In which inheritance pattern must two copies of an abnormal gene be present for the disease to develop?

The inheritance pattern where two copies of an abnormal gene must be present for the disease to develop is known as autosomal recessive inheritance. In this scenario, an individual must inherit one defective allele from each parent to express the disease phenotype. Since each parent contributes one copy of their genes, an individual with only one abnormal allele (carriers) will not exhibit the disease, as the dominant normal allele will overshadow the recessive one.

This pattern of inheritance is characteristic of numerous genetic disorders, such as cystic fibrosis and sickle cell anemia, where individuals with the condition typically have a 25% chance of being affected if both parents are carriers of the recessive allele. The requirement for two copies of the gene makes autosomal recessive conditions less common in populations, as it necessitates that both parents are carriers for a child to be affected.