3/19/12 Objective: Understand codominance and incomplete dominance Warm-Up: In pea plants, purple flowers are dominant to white flowers. Two plants are crossed and produce 800 offspring. Of the 800 offspring, 584 have purple flowers and 216 have white flowers. What are the most probable genotypes of the parents?
Codominance So far, the traits we have worked with follow a complete dominance pattern Traits with complete dominance are uncommon Codominance is a more common inheritance pattern In codominance, multiple alleles are distinctly expressed in the phenotype This means that heterozygotes have a unique phenotype
Blood Type: Phenotypes Human blood type is determined by the presence of surface proteins (called immunoglobins) on red blood cells Possible phenotypes: Type A: Individual has type A immunoglobin Type B: Individual has type B immunoglobin Type AB: Individual has both A and B immunoglobin Type O: Individual has no immunoglobin
Blood Type: Genotypes There are three alleles: A and B are codominant, and O is recessive An individual can only be type O if he or she has two O alleles Type A: AA, AO Type B: BB, BO Type AB: AB Type O: OO The heterozygote AB has a unique phneotype
Practice A man with type A blood and a woman with type B blood have a child with type O blood. Explain how this happened, using a Punnett Square to support your explanation. If a woman with type AB blood has a child with a man who is type O, will any of the children have the same blood type as one of the parents? Why or why not? You may support your answer with a Punnett Square.
Incomplete Dominance In incomplete dominance, the heterozygote has a phenotype that is “in between” the two alleles Example: In snapdragon flowers, flowers can be red, white, or pink. If a red-flowering plant is crossed with a white-flowering plant, all of the F1 will be pink.
R R RW RW W RW W RW R=allele for red; W=allele for white RR = red flowers; RW = pink flowers; WW = white flowers
This isn’t blending, though… If the F1 self-fertilizes: R W R RW RR RW W WW