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How can one defective gene cause several symptoms in a genetic disease?

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Four kittens with different fur patterns-three tabby and one black-and-white-lying together in a box, illustrating genetic variation among offspring. — heredity Each offspring is a combination of its two parents, receiving some dominant traits from its mother and others from its father.

A single defective gene can cause multiple symptoms in a genetic disease through a phenomenon known as pleiotropy. Pleiotropy occurs when one gene influences two or more seemingly unrelated phenotypic traits or affects multiple organ systems simultaneously.12

In the context of metabolic diseases, this often happens because genes direct the production of enzymes that are essential to complex, interlinked biochemical pathways. When a mutation results in a defective or missing enzyme, it can cause a "blockage" in a metabolic pathway. This leads to two primary issues that manifest as various symptoms:34
* Deficiency: The failure to produce a necessary final product, which may be required for multiple bodily functions.3
* Accumulation: The buildup of a substrate that cannot be processed, which may become toxic to various tissues and organs, leading to diverse clinical signs such as intellectual disability, seizures, or organ failure.3

Because these metabolic pathways are interconnected, a single genetic error can have widespread systemic consequences, resulting in the diverse array of symptoms observed in many hereditary disorders.3