Recessive traits do not express their effect in the presence of dominant traits. How is this fact useful in overcoming hereditary diseases in the families?

Recessive traits are usually resultant of alternation in a normal allele due to mutation. In such case, normal trait becomes dominant and recessive trait expresses itself in the form of the disease.

Each individual has two alleles for a trait and only if both alleles are recessive, the recessive trait is expressed and disease is seen. Hence, normal dominant allele prevents hereditary disease and only in absence of dominant allele i.e. in both recessive allele condition, the disease is seen.

Also, during gamete segregation alleles are also segregated in such a way that each gamete receives one of the alleles. So, even if an individual with the disease has both recessive alleles, it can only transfer one allele to progeny. Disease will be seen in progeny only if other parent also shows disease due to the presence of both recessive genes as the second allele from this parent will be also recessive.

In a similar manner, if both parents are heterozygous, there is a resultant possibility of the disease being absent in further generation.

In such a way, hereditary disease is overcome in families.