Retinoblastoma is a human eye tumor that has been shown to involve a mutation in a tumor suppressor gene called Rb-1. Deletion of this gene is a condition which is inherited in some families. When combined with a mutation in the gene, the tumor may develop. Increasing numbers of genes related to cancer are being identified.
Transposable elements are pieces of DNA which can move from one region of a chromosome to another location on the same or a different chromosome. They were first discovered by Barbara McClintock, working with maize. However, for many years their existence was not widely accepted. Subsequently, they have been found in many other organisms, including bacteria, fungi, other plants, and animals. The ends of these elements usually have repeat sequences (both direct and inverted) associated with them. Elements may contain a gene for an enzyme called a transposase, which recognizes the repeats on the ends and is involved in movement of the elements. Some elements lack this gene and rely on a gene located at a separate location in order to move. Insertion of transposable elements into genes can inactivate the gene. The existence of transposable elements is one part of the solution to the problem of extra DNA in higher organisms. The idea that these elements are "selfish DNA" which exist to copy themselves and not for the benefit of the organism has been proposed.
Bacteria and the viruses which infect bacteria (termed phage) were important genetic research organisms even before the structure of the DNA molecule was determined. They have many advantages for research, including being haploid, having short generation times, and being easily raised in very large numbers. Three types of mutations which have been studied in bacteria include antibiotic resistance/ sensitivity, ability or lack of ability to synthesize a component (such as an amino acid), and ability or lack of ability to use particular carbon sources for energy. Bacteria termed prototrophs are able to grow on minimal medium with glucose and simple salts, while auxotrophs must grow on medium with supplements because they are unable to synthesize a component themselves.
Mapping in bacteria involves introducing DNA into a recipient cell and studying the frequency of transfer, how often genes are transferred together, and the time it takes for transfer to occur in the case of conjugation.
Transformation is the transfer of naked DNA into bacterial cells. This technique was used in the demonstration that DNA was the genetic material, and is used today in recombinant DNA experiments. Frequency is on the order of 1/1000 cells. The main measure used is "cotransformation frequency". Genes near each other will be transferred together at close to the rate for a single gene, while genes far apart would be transferred at close to the product of those rates (e.g., 1/1,000,000). Thus, this method is very good for fine-level mapping of genes but not good for larger scale mapping.
Conjugation is the transfer of DNA from one bacterium to another. This involves plasmids, which are circular DNAs which can replicate independent of the bacterial chromosome. The F factor is a plasmid of about 100 kb which can exist free of the chromosome (in which case the bacteria are termed F+) or integrated (in which case the bacteria are termed Hfr). Bacteria lacking the F factor are termed F-. The F factor codes for formation of a pilus (tube, plural is pili) through which DNA can be transferred to a recipient bacterium. F+ bacteria transfer the F factor at a high rate to F- bacteria, while Hfr bacteria transfer the bacterial chromosome through the tube.