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

C W C W =white C R C R =red C R C W =pink

Codominance Both alleles are dominant Heterozygous phenotype displays both traits Example: Coat color in horses

H W H W =white H R H R =red H R H W =roan (red and white)

Multiple Alleles More than 2 alleles for a gene Example: Human Blood Types

Multiple Alleles – Blood Types Alleles: I A (dominant ), I B (dominant), i (O, recessive) Genotypes: I A I A, I A i, I B I B, I B i, I A I B, 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 bell- shaped 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 (C B ) and a tan cat (C T ) 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 H C and H S. 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.