Genetic Mapping | Research & Encyclopedia Articles

Genetic Mapping

Hundreds of scientists are currently working together toward one common goal: to decipher the coded information in the human genome. A genome includes all the DNA within a cell's chromosomes. The human genome consists of an estimated 50,000-100,000 genes, plus a massive amount of DNA of unknown function. This monumental project, launched in 1990, is estimated to cost over three billion dollars and take more than fifteen years to complete. It is by far the largest coordinated effort ever undertaken in the biological sciences. Named the Human Genome Project, this undertaking promises to revolutionize medicine and biology by creating a complete map of the location of genes on all twenty-four human chromosomes.

This has already been achieved for simple viruses with very short DNA sequences. In 1995, Craig Venter and his research team at the Institute for Genomic Research in Maryland announced that they had sequenced the entire genome for the Haemophilus influenzae bacterium. This bacterium has 1.8 million base pairs--the rungs of the DNA double helix--that make up its single circular chromosome.

By 1995, the Human Genome Project had already yielded an impressive amount of information about the human genome as well. Thanks to new research tools emerging from the project, the pace of gene discovery had nearly quadrupled. Examples of important discoveries already made by the project include the gene involved in cystic fibrosis and two genes involved in a hereditary form of colon cancer.

Coordinating the mapping of the entire human genome, with its 3 billion base pairs, requires successive approximations. First, the chromosome set and number of genes are identified. Then mapping begins by establishing which gene is linked to each chromosome, the order of the genes within a specific chromosome, and the distances between the genes on the same chromosome. Sometimes it is also possible to resolve the nucleotide sequence within a particular gene. Some physicians believe that this map will help diagnose, cure, and eventually prevent many diseases caused by faulty genes. Biologists hope to use the map to learn more about life itself. However, some opponents believe that the project's goal will reveal little about the genes' roles and suggest alternative research strategies.

Everyone does agree that this research would not be possible without the earlier contributions of several insightful biochemists. Thomas Hunt Morgan, Alfred H. Sturtevant, Frederick Sanger, and Edwin M. Southern helped develop the foundation for today's research projects.

Nearly one hundred years after Johann Gregor Mendel's work with inherited traits, Thomas Hunt Morgan began studying genetics at Columbia University. He eventually founded what was to become the most important genetic laboratory of his day. Morgan used the fruit fly as a model for his experiments because it was easy to breed and maintain. Each tiny fly produced a new generation every two weeks. Morgan initially planned to use the flies to conduct breeding experiments similar to those Mendel had carried out using pea plants in the 1860s. Morgan noted that the eyes of the fruit fly are either red or white. During his studies of this inherited trait he bred a red-eyed female with a mutant white-eyed male fly. All the offspring had red eyes. This indicated that the gene for white eyes was recessive and was masked by the dominant red-eyed trait.

Surprisingly, when the offspring of the first generation-cross were mated, several white-eyed males appeared. However, among hundreds of flies, no white-eyed females were found. To explore this phenomenon Morgan mated the original white-eyed male with a red-eyed female from the first offspring generation. Their offspring included red- and white-eyed male and female flies. Morgan wondered why the white-eyed flies didn't show up in the first generation. He reasoned that the gene that determines eye color in the fruit fly must be carried on the same chromosome that determines the sex of the fly (the X chromosome). These experiments introduced the concept of sex-linked traits. They also helped to show that genes are located on chromosomes.

Subsequent crossbreeding resulted in additional unexpected results. The ratios of traits such as eye color, wing length, and leg length did not agree with Mendel's earlier work. Morgan proposed that the two alternative forms of a gene, called alleles, must occupy the same location on similar chromosomes. He correctly reasoned that during meiosis, when the chromosomes are copied, some pieces were breaking off and rejoining with the opposite chromosome. This explained the strange variations he observed in a few of the crossbred flies. This phenomenon is known today as crossover.

With the discovery of crossovers, it became clear that genes must be positioned at particular spots, or loci, on the chromosomes. Furthermore, the alleles of any given gene must occupy corresponding loci on similar chromosomes. Morgan and his undergraduate assistant, Alfred Sturtevant, noticed that the genes for different traits were recombining at different rates. As Morgan's experiments had shown, these were fixed and predictable recombinations. It occurred to Sturtevant, however, that the percentage of recombination was probably related to the distance between genes on the same chromosome. Sturtevant postulated that genes are arranged in a linear series on chromosomes, like beads on a string. He also believed that genes located close together are less likely to separate than are genes located far apart. Sturtevant worked to determine the frequencies of recombination and plotted the sequences of the genes along the chromosome, reporting the relative distances between them. In 1913, Sturtevant began constructing chromosome maps using data from fruit fly crossover studies. By 1951, Sturtevant had completely mapped all four chromosomes of the fruit fly.

Several years later, Frederick Sanger was involved in similar work at Cambridge University. He was specifically interested in determining the exact structure of the amino acid chain of protein molecules. In 1945, he discovered a chemical that could attach itself to only one end of an amino acid chain. By attaching this agent to an amino acid and then breaking the chain down with enzymes, Sanger could tell which amino acid had been at the attached end by using a common technique called paper chromatography separation. By 1953, Sanger had found the exact order of the amino acids that comprise the insulin molecule. This paved the way for others to synthesize insulin as a treatment for diabetes. Sanger's long and very distinguished career did not stop here. In 1977, he went on to determine the entire sequence of DNA in a small virus. He is one of only three scientists to ever receive two Nobel Prizes in Chemistry--one for his insulin work and another for his viral work.

In addition to these breakthrough discoveries, a valuable lab tool was developed by Edwin Southern. The technique, known as Southern blotting, makes it possible to identify a DNA fragment by separating a mixture of fragments using electrophoresis, which allows the amino acids to be separated from proteins based on their different rates of migration in an electric field at a controlled pH. Southern then devised a way to transfer the DNA by blotting them on a special nitrocellulose sheet. A radioactive probe was then added to the DNA sequence. This probe is complementary to a specific DNA sequence and binds to the DNA fragment, which can later be visualized. This procedure makes it possible to pick out a specific fragment from a mixture containing millions of other fragments. Similar methods have recently been used to separate RNA and proteins and are whimsically referred to as Northern and Western blotting.

The knowledge that emerged from these studies has been characterized as the product of the "golden age" of genetics. Contemporary scientists are applying this knowledge to the human genome and other related projects in an attempt to better understand the mechanisms of life.