Study notes for lecture 5

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University of Toronto St. George
Human Biology
Stephen Wright

Lecture 05: Extensions of Mendelian genetics (Part 1) Reaction to Mendels work No recognition during lifetime, until 1900 Explanation o Obscure journal o Too heavy on statistics for then-audience o Results assumed pea-specific o Exceptions to 3:1 ratio exist One camp Exceptions to Mendels rules mean theyre not universally applicable Support blending inheritance As opposed to another camp Exceptions can be explained in Mendian terms of segregation and independent assortment Championed by W. Bateson o Genotype where Mendels laws always apply o Relationship between genotype and phenotype can change o Dominance and recessiveness particular to traits Exception to 3:1 ratio: incomplete dominance Cross inbred red with inbred white o F1 is pink o F2 is red, pink, and white in 1:2:1 Mendelian o Phenotypic ratios same as genotypic ratios Heterozygote has new phenotype, different from parents o Neither allele is dominant or recessive to the other o RW notation In humansfamilial hypercholesteraemia o Disease leading to high level of bad cholesterol o Heterozygote has intermediate level Phenotype distinct from either homozygous phenotype Exception to 3:1 ratio: codominance Cross inbred red with inbred white o F1 is half red, half white o Phenotypic ratios same as genotypic ratios Heterozygote has both phenotypes, not an intermediate o Equal expression of parent phenotypes o RW notation Dominance is not always complete Incomplete dominance Co-dominance Phenotype is key
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