Question

When can two recessive alleles cause a genetic defect?

Answer 1

Recessive traits can only surface when they are homologous pairs.

Answer 2

recessive alleles can cause a genetic defect when they arise in the same gene leading to wiping out of the protein they make ie homozygous recessive condition.

In a heterozygous condition where one allele is recessive ( makes no protein) and the other is dominant a genetic defect doesn't arise because the dominant allele makes sufficient amount of the protein to compensate for the loss by the recessive allele. In the case of a homozygous recessive condition since both the alleles are recessive and no protein synthesis occurs at all it leads to a genetic defect.