Here's why:

* Heterozygous individuals have one copy of the normal allele and one copy of the recessive allele. They typically don't show the harmful trait because the normal allele masks the effects of the recessive one.

* Homozygous recessive individuals have two copies of the recessive allele and therefore express the trait. While they contribute to the spread of the allele, they are relatively less common in the population compared to heterozygotes.

This is a key concept in understanding genetic disorders. Since heterozygotes are carriers and don't exhibit the disorder themselves, they can unknowingly pass on the recessive allele to their offspring. This is why genetic testing and counseling are important, especially for families with a history of genetic disorders.