Cumulative Effects of Inbreeding. Some inbreeding-related health issues can be immediately apparent, such as hip dysplasia, or blindness. Other inbreeding-related health issues take generations to become evident, but can ultimately have a measurable effect on your dog. With some body systems and genes, it isn't just one gene that controls the way that the body system functions, but a series of genes. When a series of genes control how the system develops and functions, one defective gene generally doesn't cause a complete failure of the system. Instead, it causes a slight decline in the overall performance of the body's affected organ or system. The organ or body system doesn't fail, it just doesn't perform as well as in dogs without the defective genes. Over time, these small declines in genetic well-being have a cumulative effect. They add up. If a dog with a defective gene that causes his liver to function 2% less efficiently mates with another dog with the same defective gene that causes the liver to function 2% less efficiently, the offspring's liver may function 3% less efficiently. The effect of the defective genes accumulate. Over time, this translates to measurable health problems. This slow decline in the body's efficiency can lead to a weaker immune system, shorter life spans, chronic infections, and behavioral abnormalities, including shyness and aggression. This process is called inbreeding depression and can lead to chronic health issues or serious temperament problems. Inbreeding vs. Linebreeding. In some cases, breeders deliberately inbreed their dogs. They do this to ensure that certain characteristics breed reliably, and the fastest way to do this is to deliberately breed close relatives who have very similar genes. In many cases, breed-specific inbreeding is deliberate inbreeding planned by breeders. However, many breeders attempt to distance themselves from the implications of inbreeding by referring to their breeding practices as 'linebreeding'. Linebreeding is used to refer to breeding related dogs, but not dogs with the closest relationship. For example, breeders might call breeding brother to sister inbreeding, but grandfather to granddaughter 'linebreeding'. Scientifically speaking, linebreeding is a meaningless term. Genetically, breeding close family members still results in inbreeding, even if it's not the closest possible relationship. These related family members all carry much of the same limited gene pool. By breeding them to one another, breeders increase the likelihood disease-producing genes will be more common in the puppies they produce. Linebreeding is simply a marketing term designed to distance breeders from the term 'inbreeding'. To understand this further, let's discuss how genes work.