What was Mendel’s definition of a gene?
- Mendel defined a gene as a ‘character’ that controls one specific phenotype, such as flower colour.
- It is essentially defined as the basic unit of heredity
What disease did Archibald Garrod discover was caused by a metabolic defect in 1902?`
- Alkaptonuria- individuals affected by this would produce black urine containing high levels of homogentisic acid.
- Gerrod reasoned that people suffering from alkaptonuria are unable to metabolise homogentisic acid.
- Individuals affected were homozygous for a recessive mutant allele of a single gene.
- He concluded that alkaptonuria is a genetic disease caused by the absence of an enzyme necessary to metabolise homogentisic acid.
How was Garrod’s insight supported by George Beadle and Edward Tatum in their 1941 published hypothesis ‘One gene- one enzyme’?
Through their work exploiting the fungus ‘Neurospora’
How does Neurospora normally reproduce?
Neurospora is normally haploid and reproduces mitotically by the production of asexual spores called conidia.
Neurospora can also undergo sexual reproduction. List the stages of sexual reproduction in this fungus.
Haploid individuals undergo syngamy (fusion) which results in a diploid phase. They then undergo meiosis to produce haploid ascospores. After dispersal they once again become haploid individuals.
What is the difference between wild type neurospora and mutants?
- Wild type neurospora are prototrophic which means they are able to synthesis all the molecules they need if given energy (sugar) and a source of minerals.
- Mutant neurospora are unable to synthesis specific metabolites (such as amino acids or vitamins) are called auxotrophs.
What is the process for identifying auxotrophs?
- Conidia from wild type neurospora are induced with mutations through X- Rays. This creates mutant neurospora which are then crossed with the wild type through fusion and meiosis to produce haploid ascospores.
- Single ascospores from the cross are inoculated onto complete medium. No growth = Auxotrophic mutant.
- Take conidia from auxotrophic mutant and test on supplemented media. Test for growth on minimal medium supplemented with each one of 20 amino acids. The mutant must be unable to synthesis arginine.
Is the mutant phenotype due to a mutation in a single gene?
- According to the ‘ one gene- one enzyme hypothesis’ a gene controls the production and/or activity of a single enzyme. The arg+ gene must encode an enzyme envolved in the synthesis or arginine.
- Considering a cross between arg1 and arg 2, if they are mutations in the same gene then all of the progeny will be arg auxotrophs. But if they are mutations in different genes they will complement each other.
- This means that Beadle and Tatum were able to divide the 12 arg mutants into just 4 complementation groups. Each group is corresponding to a different gene.
What is Epistasis?
- Epistasis refers to an interaction between different genes.
- It is not to be confused with dominant/ recessive relationships between alleles of the same gene.
Describe the Bombay Phenotype in humans.
- Bombay phenotype is a special phenotype in the red blood cells of humans carrying the ‘hh’ genotype. They do not produce any H antigens on their red blood cells which are essential for the production of I(A) or I(B) antigens. They are not even classified as O group as O groups still have H antigens. This means they are suitable blood donors to any blood group as they have no antigens. However they can only receive blood from other people with the Bombay Phenotype otherwise immunogloblins will attack due to the presence of foreign antigens.
- The hh allele combination at the H gene locus is epistatic to the I gene locus. That means it prevents the expression of the I genes normal phenotypic effects.
What does Epistasis mean for Mendelian ratios?
Normally we expect a 9:3:3:1 ratio in the progeny of a dihybrid cross if
- the genes segregate normally
- One allele of each gene shows complete dominance
- and they are unlinked
Even when all conditions are fulfilled, if thee gene products do not function independently then the phenotypic ratio may differ.
