Genetic instability was originally

Genetic instability was originally discovered in maize (Zea mays) in which it was found to cause altered patterns of pigmentation of the kernels. Instead of being evenly pigmented, the kernels have sectors of more intense pigmentation. The spots vary in size and colour. At the same time, the cells show chromosome breaks and other abnormalities. McClintock examined the relationship between the pigmentation pattern the kernels and chromosome changes. Variegation in the colour of the kernels was found to be parallelled by transposition of structural elements within or between chromosomes. Because transpositions result in inactivation of neighbouring genes, McClintock used the term "controlling elements" to describe the mobile chromosome structures. Another effect of transposition was chromosome breaks at points where the mobile elements were integrated.