An example for this condition is sickle-cell anaemia. The person with homozygous condition for sickle-cell anaemia does not survive beyond the age of 20 years.
The individual with normal gene (HbHb) does not have, sickle-cell anaemia but they are sensitive towards malaria.
The person with heterozygous condition for sickle-cell anaemia does not develop both sickle-cell anaemia and malaria.
Due to abnormal haemoglobin, the malarial parasite will not harm the person.
Hence, in this case, the heterozygous condition has more survival chances, an advantage with respect to the homozygous individuals.