The Human Genome Project – discovering the human blueprint
Box 2 | Gene mapping and DNA sequencing
The position occupied by a gene on a chromosome is its locus (plural loci), which is like its address. Although it is usual to talk about gene mapping, in many cases it is the loci of the genes that are being mapped. For all of the human gene loci, about a third are variable – that is, the genes on such loci can exist in different forms or variants. For example, the locus for the gene that controls ear lobe shape may have on it the arrangement of DNA that produces attached ear lobes, or the arrangement of DNA that produces free ear lobes. Variations on the same gene locus are called alleles. While humans share identical genes, individuals differ in the specific alleles they possess. So the Human Genome Project is identifying the loci for all human genes and some of the alleles for those genes that have them.
Where loci for different genes are on the same chromosome they are said to be linked because usually they will all be passed on together to the offspring. However, sometimes chromosomes can break and re-join, and the genes that are usually linked become separated. The chance of separation of linked genes depends on how close together the loci of the genes are. If they are close together crossing over between them is rare. The further apart they are the greater is the chance of crossing over happening between them. Therefore the frequency of crossing over can be used as a measure of distance between genes. From these kinds of data it is possible to produce a genetic linkage map of chromosomes.
Scientists are now producing other kinds of gene maps. After cloning large amounts of DNA from the genome, they use markers to connect the cloned pieces correctly. This information is then used to produce a physical map. The object of physical mapping is to create complete sets of cloned DNA pieces that span every region of a chromosome and even the whole human genome.
DNA sequence maps are also a kind of gene map. Knowing the sequence of bases – the chemical building blocks that make up the DNA strand – is important because it tells scientists what kind of genetic information the DNA carries in that particular segment. Scientists also use sequence information to sort out which stretches of DNA contain genes and to analyse genes for changes in sequence that may cause disease.
Box
Box 1. Genes – the basic facts
Related sites
Mapping the DNA 1
(Great moments in science, ABC Online, Australia)
Mapping the DNA 2
(Great moments in science, ABC Online, Australia)
Reading the book of life (Explore, Scientific American, USA)
Beyond biology: Instrumentation and informatics (Oak Ridge National Laboratory, USA)
Mapping and sequencing the human genome (Primer on Molecular Genetics, Department of Energy, USA)
Page updated September 2004.