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Mendelian Exceptions Mendel got lucky – all 7 traits he studied showed complete (simple) dominance. Mendelian 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 Dominance Ex: flower color R = red r = white RR x rr Incomplete Dominance F2: Rr x Rr Incomplete Dominance F2: Rr x Rr 1 red 2 pink 1 white RR Rr Alleles not blended – still able to separate. Rr rr Codominance – Both alleles are expressed equally 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 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 Genotype Phenotype Alelles = A, B, O A & B = Dominant O = Recessive AA Blood Types (ABO blood types) Example of simple dominance, codominance, and multiple alleles Genotype Phenotype Alelles = A, B, O A & B = Dominant O = Recessive AA A Blood Types (ABO blood types) Example of simple dominance, codominance, and multiple alleles Genotype Phenotype Alelles = A, B, O A & B = Dominant O = Recessive AA AO A Blood Types (ABO blood types) Example of simple dominance, codominance, and multiple alleles Genotype Phenotype Alelles = A, B, O A & B = Dominant O = Recessive AA AO A A Blood Types (ABO blood types) Example of simple dominance, codominance, and multiple alleles Genotype Phenotype Alelles = A, B, O A & B = Dominant O = Recessive AA AO BB A A Blood Types (ABO blood types) Example of simple dominance, codominance, and multiple alleles Genotype Phenotype Alelles = A, B, O A & B = Dominant O = Recessive AA AO BB A A B Blood Types (ABO blood types) Example of simple dominance, codominance, and multiple alleles Genotype Phenotype Alelles = A, B, O A & B = Dominant O = Recessive AA AO BB BO A A B Blood Types (ABO blood types) Example of simple dominance, codominance, and multiple alleles Genotype Phenotype Alelles = A, B, O A & B = Dominant O = Recessive AA AO BB BO A A B B Blood Types (ABO blood types) Example of simple dominance, codominance, and multiple alleles Genotype Phenotype Alelles = A, B, O A & B = Dominant O = Recessive AA AO BB BO AB A A B B Blood Types (ABO blood types) Example of simple dominance, codominance, and multiple alleles Genotype Phenotype Alelles = A, B, O A & B = Dominant O = Recessive AA AO BB BO AB A A B B AB (codom.) Blood Types (ABO blood types) Example of simple dominance, codominance, and multiple alleles Genotype Phenotype Alelles = A, B, O A & B = Dominant O = Recessive AA AO BB BO AB OO A A B B AB (codom.) Blood Types (ABO blood types) Example of simple dominance, codominance, and multiple alleles Genotype Phenotype Alelles = A, B, O A & B = Dominant O = Recessive AA AO BB BO AB OO A A B B AB (codom.) O Q: What combination of parents have an equal chance of having any of the 4 blood types? Q: What combination of parents have an equal chance of having any of the 4 blood types? Mexican Hairless Dogs Hairless (H) is completely dominant over hairy (h). When two hairless are crossed: 2/3 hairless 1/3 hairy Why??? Mexican Hairless Dogs Hairless (H) is completely dominant over hairy (h). When two hairless are crossed: 2/3 hairless 1/3 hairy Lethal Allele 2 copies of a lethal allele = death (usually stillborn) Hint: if offspring ratio is x/3, think lethal allele (someone is dying) Lethal Allele Q: How can we breed hairless dogs without stillborns? 3, 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? 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 Rule Make a simple 4 square cross for each trait, then multiply the results for each 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: Hair Color 2 genes are responsible for hair color: black-brown gene red-blonde gene Example: Hair Color 2 genes are responsible for hair color: black-brown gene red-blonde gene Black-brown is epistatic (dominant) over the red-blonde gene Two “exceptions” present – incomplete dominance and epistasis B = black b = brown -> controls -> R = red r = blonde B = black b = brown BB__ = -> controls -> R = red r = blonde B = black b = brown BB__ = black -> controls -> R = red r = blonde B = black b = brown BB__ = black Bbrr = -> controls -> R = red r = blonde B = black b = brown BB__ = black Bbrr = brown -> controls -> R = red r = blonde B = black b = brown BB__ = black Bbrr = brown BbR_ = -> controls -> R = red r = blonde B = black b = brown BB__ = black Bbrr = brown BbR_ = auburn -> controls -> R = red r = blonde B = black -> controls -> R = red b = brown r = blonde BB__ = black bbRR = Bbrr = brown BbR_ = auburn B = black -> controls -> R = red b = brown r = blonde BB__ = black bbRR = red Bbrr = brown BbR_ = auburn B = black -> controls -> R = red b = brown r = blonde BB__ = black bbRR = red Bbrr = brown bbRr = BbR_ = auburn B = black -> controls -> R = red b = brown r = blonde BB__ = black bbRR = red Bbrr = brown bbRr = strawberry blonde BbR_ = auburn B = black -> controls -> R = red b = brown r = blonde BB__ = black bbRR = red Bbrr = brown bbRr = strawberry blonde BbR_ = auburn bbrr = B = black -> controls -> R = red b = brown r = blonde BB__ = black bbRR = red Bbrr = brown bbRr = strawberry blonde BbR_ = auburn bbrr = blonde B = black -> controls -> R = red b = brown r = blonde BB__ = black bbRR = red Bbrr = brown bbRr = strawberry blonde BbR_ = auburn bbrr = blonde = breeds true I have brown hair and my wife has auburn hair. Our first son has strawberry blonde hair and our second has blond hair. If we had a third child, what would its hair color possibilities be and what would be the probability of each?
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