Genetics Basic Concepts
Genes and chromosomes
You can think of the DNA content of a cell as a cookbook that contains recipes describing the living organism right down to the tiniest detail. The recipes are the genes. A gene is a fragment of DNA that accounts for a specific characteristic, such as hair colour, ability to digest dairy or any other information pertaining to the organism’s appearance or functioning. It is made up of a very precise sequence of nitrogenous bases (ACTGTTAGC…), the building blocks of a DNA molecule. When decoding the sequence of a particular gene (the recipe), a cell can manufacture a specific protein. A gene can have several different forms, with one or more different nitrogenous bases. These variant forms of a gene are called alleles. In the case of eye colour, for instance, you can have alleles that contain information for brown, blue, green or grey pigment.
DNA containing our genes is located in the nucleus of the cell. It is bound to proteins to form a material known as chromatin. When a cell divides, it must start by organizing this whole mess. Chromatin then takes on a more compact form: chromosomes. Humans have 23 pairs of chromosomes; they look like 46 little rods. Cellular biologists have given each of the pairs a number. The first 22 pairs of chromosomes are called autosomes. The 23rd pair of chromosomes is different: it holds the sex chromosomes. In humans, there are two types of sex chromosomes: X and Y, which each contains information on the development of female and male genders. In females, the 23rd pair has two X chromosomes. In males, it has one X chromosome and one Y.