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Genes in Chromosome and Genetic Mapping Thomas Hunt Morgan Birthday : 1866 Death : 1945 Nationality : American Achievement : Winner of the 1933 Nobel Prize in Medicine Thomas Hunt Morgan was the first to associate a specific gene with a specific chromosome. Finished the first genetic mapping Where are genes located? Since T. H. Morgan it is known to be on chromosomes, at least in Drosophila (light microscopy) Assigning genes to individual chromosomes - chromosomal aneuploidy - sex-linked traits (white-eyed in Drosophila) - somatic cell hybrids – selective human chromosome retention in mouse/human hybrids (TK gene on chrom. 17) - G-banding / deletion mapping (DMD-gene) Double helix cell Chromosome DNA strand a gene nucleus chromatid Morgan traced a gene to a specific chromosome •Morgan choice an experimental animal, Drosophila melanogaster •fruit fly species that eats fungi on fruit •prolific breeders •generation time of two weeks. •Fruit flies have three pairs of autosomes and a pair of sex chromosomes (XX in females, XY in males). 3+1 •easier to see mutation. •displayed a clear genetic variation Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings The normal character phenotype is the wild type (red). Alternative traits are mutant phenotypes. female male Fig. 15.2 Genes located on a sex chromosome are called sex-linked genes. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Linked genes tend to be inherited together because they are located on the same chromosome Results of crosses with linked genes deviate from those expected according to independent assortment. • Morgan reasoned that body color and wing shape are usually inherited together because their genes are on the same chromosome. (b+vg+ Or b vg+) (b vg) Fig. 15.4 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Geneticists can use recombination data to map a chromosome’s genetic loci One of Morgan’s students, Alfred Sturtevant, used crossing over of linked genes to develop a method for constructing a chromosome map. This map is an ordered list of the genetic loci along a particular chromosome. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Sturtevant hypothesized : that the frequency of recombinant offspring reflected the distances between genes on a chromosome. The farther apart two genes are, the higher the probability that a crossover will occur between them and therefore a higher recombination frequency. Sturtevant used recombination frequencies from fruit fly crosses to map the relative position of genes along chromosomes, a linkage map. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Using recombination frequencies to construct a genetic map One map unit (sometimes called a centimorgan) is equivalent to a 1% recombination frequency. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 15.6 Some genes on a chromosome are so far apart, the frequency of recombination reaches is its maximum value of 50% and the genes act as if found on separate chromosomes and are inherited independently. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings More than one gene can affect a given phenotypic characteristic (eye color) Partial genetic map Fig. 15.7 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Map units (linkage map) indicate relative distance and order, not precise locations of genes. The frequency of crossing over is not actually uniform over the length of a chromosome. •Combined with other methods like chromosomal banding, geneticists can develop cytological maps. the positions of genes with respect to chromosomal features. •More recent techniques show the absolute distances between gene loci in DNA nucleotides. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings VARIATIONS IN RECOMBINATION FRACTIONS The frequency of recombination is not absolute but depends on (i) sex of gametes (is higher during oogenesis) VARIATIONS IN RECOMBINATION FRACTIONS The frequency of recombination is also dependent on (ii) relative position along the chromosom (is higher near chromosome ends)