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OAC Biology CHROMOSOME LINKAGE and Other Assorted Information Mr. Galajda Lethal Alleles: An allele that causes the death of the individual, often before birth or sexual maturity. An example: Cuenot (a biologist) crossed heterozygous yellow mice. The ratio he obtained was 2 yellow mice and one agouti mouse (he did not obtain the expected 1:2:1 phenotypic ration). WHY? A mutant gene prevented any homozygous yellow mice from being born. The mutant gene was found to affect an essential step in the embryonic development of mice. THIS phenomenon is called PLEIOTROPHY. What are some examples of this in humans? Epistasis: The interaction between nonallelic genes such that one set of genes interferes with or prevents the expression of another gene. Sample problem showing this phenomenon. . In cultivated flowers called stocks, the recessive genotype of one locus (aa) prevents the development of pigment in the flower, thus producing a white flower. In the presence of the dominant allele A, alleles at another locus may be expressed as follows: C__ = red and cc = cream.
When red stocks of the genotype AaCc are crossed to the red stocks of the genotype AaCC, what phenotypic and genotypic proportions would be expected in the offspring? Genes on the Same Chromosome - LINKAGE GENES: When T.H. Morgan (a biologist) did a dihybrid cross in fruit flies involving two autosomal recessive mutants (normal & vestigial wings and normal & purple eyes) he did not obtain the expected 9:3:3:1 ratio in the double heterozygous cross. Instead he obtained a 1:1:1:1 ratio. What's going on he asked? Well, Morgan suggested that this ratio might be reflected in the fact that the two genes were on the same pair of homologous chromosomes. The way to properly illustrate this is to show the genetic constitution of each chromosome on opposite sides of a line: vg+ pr+ ----------- where vg+ = normal wings, pr+ = normal eyes. vg pr The above is referred to as the coupling or cis configuration because the two mutants are on the same chromosome(cis = Latin meaning "on the near side") Vg pr+ ----------- vg+ pr The above is referred to as the repulsion or trans configuration because the two mutations are on different members of the homologous pair of chromosomes (trans = Latin meaning "across"). Mapping the Distance Between Genes:
Mapping genes, based upon recombination data, requires large numbers of offspring from crosses between parents differing in two or more linked genes. Thus - Morgan speculated this in 1911 and then added that closely linked genes will not recombine as much during cross-over and chiasmata than loosely linked genes. Another geneticist Sturtevant (Morgan's student) realized that these values could be used to determine the sequence of genes in a linear chromosome. He would use the "frequency of recombination as a quantitative estimate of distance". Thus - the values for percent recombination could be converted into distance values by letting each map unit (m.u.) be equivalent to a one percent recombination.
Consider the following data - Representative Recombination Percentages in Linked Genes on the X-chromosome of Drosophilia:
B = barr shaped eyes, car = carnation coloured eyes, fu = fused veins in wings, sd = scalloped wings. To put the above in a linear sequence start with the two genes closest together - (B and fu). The map distance would be 2.5 m.u. To add a third gene car to the map we know the distance from car to fu is 3.0 m.u. WITH this info there are two different sequences possible: Fu 2.5 B
Car Car 3.0 Now figure it out - if car is to the left of fu than B than the map distance between car and B should be greater than 3.0. If car is to the right, than the map distance between car and B should be less than 3.0. The table shows that the distance is in fact 5.5 , therefore the sequence must be: Car 3.0 fu 2.5 B
NOW, try and map the sd gene yourself - where is it on our gene map? Mapping illustrates that each gene occupies a specific site in the chromosome - which you refer to as the GENE LOCUS.
The One Gene-One Enzyme/Polypeptide Concept: By studying mutations affecting known metabolic pathways it was determined (these studies were done before the realization that DNA is the genetic material) that genes determine the specific amino acid sequence of proteins. In turn, these proteins play an important role in determining which metabolic reactions could occur in the body and which structures could be formed. The first to realize this concept was Archibald Garrod (English physician) who at the turn of the century (1909) studied the inheritance of enzyme deficiencies and wrote a classic book "Inborn Errors of Metabolism". The One Gene - One Enzyme Concept: It was George Beadle and Edward Tatum in the 1940's who were able to attribute a mutant genes inability to make the required functional enzyme. Thus was born the "one gene-one enzyme concept".The One Gene - One Polypeptide Concept: This concept was changed to the "one gene-one polypeptide" concept because all proteins (sickle cell anemia) can be included. Remember - genes determine proteins, many of which BUT NOT all are enzymes (some are structural!). It was in 1949 that James Neel at U of Michigan showed that individuals with sickle cell differed from the normal by one single gene difference. In this example, the change of just one amino acid has such drastic effects. It's all in the genes! |
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E-mail Mr. Galajda at: david.galajda@sac.on.ca |