Zoo 317 Heredity, Evolution and Society
|Lecture 9||Cummings 4: pp 100-103|
II. Genetic distance.
III. Physical maps.
Fig. 4.30: The caption states that gene symbols are in red and symbols for other markers are in blue. However, both are in black.
I. A genetic map is a representation of the genes on a chromosome arrayed in linear order w ith distances between loci expressed as percent recombination (map units, centimorgans). Also called a linkage map.
2. Groups of genes that are widely separated on a chromosome may show independent assortment; however, all such groups can eventually be tied together by mapping additional loci between them.
B. The farther apart two loci are, the more likely that a crossover will occur between them. Conversely, if two loci are close together, a crossover is less likely to occur between them.
C. Recombination can only be detected between two loci, both of which are heterozygous.
2. The most useful systems involve codominant alleles.
3. Efficient mapping requires polymorphic loci, i.e. loci with two or more common alleles. Loci that have a single common allele are described as monomorphic.
4. Any variations in DNA, whether in coding regions of genes or in noncoding regions, can be used as genetic markers, i.e. as a label for a particular point on a chromosome.
2. Odd numbers of crossovers create recombinant allelic combinations and are counted as one crossover.
3. A recombination rate of 50% corresponds to independent assortment. Therefore, only distances less than 50 map units can be measured directly. Greater distances can be constructed by adding up distances between closer loci.
F. If two loci are linked, the alleles that are on the same chromosome are described as coupled; alleles on opposite homologous chromosomes are in repulsion.
B. Among the techniques for constructing physical maps are the following:
2. Fluorescent or radioactive probes that bind to a particular gene can be observed microscopically and can be used to localize the gene on a metaphase spread.
3. Chromosomes from cells in metaphase can be sorted with high-speed electronic sorters. One can make preparations of a particular chromosome. If a particular gene can be shown to be in the preparation, it must be located on that chromosome.
B. It is useful in estimating genetic risk, e.g. if a gene cannot be tested directly, then variation at a closely linked locus may indicate the presence or absence of a detrimental allele.
C. A major goal of the Human Genome Project is the mapping of all human genes (as well as those of mice, Drosophila, Caenorabditis elegans (a nematode), Arabidopsis thaliana (a small plant), yeast, and the bacterium Escherichia coli. As of 1999, yeast, E. coli, C. elegans,and about a dozen other bacteria have been completely sequenced and all their genes identified, although the functions of most are unknown. Major progress has been made in mapping human genes, and a "rough draft" of the human genome is anticipated by 2000. Understanding of function of the many newly discovered human genes is being greatly aided by the studies of yeast, which has many genes similar to those of humans.
|genetic map||physical map||syntenic|
|crossing over||genetic distance||recombination|
|linkage||linkage group||map unit|