Answer
The birth defects of inbreeding are caused by deleterious alleles that are recessive. Because inbreeding is breeding with your own kin, the offspring will have homozygous recessive alleles, and will thus demonstrate the defects. However if two inbred parents conceive the offspring will be heterozygous, as they will have the recessive alleles of both parents but also the dominant.
Work Step by Step
All organisms possess both dominant and recessive alleles, with the recessive alleles typically being deleterious. A deleterious allele is one that carries a trait that would effect the organism negatively and is essentially a mutation that, although the organism possesses the trait, it is not demonstrated because it is recessive.
However with inbreeding, the deleterious alleles become prominent because the offspring will be homozygous. The offspring of two inbred parents will be healthy because they are heterozygous. As the offspring is not being bred with its kin it will not have the two recessive alleles from both parents, and therefore will not demonstrate the defect. The heterozygous offspring will have inherited both the dominant and recessive alleles of both parents, resulting in the deleterious allele not being prominent.
