How Genes Work Together. Puppies receive copies of their parents' genes when they're conceived. In simple inheritance, each puppy gets two genes for each trait, one gene from each parent and these genes function in pairs. Every living creature has some defective genes. Genes can mutate or may be inherited as defective from a parent. But because genes function in pairs, having a single normal gene paired with a defective gene typically does not result in a genetic disease. Instead, when a defective gene is paired with a normal gene the effect of the defective gene is usually canceled out and the normal gene prevails. In most cases, as long as one of the pair of genes that produces a trait is normal, the characteristic that the gene governs will be normal. Both genes must be defective to produce a genetically-based disease. For example, if a puppy has one defective gene (from one parent) and one normal gene (from the other parent) that control inherited deafness, he won't be deaf. But if the puppy inherits two defective genes (one from each parent) he'll have congenital deafness. In cases where puppies have one defective gene and one normal gene, the defect typically won't affect the puppy himself. However, with one defective gene, the puppy becomes a carrier of the defective gene and the disease. He can pass on either the defective gene or a normal gene to future generations. Its offspring can inherit his normal gene or his defective gene; it's a matter of random chance. This means that if a puppy inherits one of its parent's defective gene, and this puppy is later bred to another dog with one defective gene for the same trait, their puppy can inherit the related health problem, even though neither of the parents had the disease. This is because each parent was a carrier of the defective gene and each could pass on one defective gene to its offspring. If their offspring inherited one defective gene from each parent, this would result in the offspring having a pair of defective genes which would result in the puppy inheriting the disease. In cases where great genetic diversity exists, having a few defective genes generally don't cause medical problems. Both parents are likely to have different defective genes, so each defective gene is offset by another, normal gene and the puppy displays no medical problems. This is one reason it's so beneficial to have a large gene pool. In the case of purebred dogs that have been inbred for generations, the loss of genetic diversity means that the genes in both parents are very similar, perhaps identical. If both parents have a pair of the same defective genes, the puppy cannot inherit a 'normal' genes, so it has no choice but to develop defective gene-related health issues.