Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Genetic mapping

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single amplification reaction. For viruseswith a RNAgenome,

RT-PCR (reverse transcriptase PCR) is widely utilized for

identification and quantitation.

The anthraxoutbreak in the Unites States in the fall of

2001 illustrated the significance of these technologies.

Because an anthrax infection can mimic coldor flu symptoms,

the earliest victims did not realize they were harboring a

deadly bacterium. After confirmation that anthrax was the

causative agent in the first death, genetic technologies were

utilized to confirm the presence of anthrax in other locations

and for other potential victims. Results were available more

rapidly than would have been possible using standard micro-

biological methodology and appropriate treatment regimens

could be established immediately. Furthermore, unaffected

individuals are quickly informed of their status, alleviating

unnecessary anxiety.

The second stage of the investigation was to locate the

origin of the anthrax cells. The evidence indicated that this

event was not a random, natural phenomenon, and that an indi-

vidual or individuals had most likely dispersed the cells as an

act of bioterrorism. In response to this threat, government

agencies collected samples from all sites for analysis. A key

element in the search was the genetic identification of the cells

found in patients and mail from Florida, New York, and

Washington, D.C. The PCR studies clearly showed that all

samples were derived from the same strain of anthrax, known

as the “Ames strain” since the cell line was established in

Iowa. Although this strain has been distributed to many differ-

ent research laboratories around the world, careful analysis

revealed minor changes in the genome that allowed investiga-

tors to narrow the search to about fifteen United States labo-

ratories. Total genome sequencing of these fifteen strains and

a one-to-one base comparison with the lethal anthrax genome

may detect further variation that will allow a unique identifi-

cation to be made.

The advent of molecular technologies and the applica-

tion of genetic identification in clinical and forensic microbi-

ology have greatly improved the capability of laboratories to

detect and specifically identify an organism quickly and accu-

rately.

See alsoAnthrax, terrorist use of as a biological weapon;

Culture; Microbial genetics

GGenetic mappingENETIC MAPPING

The aim of genetic mapping is to determine the linear

sequence of genes in genetic material. The mapping can be

performed at several levels of detail (resolution) that fall into

two broad types: traditional genetic or linkage mapping and,

more detailed, physical mapping.

Linkage mapping shows the relative rather than

absolute positions of genes along a chromosome and is a tech-

nique that has been used since the early 1900s. Early geneti-

cists determined that genes were found on chromosomes.

They also reasoned that because the various forms of genes, or

alleles, could be precisely exchanged during meiosis through

crossovers between homologous chromosomes, the genes for

specific characteristics must lie at precise points along each

chromosome. It followed that the mapping of chromosomes

could, therefore, be made from the observation of crossovers.

Between 1912 and 1915, the American scientist Thomas Hunt

Morgan (1866–1945) hypothesized that if genes were

arranged linearly along chromosomes, then those genes lying

closer together would be separated by crossovers less often

than those lying further apart. Genes lying closer together

would thus have a greater probability of being passed along as

a unit. It follows that the percentage of crossovers would be

proportional to the distance between two genes on a chromo-

some. The percentage crossover can be expressed as the num-

ber of crossovers between two genes in meiosis. One genetic

map unit (m.u.) is defined as the distance between genepairs

for which one product out of 100 is recombinant (a product of

crossover). The recombinant frequency (R.F.) of 0.01 (1%) is

defined as 1 m.u. and a map unit is sometimes referred to as a

centimorgan (cM) in honor of Thomas Hunt Morgan.

As an example of how linkage mapping might work,

suppose two characteristics, A and B, show 26% crossover.

Assign 26 crossover units to the distance between these two

Technician making a genetic marker.

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