So far we have talked about simple genetics problems, where one trait is dominant and one trait is recessive and the trait only has 2 alleles. This is not how everything works. Some traits are incomplete dominance, where they mix together in the heterozygous form, or co-dominance where both traits show through, or have multiple alleles as in your blood types.
Below are the notes we took in class. After reading through the notes, watch the video to help you understand incomplete and co-dominance.
Some traits have more than 2 alleles. A great example is blood types. Although there are 3 alleles for blood type, we still only get 2, one from Mom and one from Dad. You can have type B, type A, type AB, or type O. A and B are co-dominant, while O is recessive.
GenotypesPhenotypes AA, AO Type A BB, BO Type B AB Type AB OO Type O
Ex: Cross a parent heterozygous for type A blood, with a parent that is type AB blood.
Practice Problems
1. Height is incomplete dominance. Cross two heterozygous plants. What is the percent of genotypes and phenotypes possible?
2. Cross two heterozygous tan cows. Give percent of genotypes and phenotypes. BB= Black, BH = Tan, HH = White
3. Coat color in mice is incompletely dominant. Yellow and white-colored mice are homozygous, while cream-colored mice are heterozygous. If a cream-colored mouse mates with a yellow-colored mouse, what are the possible phenotypes and genotypes?
4. In radishes, red and white are homozygous, while hybrids are purple. If a red radish is crossed with a white radish what will be the phenotypes of the offspring?
5. In snapdragons, flower color is controlled by incomplete dominance. The two alleles are red (R) and white (r). The heterozygous genotype is expressed as pink. a. What is the phenotype of a plant with the genotype RR? b. What is the phenotype of a plant with the genotype rr? c. What is the phenotype of a plant with the genotype Rr?
6. A man is homozygous for the type B blood, and a woman is type O blood. What are the possible blood types of their baby?
7. In shorthorn cattle, when a red bull (RR) is crossed with a white cow (HH), all the offspring are roan -- a spotted, red, white or milky red color. What offspring are expected from mating a roan bull and a white cow?
8. What phenotypes would you expect from a cross between a red bull and a white cow?
9. If a roan bull were crossed with a red cow, what would be the possible phenotypes of their offspring?
10. Cross a parent with heterozygous type B blood and a parent with heterozygous A blood, what is the probability that their offspring would have type O blood?