DNA, chromosomes and genes
Every cell in your body contains DNA (deoxyribonucleic acid). DNA, the genetic code for your body, exists as two long, paired strands spiraled into a double helix called a chromosome. DNA is like a blueprint that tells your body how to grow and develop.
Each cell contains two copies of each chromosome, one copy inherited from your mother through the egg and one from your father through the sperm. Each cell has a total of 46 chromosomes including a pair of sex chromosome that determines if you are male or female. Girls inherit two X-chromosomes. Boys inherit an X-chromosome and a Y-chromosome.
Genes are segments of DNA on a chromosome. Each gene is made up of a sequence of four different chemical bases (adenine, thymine, cytosine and guanine). The specific sequence determines which protein the cell will build, much as the specific sequence of letters determines words and sentences. Each gene contains specific instructions which help to build, regulate and maintain your body. Since you have two copies of each chromosome, you also have two copies of each gene. The two versions of the same gene are called "alleles.” Sometimes the two alleles are the same, other times they are different. Changes in one or both alleles can sometimes cause the gene to not work properly.