Lecture 3 BIO 311D 2nd Edition Outline of Last Lecture I Meiosis ploidy and life cycles II Sources of genetic variability in meiosis III Applying meiosis to Mendelian inheritance Outline of Current Lecture I Mendelian Inheritance Current Lecture I Mendelian Inheritance A Transmission one gene locus controls the trait only two possible alleles per gene locus B Types of Dominance 1 Simple complete dominance 2 Incomplete dominance The heterozygote has a phenotype unlike either homozygote i e pink snapdragon For example when two palominos mate they can produce offspring colors of chestnut homozygous RR cream homozygous R R or palomino heterozygous R R 3 Co dominance The ABO blood group alleles are examples of multiple alleles 3 possible at this gene locus and codominant alleles both fully expressed in the phenotype Genotype Phenotype IAi or IAIA Type A IBi or IBIB Type B IAIB Type AB ii Type O C Genotypes the two alleles an individual has for that gene locus Hint Use the same letter of the alphabet for alleles at any given gene locus For example Homozygous AA Use capital letter if simple dominance Homozygous aa Heterozygous Aa Homozygous A A Not simple dominance Heterozygous A A D Practice Problems 1 In the mating cross Rr X Rr what proportion of the offspring will have the genotype rr 1 4 2 In the mating cross Tt X TT what proportion of the offspring will have the genotype tt 0 3 A mouse shows the dominant black phenotype but has unknown genotype To do a test cross you would mate this individual with any brown mouse E Recessive Traits defined at the molecular level as loss of function mutations a missing or nonfunctional protein single gene locus 1 Hypercholesterolemia lack of LDL receptor protein 2 Cystic fibrosis Lack of CI ion transmembrane regulator protein In humans cystic fibrosis is inherited as a simple recessive trait A normal woman without cystic fibrosis who carries the gene i e she is heterozygous marries a normal man who does not carry the cf allele Child would not have CF 3 Phenylketonuria PKU missing a critical enzyme PKU symptoms due to lack of tyrosine and excessive phenylpyruvic acid and its breakdown products 4 Albinism is also a recessive trait F Heterozygous Traits 1 Achondroplasis dwarfism is heterozygous condition Homozygous dominant condition is embryonic lethal Molecular basis overactive fibroblast growth factor receptor gene FGF Receptor 3 which leads to premature differentiation from cartilage into bone
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