Gene Mapping Localizes Specific Genes to Distinct Chromosomes

Gene localizing thus can define a map of the human genome. This is already yielding useful information in the definition of human disease. Somatic cell hybridization and in situ hybridization are two techniques used to accomplish this. In in situ hybridization, the simpler and more direct procedure, a radioactive probe is added to a metaphase spread of chromosomes on a glass slide. The exact area of hybridization is localized by layering photographic emulsion over the slide and, after exposure, lining up the grains with some histologic identification of the chromosome. Fluorescence in situ hybridization (FISH) is a very sensitive technique that is also used for this purpose. This often places the gene at a location on a given band or region on the chromosome. Some of the human genes localized using these techniques are listed in Table 40-5. This table represents only a sampling, since thousands of genes have been mapped as a result of the recent sequencing of the

Figure 40-7. The polymerase chain reaction is used to amplify specific gene sequences. Double-stranded DNA is heated to separate it into individual strands. These bind two distinct primers that are directed at specific sequences on opposite strands and that define the segment to be amplified. DNA polymerase extends the primers in each direction and synthesizes two strands complementary to the original two. This cycle is repeated several times, giving an amplified product of defined length and sequence. Note that the two primers are present in excess.

Table 4Q-5. Localization of human genes.1

Gene

Chromosome

Disease

Insulin

11p15

Prolactin

6p23-q12

Growth hormone

17q21-qter

Growth hormone deficiency

a-Globin

16p12-pter

a-Thalassemia

ß-Globin

11p12

P-Thalassemia, sickle cell

Adenosine deaminase

20q13-qter

Adenosine deaminase deficiency

Phenylalanine hydroxylase

12q24

Phenylketonuria

Hypoxanthine-guanine

Xq26-q27

Lesch-Nyhan syndrome

phosphoribosyltransferase

DNA segment G8

4p

Huntington's chorea

1This table indicates the chromosomal location of several genes and the diseases associated with deficient or abnormal production of the gene products. The chromosome involved is indicated by the first number or letter. The other numbers and letters refer to precise localizations, as defined in McKusick VA: Mendelian Inheritance in Man, 6th ed. John Hopkins Univ Press, 1983.

1This table indicates the chromosomal location of several genes and the diseases associated with deficient or abnormal production of the gene products. The chromosome involved is indicated by the first number or letter. The other numbers and letters refer to precise localizations, as defined in McKusick VA: Mendelian Inheritance in Man, 6th ed. John Hopkins Univ Press, 1983.

human genome. Once the defect is localized to a region of DNA that has the characteristic structure of a gene (Figure 40-1), a synthetic gene can be constructed and expressed in an appropriate vector and its function can be assessed—or the putative peptide, deduced from the open reading frame in the coding region, can be synthesized. Antibodies directed against this peptide can be used to assess whether this peptide is expressed in normal persons and whether it is absent in those with the genetic syndrome.

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