Charles Darwin observed that changes continuously occur in populations and that these changes are critical to the success of the species. The changes Darwin noted were caused by genetic mutations, i.e., a change in the genetic code from what previously existed. Though Darwin was focused on the advantageous changes brought about by mutations, we now know that the majority of mutations in nature are not advantageous. In fact, they tend to hinder (be deleterious to) a population's success -- and a few of the mutations are quite harmful (e.g., dwarfism) or even lethal (e.g., hydrocephalus). Mutations of an extremely deleterious nature are often referred to as genetic defects or abnormalities. 

The Punnet Square is the most commonly used visual aid to explain the offspring of recessive abnormality carrier parents. (A) displays the results if both parents are carriers. (B) shows what happens if only one parent is a carrier.