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Lecture 5

Lecture 5 - From Genotype to Phenotype

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Biology 2581B
Jim Karagiannis

LECTURE 4: FROM GENOTYPE TO PHENOTYPE – DEVIATIONS FROM MENDELIAN RATIOS Key Concepts 1. Incomplete/Codominance 2. Penetrance/Expressivity 3. Complementation 4. Epistasis Deviations from Mendelian Ratios: Lucien Cuénot’s Odd Yellow Mice • Lucien Cuénot – one of the first individuals to test Mendelian principles using animals • Looked at coat color phenotypes, was able to show that the yellow allele was dominant to the agouti • However, was never ale to isolate a true-breeding yellow mouse • Should be able to isolate a homozygous yellow mouse that would give you all yellow progeny, but that never happened • Instead, matings of inbred agouti mice to yellow mice always results in a 1:1 ratio of yellow to agouti • Explanation is that homozygous yellow mouse was never born – example of a lethal mutation – when it is present in two copies, it is never able to develop • Matings of yellow to yellow mice always results in a 2:1 ratio of yellow to agouti? • Also an example of pleiotropy – a single locus affecting two traits, yellow allele not only affects coat color, but also viablity • Pleiotropy: phenomenon in which a single gene determines a number of distinct and seemingly unrelated characteristics y • A is dominant toAwith respect to coat color • A is recessive toAwith respect to viability Deviations from Mendelian Ratios • Simple extensions of Mendel’s core model can readily explain the relationship between genotype and phenotype in cases where we see deviations from Mendelian ratios • Types of lethal alleles: o Early onset: the gene is necessary for cellular function, death at embryogenesis or early on in life o Late onset: the gene is essential for survival, but not until the individual has matured o Semi lethal: kills some mutant individuals in the population, but not all. o Conditional: the mutation is only lethal under certain environmental conditions e.g. temperature The Chi-Square Test • Are the deviations from the expected 3:1 phenotypic ratio due to sampling error? • Null hypothesis: the observed deviation from the expected 3:1 phenotypic ratio is due to sampling error • Statistic is converted into a probability, using a chi- square table • To use the table, you need to determine the degrees of freedom (number of classes – 1) • If you can show that p < 0.05, generally you conclude that there must be some sort of biological explanation as deviation is not sue to standard error • What is the biological explanation? In this case, this is an example of a semi-lethal allele (mutants die off at a greater rate than the wildtype, which is why a skew from the 3:1 ratio is evident) • Degrees of freedom = 1 and p-value is 0.05, so 3.841 • Therefore, we conclude that the observed differences from the expected 3:1 ratio are not due to sampling error i.e. we reject the null hypothesis Penetrance and Expressivity • Ovals represent individuals. In each example all individuals have the identical genotype • The probability of penetrance and the level of expressivity can not be derived from Mendelian principles and thus must be determined empirically • Penetrance: proportion of members of a population with a given genotype showing the expected phenotype • Expressivity: intensity with which a particular genotype is expressed • Neurofibromatosis is a dominant disease that shows 50-80% penetrance and variable expressivity o Mildest form – “café-au-lait” spots on the skin o In more severe cases, neurofibromas are also seen Deviations from Mendelian Ratios: Traits Determined by One Gene Extensions to Mendel: Single Gene Inheritance • (1) Complete dominance – two different phenotypes (blue and white) results in heterozygote (white phenotype) • (2) Complete dominance – opposite, blue phenotype is dominant to white phenotype • (3) Incomplete dominance – traits are blended, results in light blue phenotype • (4) Codominant – both traits are observed in the progeny • Single gene inheritance: incomplete dominance o Heterozygote expresses a phenotype that is different than either of the parents o Two alleles –Acodes for wildtype enzyme that produces pigment and a is a no
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