Complete and Incomplete Dominance

In the example where the red flower could have the genotype AA or Aa, the Aa genotype shows that the A allele exhibits complete dominance over the white allele because only the allele for red flowers (A) is expressed in the phenotype.

However, Mendel also observed what is called incomplete dominance.  Incomplete dominance is when the progeny exhibits an intermediate phenotype (one that is an intermediate between the two parents.

For example, a cross resulting in intermediate phenotypes between to homozygous true breeding parents for two different flower colors: AA (red)  x   aa  (white)

The resulting Progeny (if incomplete dominance):  The phenotype would be a pink flower (dark or light pink, but not white or red).



Below, I am utilizing the Genetics textbook by Pierce to support the information that I'm providing you.

 

 

 

Here is the answer from Chegg's Cramster on section 5.1 comprhension questions:

1. What is the difference between complete dominance and incomplete dominance?
2. What is incomplete penetrance and what causes it?

However, they don't really answer #2. So, here's my answer:

Penetrance is defined as the percentage of individuals with a specific genotype that express the phenotype expected from having that genotype. Incomplete Penetrance is the term geneticists use to describe the circumstance where the genotype does not produce the expected phenotype in all individuals of a population. Incomplete penetrance and variable expressivity are as a result of the effects of the environment and other genes that may alter or mask the expression of a certain gene.