NOTES 24 – Genetic Disorders and Hereditary Diseases

What is a genetic disorder? A genetic disorder is a condition caused by any abnormality in a person’s genes or chromosomes Genetic disorders have 2 main causes

Cause #1 An error during meiosis causes a sperm cell or an egg cell to have some defect. If one of these gametes is involved in fertilization, it usually results in miscarriage, but in a few cases, a baby will develop and be carried to term with a genetic disorder EX. Down Syndrome is caused by an extra 21st chromosome in the sperm or egg cell

Cause #2 A defective gene present in one or both of the parents is inherited directly by the child causing a disorder These kinds of disorders are calle hereditary diseases and follow the same rules of inheritance as other human traits

Hereditary Diseases Caused by Dominant Genes Only one copy of the gene (from either parent) is needed for a person to have the disease A person with a dominant hereditary disease usually has at least one affected parent Parents have a 50% chance of passing these types of diseases to children

Ex. Huntington’s Disease Caused by the inheritance of a dominant gene Affects the nervous system Produces incorrect protein that eventually leads to destruction of brain cells Symptom’s include abnormal body movements, lack of coordination, impaired mental abilities and altered personality Patients generally live 15 to 20 years after the appearance of the first symptoms Currently no cure

Hereditary Diseases Caused by Recessive Genes Two copies of the defective gene must be inherited (one from each parent) Affected persons usually have two non-affected parents who are carriers Two carrier parents have a 25% chance of having a child with the disease

Ex. Sickle-Cell Anemia Caused by the inheritance of two recessive genes Causes red blood cells to be abnormally shaped Cells may stick to each other or become stuck in blood vessels preventing oxygen flow which may lead to organ damage or stroke Individuals are often well, but experience painful, possibly fatal attacks from time to time

Sickle Cell Anemia

Hereditary Disease Caused by Sex-Linked Recessive Genes Caused by defective genes carried on the X chromosome Males affected more than women Males inherit one defective gene on X from mother to have disorder Females only affected if they inherit 2 defective genes (one from each parent)

Ex. Hemophilia Caused by the inheritance of one recessive gene on the X (males) or two recessive genes on both X’s (females) Affects blood – blood does not clot Leads to severely increased risk of bleeding from common injuries Potentially fatal

Hemophilia