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Transcript
Exceptions to Mendelian Inheritance Mendelian Inheritance means… • one gene = one trait • 2 different alleles (versions) for a gene • one dominant and one recessive allele (heterozygous = dominant phenotype) • segregation • independent assortment • genotype determines phenotype Exceptions to Mendel • • • • • • • Linkage X-linked traits Incomplete dominance Codominance Multiple Alleles Polygenic Traits Environmental Influences Incomplete Dominance • Neither allele is dominant • Heterozygous phenotype is intermediate Example: Flower color in snapdragons CWCW=white CRCR=red CRCW=pink Codominance • Both alleles are dominant • Heterozygous phenotype displays both traits Example: Coat color in horses HRHR=red HWHW=white HRHW=roan (red and white) Multiple Alleles • More than 2 alleles for a gene Example: Human Blood Types Multiple Alleles – Blood Types • Alleles: IA (dominant), IB (dominant), i (O, recessive) • Genotypes: IA IA, IA i, IB IB, IB i, IAIB, ii • What phenotype is produced by each of these genotypes? Polygenic Traits • Traits that are determined by multiple genes produce a range of phenotypes on a bellshaped continuum (applies to most human traits). Example: Human Skin Color Modes of Selection Environmental Influences Environmental factors can influence an individual’s phenotype (without changing the genotype). 1. A cross between a black cat (CB) and a tan cat (CT) produces a tabby pattern (black and tan colors together). a. What type of inheritance does this illustrate? b. What is the probability of producing a tabby kitten if a tabby cat is crossed with a tan cat? 2. In humans, straight and curly hair textures demonstrate incomplete dominance. The heterozygous individuals have wavy hair. The alleles are HC and HS. A woman and her husband who both have wavy hair desire a child with curly hair. What are the chances that their first child will have curly hair? Show the Punnett square. 3. A man is suing his wife for divorce on grounds of infidelity (he thinks she is having an affair). Their 1st and 2nd children, whom they both claim, have blood types O and AB, respectively. The 3rd child, whom the father disclaims, is blood type B. Could he be the father of the third child? Show the Punnett square.