These are what the chromosomes of a human look like. Chromosome pairs like these are in every human cell. The last pair determines the gender of the person. Click on image for full size (53K JPG) Courtesy of the National Library of Medicine and NIH

Do you look a bit like your brothers and sisters? Do you look a bit like your parents? The similarities are because, unless you were adopted, you and the other members of your family have genetic material in common.

Some characteristics, or traits, result from interactions with the environment, others are determined from the genetic material in your chromosomes. Chromosomes are the keepers of the genetic material in eukaryotic cells. An organism has the same chromosomes for its entire life. The chromosomes are located within each cell nucleus. They provide the directions for how the cell is supposed to function and determine some characteristics about how the individual looks. Each chromosome contains a very complex molecule called DNA. The DNA molecule contains genes, which direct how an organism’s body is built and maintained.

Heredity is the passage of DNA from the chromosomes of one generation to the chromosomes of the next. Chromosomes in your body are in pairs. One chromosome of each pair came from your mother and one from your father. There are many genes within each chromosome. Different genes determine different traits. For each gene there can be many varieties. For instance, one variety might tell the body that eyes should be blue while another variety tells that eyes should be brown. (The dominant gene determines the actual color of the person’s eyes.) Each variety is called an allele.

There might be many types of alleles for each type of gene. Each animal or plant has two alleles of each gene. One allele is inherited from its mother and the other is inherited from its father. This means that an organism has two varieties of every gene, but they might not be the same type.