Final answer:
Some genes have multiple effects on an organism through pleiotropy, where one gene influences many phenotypic traits. Contrastingly, polygenic inheritance involves multiple genes contributing to a single trait, and epistasis describes the situation where the expression of one gene is influenced by other genes.
Step-by-step explanation:
Some genes can have multiple effects on an organism, a concept known as pleiotropy. Pleiotropy occurs when one gene influences two or more seemingly unrelated phenotypic traits. An example of pleiotropy is Marfan syndrome, a genetic disorder where a single gene affects the connective tissues across various parts of the body, leading to symptoms in the skeletal system, vision, and cardiovascular health.
In contrast to pleiotropy, polygenic inheritance involves multiple genes contributing to a single trait, as we see in human skin color or height. Meanwhile, epistasis occurs when one gene's expression is affected by the presence or absence of one or more other genes. Mendelian genetics first suggested that single genes control individual traits, but we now understand that traits like eye color result from the interplay of multiple genes, demonstrating the complexity of genetic inheritance.