Even in simple Mendelian genetics, an offspring’s traits can be hard to predict. For example, two parents that do not exhibit the sickle cell disease trait can produce an offspring with sickle cell disease. This indicates that the sickle cell trait must be what kind of trait?
The sickle cell trait must be a recessive trait. In order for an offspring to exhibit sickle cell disease, they must inherit two copies of the recessive allele for sickle cell from both parents. Even if both parents do not exhibit the disease themselves, they can still be carriers of the recessive allele and pass it on to their offspring.
