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Lecture 9

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Biology 1001A
Tom Haffie

October 13, 2011 Biology Chapter 11 (Lecture 9) Epistasis • Genes interact, with one or more alleles of a gene at one locus inhibiting or masking the effects of one or more alleles of a gene at a different locus • The result of epistasis is that the expected PHENOTYPES do not appear among offspring o Does NOT affect the genotype (9:3:3:1) but affects the phenotype. Many other dihybrid crosses that involve epistatic interactions produce distributions that differ from the expected 9:3:3:1 o A good example is hair colour in Labs. B and b are alleles for the amount of melanin production. E and e are alleles for pigment deposition. B and b need E to be expressed. If they get e then the fur lacks melanin and appears yellow. (Page 242 of text book) o It is thought that epistasis is an important factor in determining an individual’s susceptibility to common human diseases Polygenic Inheritance • Continuous distribution; several different genes contribute to the same character • These characters are also known as quantitative traits o Example is height, there’s a continuous distribution of gradations in height between limits of about 1 and 2 metres • Often modified by the environment Drosophila genetic notation • For Drosophila there is a ‘wild type’ • Mutant alleles are named after the new phenotype they cause • Dominant mutant alleles are uppercase • Recessive mutant alleles are lowercase • Normal alleles are indicated by adding ‘+’ o The normal allele of a Dominant mutation is recessive* Linked Genes • If two genes are linked they do not assort independently because the chromosome is inherited as a singl
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