Also known as cytogenetic mapping or genetic mapping, chromosome mapping is a technique used to locate particular characteristics (coded for by specific genes) on specific chromosomes and regions within those chromosomes. By knowing the location of genes on a chromosome, predictive statements can be made about the inheritance of their characteristics. Initially, the maps of chromosomes were produced by looking at the inheritance of mutant conditions. Presently, a variety of techniques, ranging from physical observation of chromosomes to breeding experiments where the relative frequencies of specific offspring are considered, are used to map chromosomes.
If two characteristics are considered (and hence their controlling genes), both of which are present in one parent, then the frequency in which they occur together in the offspring is used in constructing the map. For example, Gregor Johann Mendel studied the flower color and plant height of peas and found that all possible combinations were in existence. Particular heights were found just as frequently with white flowers as with other colored flowers; similarly, dwarf plants occurred just as frequently with the two flower types. Mendel concluded that the forms of the two genes were transmitted from parent to offspring independently of each other. Named the Law of Independent Assortment by Mendel and considered the most basic form of chromosome mapping, it shows that the genes are on two separate chromosomes.
When genes occur together more frequently than would be expected, they are linked. Present on the same chromosome and members of the same linkage group, the closer the linked genes are to each other on the chromosome the more likely they are to be transmitted together, because there is less likely to be a cross over event between them during meiosis.
High resolution can be achieved on chromosome maps by using three, rather than two, linked genes. This allows a double crossover between two linked genes (which would give a false frequency) to be overcome. A double crossover is more likely the further apart the genes are on the chromosome. The more breeding experiments that are conducted, the greater the detail of the genetic map. As a consequence of such experiments, detailed maps exist for the classic organisms of genetics such as the fruit fly Drosophila melanogaster and the ascomycete fungus Neurospora crassa. For practical reasons, this method of chromosome mapping is not used for human beings. The majority of loci (positions of genes on chromosomes) that are known for humans are based on mutant alleles (alternative forms of a gene), which produce a particular disease or syndrome. Most mapping of non-mutant forms has developed from the studies of extended families.
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