Explain why the corn kernel in Figure 18.34d is variegated, with some areas colo

Question

Explain why the corn kernel in Figure 18.34d is variegated, with some areas colored and some areas lacking pigment? An active Ds element causes an Ac element to transpose out of the C allele during kernel development, allowing colored sectors produced in an otherwise colorless (yellow) kernel. An active Ds element causes an Ac element to transpose into the C allele, which results in colored sectors in an otherwise colorless (yellow) kernel. Ac and Ds elements simultaneously transpose into the C allele, but later Ac transposes back out, which results in colored sectors in an otherwise colorless (yellow) kernel. An active Ds element that enhances transcription of the C allele is suppressed by an Ac element, which results in colored sectors in an otherwise colorless (yellow) kernel. An active Ac element causes an Ds element to transpose out of the C allele during kernel development, allowing colored sectors in an otherwise colorless (yellow) kernel.

Explanation / Answer

The presence of transposable elements was first discovered by Barbara McClintock in maize.The edevlopment of kernal in corn was noticed to be under the influence of the genes  Ds (dissociation or a locus were breakage of chromosomes ) and  Ac, (activator).

Kernal coloration in corn is under the influence of the following genes

CI = dominant allele which prevents the expression of color in the aleurone layer.

C = recessive allele that allows the color development in the aleurone layer.

Bz = dominant allele that is responsible for purple coloration .

bz = recessive allele that produces a dark brown to purple-brown aleurone color.

Ds = a genetic location where chromosome breakage occurs.

when she crossed Homozygous stocks CC bzbz female with CICI BzBz DsDs males. The aleurone layer of the endosperm would be colorless because of the presence of the inhibitor allele,with the genotype  CICC Bzbzbz --Ds.but a few kernels had dark brown colored and she concluded that in some manner the CI and Bz alleles were lost because chromosome breakage had occurred at the Ds locus. Further the breakage was found to be not during the   gamete formation, but had occurred after fertilization and during the development of the seed and explained that This breakage and loss of genes occurred in a single cell, but all cells that developed from mitotic division of that cell did not contain the inhibitor gene, so the color expression was controlled by the recessive bz allele in those cells. further she realized that Ds alone could not induce the breakage. A second factor, Ac, short for activator, was also necessary and concluded that Ds requires some factor provided by Ac to move, whereas Ac is independent.

with this background the answer to the above question would be:

An active element Ac causes Ds elemnt to transpose out of the C allele during kernal; development , allowing colored sectors in an otherwise colorless sector.

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