Genetic Disorders & Inheritance

A genetic disorder is caused by a fault in the DNA: either a single faulty gene (an allele) or the wrong number of chromosomes. Because the fault is in the DNA, it can be passed from parents to children through the gametes. Knowing the pattern lets doctors predict the risk for each child.

A recessive allele only causes the disorder when both copies are faulty (aa). A person with one faulty and one normal allele (Aa) is a healthy carrier. Two carriers can have an affected child even though neither parent is ill, so the disorder seems to skip generations. Cystic fibrosis works this way.

A dominant allele causes the disorder with only one copy (Aa). There are no silent carriers, so the disorder appears in every generation that carries the allele. Huntington's disease is dominant, but its symptoms only start in middle age, so a parent may have children before knowing they carry it.

Sometimes chromosomes fail to separate properly during meiosis (non-disjunction), so a gamete carries an extra chromosome. Down syndrome is caused by an extra chromosome 21 (three copies, trisomy 21). This is not usually inherited from a carrier parent; it is an error in cell division.