Hereditary diseases

Hereditary diseases are diseases that are passed down from the parents' genes. Depending if the disease is dominant or recessive, and if both parents have it or are carriers, a child may be completely unaffected, a carrier or sick.

Recessive diseases
A recessive gene is only expressed if the alleles are homozygous for recessive, which means that both are the same. Carrying only one recessive gene, meaning that your alleles is heterozygous, means that you are a carrier for the disease. You can pass the gene for the disease on to your children, but it will only be expressed if the other parent also passes down a recessive gene. Therefore, two healthy carrier parents can have a child with the disease they both carry.

The Punnett square shows the potential offspring of two parents with heterozygous alleles. The children have a 50% of being a carrier for the disease, and a 25% chance for both having the disease and for not being affected at all.

Some examples of recessive diseases are cystic fibrosis and Hartnup's disease.

Dominant diseases
Genes that are dominant will always be expressed if they are present. This means that if the parents' alleles are homozygous for dominant, the child is guaranteed to have the disease. You cannot be a carrier of a dominant disease; you can only be affected or unaffected, sick or healthy, because if you have the gene, it will be expressed.

However, if the affected parent's alleles are heterozygous, the child has a 50% chance of having the disease as well, as shown in the Punnett square. Depending on the severity of the illness, this can be a factor for an affected individual in deciding whether or not they will have children, because a child is unfortunately quite likely to be affected as well.

Huntington's disease is one of the most common dominantly transmitted diseases.