Mendelian ExceptionsMendelian Exceptions Mendel got lucky all 7 traits he studied showed complete (simple) dominance. Mendelian ExceptionsMendelian Exceptions Mendel got lucky all 7 traits he studied showed complete (simple) dominance. One allele is completely dominant over the other allele Homozygous dominant and heterozygous = same phenotype 1 allele is enough for full expression of dominant trait Incomplete Dominance = the norm Heterozygote = intermediate phenotype 1 allele is not enough for full expression Incomplete DominanceIncomplete Dominance Ex: flower color R = red RR x rr r = white Incomplete DominanceIncomplete Dominance F 2 : Rr x Rr Incomplete DominanceIncomplete Dominance F 2 : Rr x Rr Alleles not blended still able to separate. RR Rr rr 1 red 2 pink 1 white Codominance Both alleles are expressed equally Ex: Cows B = black W = white BB = Black WW = White BW = ??? Codominance Both alleles are expressed equally Ex: Cows B = black W = white BB = Black WW = White BW = Black and white Other examples: calico cats, streaked flowers Multiple allelesMultiple alleles Allele: Alternate forms of a gene Only 2 possible alleles in an individual, BUT any # of alleles may be present in a population due to mutations Blood Types (ABO blood types) Example of simple dominance, codominance, and multiple alleles Blood Types (ABO blood types) Example of simple dominance, codominance, and multiple alleles Alelles = A, B, O A & B = Dominant O = Recessive Blood Types (ABO blood types) Example of simple dominance, codominance, and multiple alleles Alelles = A, B, O A & B = Dominant O = Recessive Genotype Phenotype AA Blood Types (ABO blood types) Example of simple dominance, codominance, and multiple alleles Alelles = A, B, O A & B = Dominant O = Recessive Genotype Phenotype AAA Blood Types (ABO blood types) Example of simple dominance, codominance, and multiple alleles Alelles = A, B, O A & B = Dominant O = Recessive Genotype Phenotype AAA AO Blood Types (ABO blood types) Example of simple dominance, codominance, and multiple alleles Alelles = A, B, O A & B = Dominant O = Recessive Genotype Phenotype AAA AOA Blood Types (ABO blood types) Example of simple dominance, codominance, and multiple alleles Alelles = A, B, O A & B = Dominant O = Recessive Genotype Phenotype AAA AOA BB Blood Types (ABO blood types) Example of simple dominance, codominance, and multiple alleles Alelles = A, B, O A & B = Dominant O = Recessive Genotype Phenotype AAA AOA BBB Blood Types (ABO blood types) Example of simple dominance, codominance, and multiple alleles Alelles = A, B, O A & B = Dominant O = Recessive Genotype Phenotype AAA AOA BBB BO Blood Types (ABO blood types) Example of simple dominance, codominance, and multiple alleles Alelles = A, B, O A & B = Dominant O = Recessive Genotype Phenotype AAA AOA BBB BOB Blood Types (ABO blood types) Example of simple dominance, codominance, and multiple alleles Alelles = A, B, O A & B = Dominant O = Recessive Genotype Phenotype AAA AOA BBB BOB AB Blood Types (ABO blood types) Example of simple dominance, codominance, and multiple alleles Alelles = A, B, O A & B = Dominant O = Recessive Genotype Phenotype AAA AOA BBB BOB ABAB (codom.) Blood Types (ABO blood types) Example of simple dominance, codominance, and multiple alleles Alelles = A, B, O A & B = Dominant O = Recessive Genotype Phenotype AAA AOA BBB BOB ABAB (codom.) OO Blood Types (ABO blood types) Example of simple dominance, codominance, and multiple alleles Alelles = A, B, O A & B = Dominant O = Recessive Genotype Phenotype AAA AOA BBB BOB ABAB (codom.) OOO Q: What combination of parents have an equal chance of having any of the 4 blood types? Mexican Hairless DogsMexican Hairless Dogs Hairless (H) is completely dominant over hairy (h). When two hairless are crossed: 2/3 hairless 1/3 hairy Why??? Mexican Hairless DogsMexican Hairless Dogs Hairless (H) is completely dominant over hairy (h). When two hairless are crossed: 2/3 hairless 1/3 hairy Mexican Hairless DogsMexican Hairless Dogs Hairless (H) is completely dominant over hairy (h). When two hairless are crossed: 2/3 hairless 1/3 hairy HHHh hh Mexican Hairless DogsMexican Hairless Dogs Hairless (H) is completely dominant over hairy (h). When two hairless are crossed: 2/3 hairless 1/3 hairy HHHh hh Lethal AlleleLethal Allele 2 copies of a lethal allele = death (usually stillborn) Hint: if offspring ratio is x/3, think lethal allele (someone is dying) Lethal AlleleLethal Allele Q: How can we breed hairless dogs without stillborns? 3, 4, 5, etc. trait crosses3, 4, 5, etc. trait crosses If a pea plant that is TTRrPp is crossed with a plant that is TtrrPp, what percent of the offspring will be heterozygous for all three traits? Is there an easier way??? Product RuleProduct Rule Make a simple 4 square cross for each trait, then multiply the results for each PPPp pp Rr rr TT Tt Epistasis The process of one gene controlling the expression of another An epistatic gene can completely mask the effects of another gene 2 genes -> 1 trait Example: Lab Fur ColorExample: Lab Fur Color 2 genes are responsible for fur color: Black gene Brown-blonde gene Example: Hair ColorExample: Hair Color 2 genes are responsible for hair color: Black gene Brown-blonde gene Black is epistatic (dominant) over the red-blonde gene Polygenic InheritancePolygenic Inheritance Most traits (especially visible ones) are due to the interaction of multiple proteins, thus multiple genes As such, most traits do not follow simple Mendelian ratios