Introduction to Genetics

The following is a basic overview of the foundations of genetics. For more in-depth explanations, please refer to the links at the bottom of the page.

Genetics is defined as the biological study of the variations in organisms; it's the study of the biological differences that make all living things unique. To begin, it's easiest to start with describing the basic building blocks of all living organisms, cells. The human body is composed of trillions of cells. They provide structure for the body, take in nutrients from food, convert those nutrients into energy, and carry out specialized functions. Each cell is made up of several pieces, one of which is called the nucleus. A cell's nucleus contains the majority of hereditary material known as DNA.

DNA, or deoxyribonucleic acid, is the hereditary material in humans and almost all other organisms. The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Human DNA consists of about 3 billion bases, and more than 99 percent of those bases are the same in all people. The order, or sequence, of these bases determines the information available for building and maintaining an organism, similar to the way in which letters of the alphabet appear in a certain order to form words and sentences.

DNA bases pair up with each other, A with T and C with G, to form units called base pairs. Each base is also attached to a sugar molecule and a phosphate molecule. Together, a base, sugar, and phosphate are called a nucleotide. Nucleotides are arranged in two long strands that form a spiral called a double helix. The structure of the double helix is somewhat like a ladder, with the base pairs forming the ladder's rungs and the sugar and phosphate molecules forming the vertical sidepieces of the ladder. Long ladders of DNA are bundled with proteins into organized structures called chromosomes, which are responsible for carrying genetic material. Humans have 46 chromosomes.

Genes are specified portions of the DNA ladder, which act as instructions to make molecules called proteins. In humans, genes vary in size from a few hundred DNA bases to more than 2 million bases. The Human Genome Project has estimated that humans have between 20,000 and 25,000 genes. Every person has two copies of each gene, one inherited from each parent. Most genes are the same in all people, but a small number of genes (less than 1 percent of the total) are slightly different between people. Alleles are forms of the same gene with small differences in their sequence of DNA bases. These small differences contribute to each person's unique physical features.  The set of all genes that specify your traits is known as your genome. The composition of the genes is called a genotype.

To learn more about genetics, check out the following links:

http://learn.genetics.utah.edu/content/begin/tour/
http://www.dnalc.org/websites/