Biography
Harriet Baldwin Creighton
Harriet Baldwin Creighton was an American botanist, geneticist, and educator. She was born in Delavan, Illinois on June 27, 1909. She completed her Ph.D at Cornell University in 1933. During her time at Cornell, she worked in the field of maize cytogenetics with Barbara McClintock. She helped prove that genes are located on chromosomes.
Barbara McClintock
Barbara McClintock was an American scientist and cytogeneticist, who was awarded the 1983 Nobel Prize in physiology or medicine. Born on June 16, 1902, McClintock studied maize chromosomes and how they changed during reproduction. She received her Ph.D in botany at Cornell University, where she met Harriet Creighton. She developed a technique for visualizing maize chromosome and used microscopic analysis to demonstrate many fundamental genetic ideas. She was recognized among the best in the field, awarded prestigious fellowships, and elected a member of The National Academy of Sciences 1944. She produced the first genetic map for maize, linking regions of the chromosomes to physical traits.
Creighton and McClintock
After Creighton graduated Cornell University as a botany graduate student, she became a laboratory assistant in botany. At that time, Barbara McClintock was an instructor at Cornell. The two women immediately became friends and began working together to solve one of the most complex genetics problem; since the beginning the twentieth century, cytologists theorized that chromosomes carried and exchanged genetic information to produce new combinations of physical traits, but cytological evidence to prove their hypothesis was lacking. They designed an experiment that showed that the exchange of genes was accompanied with a physical aspect. This physical aspect was the exchange of sections of chromosomes.
McClintock and Creighton believed that there was an established relationship between the process of physical exchange and the occurrence of genetic recombination. In order to prove that this was true they would need three things. They would need to discover heteromorphic pairs of homologous chromosomes so that cytological markers could be used to observe the physical exchanges between them. Once they did this the results would have to show that initial crosses involving cytologically marked chromosomes would have to be made to demonstrate that exchanges between the chromatid arms d occur. Lastly once these physical exchanges were observed they would have to determine if there is a stable and reliable correlation between the exchange processes.
Creighton and McClinktock first obtained a strain of corn with an abnormal chromosome belonging to homologous pair number nine.This chromosome carried a distinct knob on one end and a detectable translocation on the other. By crossing corn carrying only the abnormal chromosomes with corn carrying only normal chromosomes they were able to obtain plants with heteromorphic pairs of chromosomes (one normal and one abnormal chromosome number nine). Then they carried out crosses involving plants with heteromorphic chromosomes pairs in order to find the occurrence of recombination. Having demonstrated that physical exchange between chromatids exist, they established a relationship between the process of physical exchange and genetic recombination. They mapped out the two loci that occurred on chromosome 9 between the knob and the translocation.
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They used corn chromosome 9 markers:
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c = colorless seed
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wx = waxy endosperm
They created a heterozygote with the following characteristics:
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repulsion configuration of genetic markers
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cytological landmarks on both ends of one chromosome
They next performed a test cross to this stock with a cc wx wx tester.
If crossing over involves the physical exchange of chromsomal material ,
then the recombinant phenotypes should each contain one of the cytological
landmarks.
The behavior of the genes as revealed by the phenotypes
produced was shown to be directly related to the behavior of chromosomes
as seen under the microscope. The recombination of genes occurs when
homologous chromosomes exchange parts.In conclusion they showed that genes could move from place to place on a chromosome. They showed/proved that pairing chromosomes, heteromorphic in two regions, have been known to exchange parts at the same time they exchange genes.
"Genetic Linkage." Genetic Linkage. N.p., n.d. Web. 21 Jan. 2016.
"The Evidence of Creighton and McClintock." Creighton and McClintock. N.p., n.d. Web. 21 Jan. 2016.
"Classic Papers in Genetics." Classic Papers in Genetics. N.p., n.d. Web. 21 Jan. 2016.
