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 learn that a gene is simply a section of DNA that contains the instructions for making a protein. The genetic makeup of an organism, contained in its DNA, is known as the organism’s genotype. The observable physical and biochemical characteristics, or traits, of an organism are known as the organism’s phenotype. How genotype becomes phenotype how genes become traits is one of the subjects of this chapter.
But why do so many of an organism’s traits depend on genes and therefore, presumably, on proteins? Because, as we learned in the previous chapter, proteins play a huge variety of roles in living organisms they provide structure, they act
as hormones, they transport molecules, they function in cell signaling, and they protect organisms from disease. In addition, the all-important enzymes, required for practically every chemical reaction that occurs in cells, are proteins as well.
Chromosomes: Packages of Genetic Information
Let’s begin by asking where the genes are. In eukaryotic organisms, DNA is found in the cell nucleus, where it is packaged in linear structures known as chromosomes (Figure 1). Each chromosome consists of a single long piece of DNA as well as small proteins called histones. DNA is wrapped around histone “spools” like thread. Because the DNA in each chromosome is so long a single human cell contains 7 feet of DNA the histones help to keep it from becoming hopelessly tangled.
Most cells have two of each kind of chromosome, like a pair of matched shoes. These cells are referred to as diploid, and their matched chromosomes are known as homologous chromosomes. Some cells such as sperm and egg cells have only one of each kind of chromosome these cells are referred to as haploid. Different organisms have different numbers of chromosomes. Chickens have 78 (39 pairs), mosquitoes have 6 (3 pairs), lettuces have 18 (9 pairs), and yeast have 32 (16 pairs). In humans, there are 46 chromosomes, or 23 pairs, as shown in Figure 2. One pair of sex chromosomes determines sex.
|Figure 2 Humans have 23 pairs of chromosomes. These are the chromosomes of a human male.|
Females have two X sex chromosomes, whereas males have one X and one Y sex chromosome. The other chromosomes are known as autosomal chromosomes.