1

2

3

4

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

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

• 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

• 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

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

9Figure 7.6

10

The Data

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

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 typeClones 4, 7 & 8 expressed only the fast type

12

• These results are consistent with the hypothesis that– X inactivation has already occurred in any given

epithelial cellAND

– 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