chromosomes."— Presentation transcript:
1KEY CONCEPT Genes can be mapped to specific locations on chromosomes.
2Gene linkage was discovered by deviations from Mendel’s rules Mendel crossed pea plants with different discrete traitsAlleles assort independently with 9:3:3:1 ratio
3R.C. Punnett and William Bateson Developed Punnett squares. Crazy chicken breeder.Found that rates often did not follow 9:3:3:1 proportionsTherefore, some genes are linkedCream Legbar chicken.Originally bred by Punnett
4Gene linkage was explained using fruit flies. Thomas Hunt Morgan studied fruit fliesFound some traits were inherited togetherLinked traits fell into four groupsDetermined fruit flies have four pairs of chromosomesChromosomes, not genes, assort independently during meiosis.Wild typeMutant
5Linked genes are not inherited together every time. Chromosomes exchange homologous genes during meiosis.Genes that are closer together will more often cross over together.
6Alfred Sturtevant (student of Morgan) Hypothesis:Frequency of genes crossing-over together depends on distance between the genes on the chromosomesExperiments:Found 3 linked traits in fruit flies: body color, eye color, wing size.Recorded how many times the genes crossed over together. Number out of 100 (i.e. percent)
7Alfred Sturtevant (student of Morgan) Conclusions:Determined rates of crossover between each of the three genes.Linkage Maps show relative distance between genes on a chromosomeIf you know crossover rates of three or more genes, you can determine their relative locations.
8Linkage maps estimate distances between genes. The closer together two genes are, the more likely they will be inherited together.Linkage maps show the relative locations of genes.
9Cross-over frequencies can be converted into map units. gene A and gene B cross over 6.0 percent of the timegene B and gene C cross over 12.5 percent of the timegene A and gene C cross over 18.5 percent of the time