253
Genes are subject to random mutations that give rise to new alleles; thus many genes have more than two alleles within a population. Dominance is not necessarily an all-
learning outcomes
You should be able to:
Analyze genetic crosses involving incomplete dominance.
Explain the concept of codominance.
Distinguish between incomplete dominance, codominance, and pleiotropic effects.
What is the difference between codominance and incomplete dominance?
In codominance, in a heterozygote both alleles are evident in the phenotype. In incomplete dominance, in a heterozygote an intermediate phenotype between the parents is seen.
Explain how blood type AB results from codominance.
In blood type AB, both the A and B alleles are expressed. On the red blood cell surface, two types of proteins are seen.
In a particular plant species, two alleles control flower color, which can be yellow, green, or blue. Crosses of these plants produce the following offspring:
| Parental phenotypes | Offspring phenotypes (ratio) |
|---|---|
| Yellow × yellow | All yellow |
| Green × yellow | Green or yellow (1:1) |
| Green × blue | Blue or green (1:1) |
| Blue × blue | All blue |
What will be the phenotype, and ratio, of the offspring of a cross of green × green?
Yellow, blue, and white in a 1:2:1 ratio.
Thus far we have discussed phenotypic characters that are affected by single genes. In many cases, however, several genes interact to determine a phenotype. To complicate things further, the physical environment may influence which genes of an individual are expressed, meaning that phenotype is sometimes determined by both genes and the environment.