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BMS2042 (2)
Lecture 3

Lecture 3 and 4 notes

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Monash University
Biomedical Sciences
Peter Boag

Lecture 3 notes: Readings: Chapter 4, pp221-230, 94-95, 519-523 In Mendel’s studies: - Single gene effects - 2 alleles only Not always simple, transmission of many traits don’t produce Mendelian ratios: - Inheritance of many traits more complex - Extensions to Mendel, the laws still apply but different ratios - For non Mendelian inheritance, Mendel’s laws are not followed Extensions to Mendel for single genes: Mendel saw complete dominance – in heterozygote only one allele is expressed. Due to 50% gene product being enough for normal phenotype Sometimes one allele not completely dominant over the other: Incomplete dominance: heterozygote has phenotype intermediate between two homozygotes. Co-dominance – heterozygote exhibits both the phenotypes of the two homozygotes For both of these you see 1:2:1 F2 Phenotypic ratio (i.e. not Mendelian 3:1) (From Klug) In this example, the genotype C C IS NOT enough for a red phenotype or white phenotype and so this is an incomplete dominance pattern where the f1 generation is intermediate of the two colours producing pink. - If you then cross the two pink heterozygous genotypes, you will get a genotypic AND phenotypic ratio in f2 of 1:2:1 - This only happens in incomplete dominance, in complete dominance: genotypic and phenotypic ratios do not coincide Co-dominance: Both alternative traits are visible in heterozygote; neither phenotype is dominant over the other. - Genotypic and phenotypic ratios of F2 coincide: 1:2:1 - E.g. ABO blood system - Lethal alleles: - Lethal allele is one that has potential to cause death in an organism - A lethal allele can produce ratios that deviate from Mendelian ratios - In mice yellow colour pattern is recessive lethal. Homozygote yellow = death (not born) o In homozygote, no protein results in lethality; therefore it is an essential gene. NB: this gene is pleiotropic – one gene affects multiple traits, a visible phenotype and also viability. Penetrance and Expressivity: Sometimes individuals carrying a dominant allele do not exhibit the trait – called incomplete penetrance Penetrance: proportion of individuals of specified genotype that reveal the expected phenotype Expressivity: degree of expression of phenotype in individual of specified genotype E.g. polydactyly Sex-influenced traits: - Gender can govern the inheritance pattern for certain traits. An allele is dominant in one sex and recessive in the other! - E.g. pattern baldness – autosomal trait
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