What Are Inherited Traits Passed on Through?

Single-Gene Traits

Single-gene, or monogenic, traits are controlled by a single gene pair. Cystic fibrosis, for instance, is controlled by a single gene on human chromosome 7. A given single-gene trait may be dominant or recessive. An organism with two matching genes will express that genetic trait. An organism with a dominant and a recessive gene will express the dominant trait. In cystic fibrosis, the disease-causing gene is recessive. This means a child born with only one copy of that gene will not have cystic fibrosis; instead, the dominant non-cystic fibrosis gene will be expressed. A child with two copies, however, will have the disease.

Most traits are controlled by multiple genes. Human eye color, for example, is controlled by at least three different genes. The way these genes blend together in a child determines what his eye color will be. Also, not all single-gene pairs are simply dominant or recessive; human blood typing depends on two dominant genes, with the recessive trait being the absence of these two genes. Human blood may have an A antigen, a B antigen, or neither. Children born with a single or double A will have type A blood --- but neither A nor B is dominant. Children with both an A and a B antigen gene will have the AB blood type.

In all higher organisms, a special pair of chromosomes, referred to as X and Y because of their shapes, determine the sex of an organism. In male organisms, the Y chromosome is missing a large number of genes. This means genes found on the X that would be paired with one of the missing genes on the Y will automatically be expressed in the male organism. Traits in humans like color blindness and hemophilia are found on the sex chromosomes and thus are far more likely to be expressed in a male.

The genes in a person's DNA, whether those genes are expressed or not, are referred to as a genotype. Traits that are expressed are called the phenotype. This is important in diagnosing genetic disease. A person who is perfectly healthy may have one copy of the cystic fibrosis gene; as this gene is recessive, the person will show no signs of the disease. However, he is a carrier. If he marries another person with a single copy of the same gene, there is a 25 percent chance any child they conceive will also have cystic fibrosis, and a 50 percent chance the child will have a single copy. Families that intermarry frequently, like the royal houses of Europe, are more likely to have expressed genetic disorders due to the higher chance of carriers of the same gene conceiving children together.