Barbara Mcclintocks Search for the Patterns of Genetic Control Review

© 2002 Nature Publishing Group Feschotte, C. et al. Establish transposable elements: where genetics meets genomics. Nature Reviews Genetics 3, 330. All rights reserved.

McClintock worked with what is known as the Ac/Ds system in maize, which she discovered by conducting standard genetic breeding experiments with an unusual phenotype. Through these experiments, McClintock recognized that breakage occurred at specific sites on maize chromosomes. Indeed, the first transposable element she discovered was a site of chromosome breakage, aptly named "dissociation" (Ds). Although McClintock eventually found that some TEs can "jump" apart, she initially noted that the movements of Ds are regulated by an autonomous element called "activator" (Ac), which can also promote its ain transposition.

Of course, these discoveries were preceded by extensive breeding experimentation. Information technology was known at the time from previous work by Rollins A. Emerson, another American maize geneticist and the "rediscoverer" of Mendel's laws of inheritance, that maize had genes encoding variegated, or multicolored, kernels; these kernels were described as colorless (although they were actually white or xanthous), except for spots or streaks of imperial or brownish (Effigy two). Emerson had proposed that the variegated streaking was due to an "unstable mutation," or a mutation for the colorless phenotype that would sometimes revert back to its wild-type variant and result in an surface area of color. However, he couldn't explicate why or how this occurred. As McClintock discovered, the unstable mutation Emerson puzzled over was actually a four-gene system, as outlined in Table 1.

Table 1: Maize Genes Studied by Barbara McClintock

Factor	Clarification
C'	Dominant allele on the short arm of chromosome 9 that prevents color from being expressed in the aleurone layer of the maize kernel, causing a and so-called "colorless" phenotype (which is actually white or yellow in colour). This is as well known every bit the inhibitor allele.
C	Recessive allele on the brusque arm of chromosome 9 that leads to colour evolution.
Bz	Dominant allele on the curt arm of chromosome 9 that leads to a majestic phenotype.
bz	Recessive allele on the short arm of chromosome 9 that leads to a dark brownish phenotype.
Ds	Genetic location on the brusque arm of chromosome 9 at which chromosomal breakage occurs.
As	A cistron of unknown location (at least when McClintock was conducting her inquiry) that impacts the expression of Ds.

Adapted from McClean, 1997

In her experiments, McClintock bred females that were homozygous for C and bz and that lacked Ds (denoted CCbzbz--, where the dashes indicate the absence of Ds alleles) with males that were homozygous for C', Bz, and Ds (denoted C'C'BzBzDsDs) to yield heterozygotes with an aleurone layer that had the genotype C'CCBzbzbz--Ds. (Remember, in double fertilization, the sperm provides one fix of alleles, and the egg provides two.) Because of the presence of the dominant inhibitor allele C', the offspring kernels were expected to exist colorless, no matter what their genetic makeup at the Bz/bz locus. In fact, upon crossbreeding, many of these kernels were indeed colorless. However, McClintock likewise observed many kernels with colorless backgrounds and varying amounts of night brownish spots or streaks, and she concluded that individual cells in those kernels had lost their C' and Bz alleles considering of a chromosomal break at the Ds locus. Without either the C' allele (to foreclose colour expression) or the Bz (regal) allele, the cells that had experienced a breakage at the Ds locus ended up with some brown coloring.

Within the affected seeds, the amount of colored streaking or spotting depended upon when during seed development the somatic jail cell mutation at Ds occurred. If this mutation occurred early on in development, so, equally the one mutant jail cell continued to divide, more than cells in the mature kernel would have the brownish phenotype, and the spot or streak of color on the kernel would exist larger. On the other hand, if the mutation occurred later in development, the spotting would exist smaller, considering the kernel would undergo less cell division prior to maturity.

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Source: http://www.nature.com/scitable/topicpage/barbara-mcclintock-and-the-discovery-of-jumping-34083

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