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Genetic testing does not just involve breeding animals. We see genetic disease in practice every day. Every nucleated cell in the body has the same DNA instruction manual. Cells in different tissues have different genes turned on and off to control their maturation and function. Genes get turned on and off, and their effects are mitigated based on the effect of other genes, diet, drugs, inflammation, infection, and surgery.

All genes come in pairs – one from the sire and one from the dam (see 3.1 Genetic Basics). Simple Mendelian genetic diseases are caused by the effects of only one gene pair. Dominant disease can be caused by one mutated gene in the pair, and recessive disease requires two mutated genes. Sex‐linked/X‐linked disease involves genes on the X chromosome. As males have only one X chromosome, any mutation on the X chromosome can cause disease. Females, with two X chromosomes, can be carriers of recessive disease with one copy of a mutation, or can be affected if they have two copies (see 3.2 Modes of Inheritance).

Complexly inherited disease, also referred to as polygenically inherited traits, involves the combined effect of multiple gene pairs and usually an environmental component. The majority of genetic diseases and the most commonly seen diseases (e.g., hip dysplasia, feline inflammatory bladder disease, allergies, hypothyroidism, etc.) are complexly inherited and currently have no mutation‐based DNA tests available.

Genetic tests vary on what they are able to identify, and therefore how they can be used in managing genetic disease. To understand how we can use genetic tests, we have to understand the types of tests that are available, what they can tell us, and their limitations (see 3.11 Integrating Genotypic and Phenotypic Testing).