Mary Gasseling experimented with chicken eggs and took the patch of tissue from the pinky and put it on the first finger. The chicken resulted in having a new set of fingers that were mirror images of the normal set. The gene inside the tissue was able to control the development of the pattern of the fingers. (mirror image duplication)
The patch of tissue is the zone of polarizing activity (ZPA) How did it control the formation of fingers and toes? There is the concentration-dependent idea which explains that there is a high concentration of ZPA.
Denis Summerbell experimented with the idea and put foil between the ZPA patch and the rest of the limb. The foil was a barrier to prevent any molecule from diffusing to the other side. It resulted in cells on the ZPA side forming digits and the opposite side had barely formed digits. Something spread out of the ZPA that controlled how digits formed.
That something was the sonic hedgehog gene. It is what causes the ZPA tissue to do what it does. When things go wrong with sonic hedgehog, like in this picture, hands become messed up because the gene is not turned on properly during the 8th week of human development.
Errors sometimes happen when the DNA is copied from the parent to the daughter cell. The error is called a mutation and it is not done randomly. Evolution favors a mutation that helps an organism discover adaptations to help it survive. So this flower's mutations were triggered by outside influences. McClintock discovered "jumping genes" (transposons) that proved that mutations are not just random. They remain in an active gene after inserted and make a difference. There are two different ties of jumping genes: DNA transposons which cut and paste, and Retrotransposons which copy and paste. This flower was affected by the DNA transposons where the code of the gene splits and it loses its trait which causes a change in the physical trait.
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