What are three exceptions to Mendel's observations?

The three exceptions to Mendel's observations are:

Law of co-dominance, Law of incomplete dominance, and Pleiotropy.

1. Law of Co-dominance:

1. In co-dominance heterozygotes express the phenotype of both parents e.g. ABO blood groups and sickle cell anemia.
2. In co-dominance, there is an independence of allele function.
3. Neither allele is dominant or partially dominant over the other.

2. Law of incomplete dominance:

1. One allele is not completely dominant to another. F1 is intermediate between both parents.
2. The phenotype of the heterozygote is intermediate between the phenotypes of homozygotes for each allele.
3. Example - flower color in the four o'clock plant (Mirabilis jalapa).

3. Pleiotropy:

1. The term pleiotropy is derived from the Greek word “pleio”, which means “many”, and tropic, which means “affecting”.
2. Genes that affect multiple, apparently unrelated, phenotypes are thus called pleiotropic genes.
3. Example of pleiotropy in humans in phenylketonuria.