― What Is a Gene? | Genetics - Genes determine all sorts of traits in organisms the colors of an orchid’s ﬂowers, the length of a cat’s tail, the substances that make up a crab’s carapace or a bacterium’s cell wall. In humans, genes affect what color eyes we have, whether we are tall or short, and whether our hair is straight or curly. Genes are even believed to inﬂuence human personality traits at least to some degree. But what is a gene, and how does it determine a trait? It may surprise you to [...].