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4 7.20 Somatic-cell hybridization is used to assign a gene to a particular human chromosome. A panel of six cell lines, each line containing a different subset of human chromosomes, is examined for the presence of the gene product (such as an enzyme). A plus sign means that the gene product is present; a minus sign means that the gene product is missing. Four of the cell lines (A, B, D, and F) have the gene product, indicating that the gene is present on one of the chromosomes found in these cell lines. The only chromosome common to all four of these cell lines is chromosome 4, indicating that the gene is located on this chromosome.

Two genes determined to be on the same chromosome with the use of somatic-cell hybridization are said to be syntenic genes. This term is used because syntenic genes may or may not exhibit linkage in the traditional genetic sense — remember that two genes can be located on the same chromosome but may be so far apart that they assort independently. Syntenic refers to genes that are physically linked, regardless of whether they exhibit genetic linkage. (Synteny is sometimes also used to refer to

The gene product (an enzyme) is present when there is an intact chromosome 4.

The gene product is absent when the entire chromosome 4 is absent.

.or its short arm is missing.

The gene product (an enzyme) is present when there is an intact chromosome 4.

The gene product is absent when the entire chromosome 4 is absent.

Cell line 1

Cell line 2

Cell line 3

Conclusion: If the gene product is present in a cell line with an intact chromosome but missing from a line with a chromosome deletion, the gene for that product must be located in the deleted region.

7.21 Genes can be localized to a specific part of a chromosome by using somatic-cell hybridization.

gene loci in different organisms located on a chromosome region of common evolutionary origin.)

Sometimes somatic-cell hybridization can be used to position a gene on a specific part of a chromosome. Some hybrid cell lines carry a human chromosome with a chromosome mutation such as a deletion or a translocation. If the gene is present in a cell line with the intact chromosome but missing from a line with a chromosome deletion, the gene must be located in the deleted region ( FIGURE 7.21) Similarly, if a gene is usually absent from a chromosome but consistently appears whenever a translocation (a piece of another chromosome that has broken off and attached itself to the chromosome in question) is present, it must be present on the translocated part of the chromosome.

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