| Actually, it is our perception of what recessive traits are that is changing. At one point, the scientific community believed that the recessive trait was not manifested in a person who was heterozygous for the particular trait.
As the knowledge of genetics has been expanding, also has our knowledge of the role of recessive genes. Generally speaking, most scientists agree that the recessive trait has some manifestations, even in the presence of the dominant trait.
For example, sickle cell anemia (SCA) is a recessive disease. If an individual has both the dominant trait and the recessive trait for SCA, the recessive trait still plays an active role in the person's life. It has been shown that the recessive trait for SCA helps an individual survive malaria. Although the trait does not prevent malaria, it allows it to be treated more easily and lessens the severity of the disease.
Recessive genes for Cystic Fibrosis are another example of those genes that still manifest themselves in the presence of a dominant gene of the same type. The recessive trait for Cystic Fibrosis helps a person fight cholera.
With the new developments in genetics, it is becoming more obvious that recessive traits have a more important role than we have previously given them credit for.
| 
