基因一词来自希腊语，意思为“生”。是指携带有遗传信息的DNA串行，是控制性状的基本遗传单位，亦即一段具有功能性的DNA串行。基因通过指导蛋白质的合成来表现所携带的遗传信息，从而控制生物个体的性状（差异）表现。人类约有两万至两万五千个基因。

染色体在体细胞中是成对存在的，每条染色体上都带有一定数量的基因。一个基因在细胞有丝分裂时有两个对列的位点，称为等位基因，分别来自父与母。依所携带性状的表现，又可分为显性基因和隐性基因。

一般来说，同一生物体中的每个细胞体都含有相同的基因，但并不是每个细胞中的所有基因携带的遗传信息都会被表现出来。职司不同功能的细胞中，活化而表现的基因也不同。

A gene is a locus (or region) of DNA that encodes a functional RNA or protein product, and is the molecular unit of heredity. The transmission of genes to an organism's offspring is the basis of the inheritance of phenotypic traits. Most biological traits are under the influence of polygenes (many different genes) as well as the gene–environment interactions. Some genetic traits are instantly visible, such as eye colour or number of limbs, and some are not, such as blood type, risk for specific diseases, or the thousands of basic biochemical processes that comprise life.