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4. 4 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

5. 5 In 1963, Ronald Davidson, Harold Nitowsky and Barton Childs set out to test the Lyon hypothesis at the cellular level To do so they analyzed the expression of a human X-linked gene –The gene encodes glucose-6-phosphate dehydrogenase (G-6-PD), an enzyme used in sugar metabolism The Lyon Hypothesis Put to the Test Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

6. 6 Biochemists had found that individuals vary with regards to the G-6-PD enzyme –This variation can be detected when the enzyme is subjected to agarose gel electrophoresis –One G-6-PD allele encodes an enzyme that migrates very quickly The “fast” enzyme –Another allele encodes an enzyme that migrates slowly The “slow” enzyme –The two types of enzymes have minor differences in their structures These do not significantly affect G-6-PD function Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

7. 7 Figure 7.5 illustrates the mobility of G-6-PD proteins from various individuals Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Thus heterozygous adult females produce both types of enzymes Hemizygous males produce either the fast or the slow type

8. 8 The Hypothesis –According to the Lyon hypothesis, an adult female who is heterozygous for the fast and slow G-6-PD alleles should express only one of the two alleles in any particular somatic cell and its descendants, but not both Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Testing the Hypothesis –Refer to Figure 7.6

9. 9 Figure 7.6

10. 10 The Data Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

11. 11 Interpreting the Data Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display All nine clones expressed one of the two types of G-6-PD enzyme, not both These epithelial cells were used to generate the nine clones (as described in steps 2 to 4) The heterozygous woman produced both types of G-6-PD enzymes Clones 2, 3, 5, 6, 9 & 10 expressed only the slow type Clones 4, 7 & 8 expressed only the fast type

12. 12 These results are consistent with the hypothesis that –X inactivation has already occurred in any given epithelial cell AND –This pattern of inactivation is passed to all of the cell’s progeny Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Interpreting the Data