Genetics Lecture A

8/8/2019 Genetics Lecture A

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Classical Papers in Genetics CS374 Fall 2004 Lecture 10, 10/28/04Lecturer: Olga Russakovsky Scribe: Stephen Guo

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Due to the amount of variety observed between generations, biologists concluded that it was impossible fora chromosome to be limited to carrying only one allele. By the pigeonhole principle, some chromosomesmust carry multiple alleles to explain the quantity of diversity.

2 Is a chromosome entirely dominant, or can some parts be dominant and other parts recessive?The latter theory was confirmed by observations that in the same chromosome, sometimes both traits were

dominant over the homologue, while other times only one was dominant.

Example that demonstrates (part dominant-recessive) theory:

Key: A = green color of seeda = brown color of seedB = big leavesb = small leaves

If Chromosome 1 is AB and Chromosome 2 is ab, there will be observed correlation between the traits of the offspring (green-big, brown-small).

However, if Chromosome 1 is Ab and Chromosome 2 is aB, observed correlation will be opposite fromabove.

3 What is a Gene? [1933]

After studying chromosomes, the logical next step is to study the gene.At the time of the paper, it was known that a gene was:-a minute organic particle-capable of reproduction-located in a chromosome

-responsible for the transmission of a hereditary characteristic

The goal of the paper was to expand the definition of the gene, and summarize all knowledge known aboutgenes.

3.1 Gene definition analyzed

Size: the first question biologists had about the gene was its size. Early studies estimated size bydetermining the volume of a chromosome, approximating the number of genes in the chromosome, anddividing the two numbers to get the volume of a gene. Using this method, proposed upper limits fromvarious biologists on the size were 10, 20, 50, 60, 70 millimicrons (size of a few organic molecules). As isapparent from the large range of upper bounds, this method was not particularly accurate.

Biologists understood the limitations of the previous estimates as well. They had some direct evidence thata gene was smaller than their estimates, because chromosomes were not visible inside nuclei until division.


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Separate studies using X-rays observed that genes reacted to being hitby photoelectrons. Using this more refined X-ray technique, biologistsestimated the size of a gene as a single organic molecule.

Capacity of reproduction: at the time, biologists knew little about genereproduction. Their only notable observation was based on the study of unstable genes: new genes tended to form next to old genes, rather thanold genes splitting up in two.

Transmission of hereditary characteristics: biologists understood thatthe overall hereditary process depended upon the presence of all genes.

Location of genes: It was observed that genes were present on achromosome in linear order, with each gene appearing in a permanentlocus on the chromosome. Changes in the order of genes on achromosome corresponded to genetic abnormalities. Also, it was foundthat genetic distance between genes did not correspond to the actualphysical separation distance.

3.2 Stability of the gene

Another area of gene studies is how genes are mutated. It was observedthat genes change naturally from one allele to another, with very lowprobability. The use of X-rays and radiation increased this mutationrate. Different genes had different mutation rates, genes with highmutation rate were labeled as unstable genes. The mutation rate of genes in general was observed to be a continuous series, ie plotting themutation rates of all genes roughly forms a line. Regarding the

different mutation rates of different genes, biologists observed that gene mutation rates were correlatedwith what tissues the genes were located in, as well as the stage of development of the cell. Examplemutation rates are in the table below:

Height

Hair color

Finger length

Nail shape

Eye color


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Experiment demonstrating correlation between gene mutation and stage of development: The flower Delphinium ajacis has two unstable genes that affect the color of the flowers, corresponding to rose andlavender. Both of these genes eventually change to the color purple. It was observed that the number of purple spots on a flower was directly related to the number of gene mutations. And the size of the spots onthe flower indicated the stage of ontogeny in which the stage occurred. Additionally, this flower onlyexisted in two forms: lavender with small spots and rose with varying size spots. From these facts, theconclusion was that the lavender gene was unstable late in the cell development process, while the rose

gene was equally unstable at all stages of development.

Regarding the different types of gene mutations, it was observed that given a germ cell and somatic ellwith equal mutation probability, the germ cell was more stable. In general, unstable genes mutated withhigher frequency than stable genes. Biologists also noted that unstable genes were more likely to mutate intissues that were more easily observable.

3.3 Role of genes


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The most important observation about genes was discussed at the end of the paper: any radical changeeliminates the gene from the gene complex, and the elimination of a single gene usually is lethal.

From this observation, the author derived the crux of the paper: the primary function of the gene is not theone by which we recognize it (determination of phenotypes), but the regulation of life processes of the cell.

4 Central Dogma of Molecular Biology [1970]

The Central Dogma of Molecular Biology can be summarized with one picture. Black arrows indicateknown routes of information transfer. Dotted lines indicate unlikely information transfer routes (occur inonly a limited number of cells). Arrows not present indicate information routes postulated to not exist.

Genetics Lecture A

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