Final answer:
Incomplete dominance is the genetic cross where a blend of traits from both parents is expressed, like the blend of red and blue alleles leading to purple flowers in certain plants.
Step-by-step explanation:
When a red flower blends with a blue flower to make a purple flower, this type of genetic cross is called incomplete dominance. The term refers to a situation in which the phenotype of the heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele. For instance, in the case of the snapdragon, Antirrhinum majus, crossing a plant with homozygous red flowers (CRCR) with one with homozygous white flowers (CWCW) results in offspring with pink flowers (CRCW), demonstrating incomplete dominance, as the flowers are neither completely red nor white, but a blend of both, hence purple in this scenario.
In terms of genetics, this means that neither the red nor the blue allele is completely dominant over the other. When red (R) and blue (B) alleles are present together (RB), the flowers display a mixed purple color. Therefore, the genotypic ratio from a self-cross of heterozygotes in incomplete dominance follows a 1:2:1 pattern, which translates to a phenotypic ratio for red:pink:blue or red:purple:blue, depending on the alleles involved.