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Lesson #1.7-Incomplete Dominance Codominance Dihybrid Crosses Exceptions to Mendel’s principles • So far, offspring have either the phenotype of one parent or the other. • Sometimes, there is no dominant or recessive gene, or the trait is controlled by many alleles or genes. Incomplete Dominance • Sometimes two alleles can be equally dominant; they interact to produce a new phenotype • The new phenotype in the offspring is a mix of the two parent phenotypes. Incomplete Dominance • A third (new) phenotype appears in the heterozygous condition Incomplete Dominance Incomplete Dominance Sample Problem 1: • Flower color in snapdragons: Trait: colour in snapdragons CR = red CR CR = red CW = white CW CW = white CR CW = pink Problem: Incomplete Dominance • Show the cross between a red and a white flower. F1 PHENOTYPES: 100% pink F1 GENOTYPES: 100% CRCW CR CR CW CR CW CR C W CW CR CW CR C W Codominance • The new phenotype in the offspring is a combination of the two parent phenotypes (i.e. both alleles are expressed at the same time) • The heterozygous condition, both alleles are expressed equally Sample Problem 1: Sickle Cell Anemia in Humans Trait: Red Blood Cell (RBC) shape BNBN = normal BSBS = sickle BNBS = cells cells normal and sickle Problem: Codominance • Show the cross between an individual with sickle-cell anemia and another who is a carrier but not sick. N S B B BNBS x BSBS BS BNBS BSBS F1 PHENOTYPES: 50% normal and sickle 50% sickle F1 GENOTYPES: 50% BNBS 50% BSBS BS BNBS BSBS Codominance • In codominance, neither allele is dominant; both are expressed. A cross between organisms with two different phenotypes produces offspring with has both phenotypes of the parental traits shown. • Both alleles contribute to the phenotype 2. Codominance – Example: In come chickens Black Chicken x White Speckled Chicken Roan Horse http://search.vadlo.com/b/q?rel=2&keys=Dominance+Incomplete+Dominance+Codominance+PPT YOU tell me which type of dominance… Codominance! Type of Dominance? Incomplete Dominance! Type of Dominance? Incomplete Dominance! Dihybrid crosses are those where we consider the inheritance of two characteristics at the same time. • Mendel had observed that the seeds of his pea plants varied in several ways – among them, whether the peas were round or wrinkled and whether they were green or yellow. • He decided to look at how both traits were inherited together…. It appears that the inheritance of seed shape has no influence over the inheritance of seed colour The two characters are inherited INDEPENDENTLY This follows Mendel’s Law of Independent Assortment There are four possible combinations of the two characteristics. Peas can be: 1) round and yellow 2) round and green 3) wrinkled and yellow 4) wrinkled and green From his monohybrid trials, Mendel had discovered that the allele for yellow colour was dominant to that for green. He had also found that the allele for round peas was dominant to the allele for wrinkled peas. Sample Problem 1: Two heterozygous pea plants are crossed for two traits: pea colour & pea texture. (Yellow is dominant to green and round is dominant to wrinkled) Trait: Pea shape and colour R = Round r= wrinkled Y = Yellow y = green DO NOT COPY PARENT: Phenotype Round, yellow Genotype RRYY Gametes F1 RY, RY, RY, RY all X Wrinkled, green X rryy X ry, ry, ry, ry RrYy All of the F1 generation would be heterozygous for both characteristics, meaning that they would all be round and yellow. As with monohybrid crosses, he then crossed two of the F1 generation together… Sample Problem 1: Two heterozygous pea plants are crossed for two traits: pea colour & pea texture. (Yellow is dominant to green and round is dominant to wrinkled) Phenotype: Round, yellow Genotype: RrYy Gametes: RY, Ry, rY, ry F2 X Round, yellow RrYy RY, Ry, rY, ry We really need a punnet square for this… RY RY Ry rY ry Ry rY ry RY Ry rY ry RY Ry rY ry RRYY RRYy RrYY RrYy RRYy RRyy RrYy Rryy RrYY RrYy rrYY rrYy RrYy Rryy rrYy rryy 9 : 3 : 3 : 1 Round Round Wrinkled Wrinkled Yellow Green Yellow Green This is the typical ratio expected in a dihybrid cross. This shows that traits are inherited INDEPENDENTLY!
