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26. A panel of cell lines was created from mouse - human *27. A panel of cell lines was created from mouse - human somatic-cell fusions. Each line was examined for the presence of somatic-cell fusions. Each line was examined for the human chromosomes and for the production of an enzyme. The presence of human chromosomes and for the production of following results were obtained: three enzymes. The following results were obtained.

_Human chromosomes_ Cell Enzyme _Human chromosomes_

Cell line Enzyme 1 2 3 4 5 6 7 8 9 10 17 22 line 1 2 3 4 8 9 12 15 16 17 22 X

A - +--- + -- -- - + - B + -- -- +- - + + -B + + + -- -- - + -- + + C - + + +- ---- + - +

C - +--- + -- -- -- + d - + + + + -- - + -- + D - ---+--------

On the basis of these results, which chromosome has the On the basis of these results, give the chromosome location gene that codes for the enzyme? of enzyme 1, enzyme 2, and enzyme 3.

^CHALLENGE QUESTION^_

28. In calculating map distances, we did not concern ourselves with whether double crossovers were two stranded, three stranded, or four stranded; yet, these different types of double crossovers produce different types of gametes. Can you explain why we do not need to determine how many strands take part in double crossovers in diploid organisms? (Hint: Draw out the types of gametes produced by the different types of double crossovers and see how they contribute to the determination of map distances.)

SUGGESTED READINGS]_

Creighton, H. B., and B. McClintock. 1931. A correlation of cytological and genetical crossing over in Zea mays. Proceedings of the National Academy of Science U. S. A. 17:492 - 497. Paper reporting Creighton and McClintock's finding that crossing over is associated with exchange of chromosome segments.

Crow, J. 1988. A diamond anniversary: the first genetic map. Genetics 118:1 - 3.

A brief review of the history of Sturtevant's first genetic map. Ruddle, F. H., and R. S. Kucherlapati. 1974. Hybrid cells and human genes. Scientific American 231(1):36 - 44. A readable review of somatic-cell hybridization.

Stern, C. 1936. Somatic crossing over and segregation in Drosophila melanogaster. Genetics 21:625- 631. Stern's finding, similar to Creighton and McClintock's, of a correlation between crossing over and physical exchange of chromosome segments. Sturtevant, A. H. 1913. The linear arrangement of six sex-linked factors in Drosophila, as shown by their mode of association. Journal of Experimental Zoology 14:43 - 59. Sturtevant's report of the first genetic map.

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