When scientists first observed chromosomes in mitosis and meiosis, it was suggested that chromosomes were likely to carry the genetic material. Some direct evidence came from studying X linked genes in fruit flies: the traits (such as a trait for eye color) followed the X chromosome. When researchers found genetic evidence for recombination, they wanted to see whether they could correlate physical crossing over on the chromosome withe genetic recombination. In order to do, this scientists took advantage of some characteristics of corn chromosomes.
To understand their research it helps to review a few key corn traits
The colored trait is coded for by a dominant allele (C) and the recessive allele (c) gives yellow kernels
A second gene called Waxy effects the type of starch in the kernels. Wx is a dominant allele that makes 2 kinds of starch (Amylose starch and amylopectin) while corn plants homozygous for the recessive allele (wx) make only amylopectin. The Wx gene and C gene are on chromosome 9 of corn.
Corn chromosomes can also have knobs, which are darkly staining regions at the end of chromosomes. However, for a given chromosome (such as chromosome 9) the knob may or may not be present and it acts like a genetic marker. In other words, a corn plant that has knobs on both chromosome 9s will pass this chromosome to all of its offspring. If such a plant is crossed to a strain that lacks the knob, then the offspring will be heterozygous for the knob trait. Below is a picture of knobs in corn chromosomes. Chromosome Knobs are shown below:
the first critical experiment involved the cross shown below in which plants that were heterozygous for the Knob and the C gene were crossed to plants that lacked the knob and were homozygous for the recessive c allele. These experiments were carried out by Harriet Creighton and Barbara McClintock
From this experiment, the researchers concluded that the chromosome 9 Knob and the C gene were on the same chromosome and linked closely.
The second experiment used a strain of corn that was heterozygous for the Wx gene, the C gene and the knob. Creighton and McClintock carried out the following cross:
In your own words, state what the results of this cross says about the relationship between recombination and physical crossing over.
Image Credits
FIGURE 3. Carmine-stained pachytene chromosomes of the 441123 × 444331 hybrid showing cytologically visible knobs on chromosomes 3(B), 5(C), 6(F), 7(G), 8(I), and 9(J)
From Mondin etal
Karyotype variability in tropical maize sister inbred lines and hybrids compared with KYS standard line
http://journal.frontiersin.org/article/10.3389/fpls.2014.00544/full
Image URL: http://www.frontiersin.org/files/Articles/111039/fpls-05-00544-HTML/image_m/fpls-05-00544-g003.jpg
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