Exceptions to Mendel’s principles: • Phenotype can be affected by more than genotype. • Not all genes show a pattern of dominance and recessiveness. • For some genes, there are more than two alleles. • Many times, traits are controlled by more than one gene. Incomplete Dominance or Nondominance All traits are not fully dominant or recessive traits. Some genes appear to: blend together. For example: In some flowers, such as snapdragons, a homozygous red flower crossed with a homozygous white flower yields a heterozygous pink flower. This is known as incomplete dominance or nondominance. In this case, no allele is dominant or recessive they blend together in the offspring. Since there is no recessive allele, use only capital letters. For example: A red flower would be RR, a white flower would be WW and the pink hybrid would be RW. RR R R W RW RW W RW RW WW RW Notice there is no Dominant trait here and so there is a blending of both traits in the F1 generation. This is referred to as Incomplete Dominance 4 RW 4 Pink Or Or 100% RW 100% Pink What type of offspring might be produced by two pink flowering plants? What are the genotypes of the parents? R R W RR RW W RW WW 1 RR 2 RW 1 WW 1 Red 2 Pink 1 White RR R WW RW R W RW RW W RW RW In this scenario each Allele is going to be of equal strength (there is no complete dominance and also no blending). With no dominance here we see an equal distribution of both Alleles in the phenotype: The cow has red hairs and white hairs Do the following cross using codominance A blue bird is crossed with a red bird R R B BR BR B BR BR B R B BB BR R BR RR F1= 4 BR / 100% BR 4 BLUE AND RED BIRDS/ 100% BLUE AND RED BIRDS F2= 1 BB 2 BR 1 RR 1 BLUE 2 BLUE AND RED 1 RED Do the following cross using codominance A BLUE FISH is crossed with a YELLOW FISH Y Y B BY BY B BY BY B Y B BB BY Y BY YY F1= 4 BY / 100% BY 4 BLUE AND YELLOW FISH/ 100% BLUE AND YELLOW FISH F2= 1 BB 2 BY 1 YY 1 BLUE 2 BLUE AND YELLOW 1 YELLOW In humans, four blood types are possible: A, B, AB, and O There are three alleles that determine blood type. These three alleles are written as follows: IA, IB, and i. Alleles IA and IB are codominant, and the allele “i” is recessive. Codominance: Both dominant alleles are seen in the phenotype of the heterozygous offspring. IA i IB IB IB i IA IB ii IA IA What types of offspring might be expected if one parent has type AB blood and the other parent is heterozygous for type A blood? What is the genotype of the first parent? What is the genotype of the second parent? IA IB IA IA IA IA IB i IA i IB i IA IA IA IB IA i IB i 2 with Type A blood 1 with Type AB blood 1 with Type B blood What types of offspring might be expected if one parent is heterozygous for type B blood and the other parent is heterozygous for type A blood? What is the genotype of the first parent? What is the genotype of the second parent? IB i IA IA IB IA i i IB i ii IA IB IA i IB i ii 1 with Type AB blood 1 with Type A blood 1 with Type B blood 1 with type O blood What types of offspring might be expected if one parent is type AB blood and the other parent is heterozygous for type B blood? What is the genotype of the first parent? What is the genotype of the second parent? IB IA IB i IA IB IA i IB IB I Bi IA IB IA i IB i IB IB 1 with Type AB blood 1 with Type A blood 2 with Type B blood
