Genes 101

How are genetic diseases inherited?

Most genetic diseases affecting children are recessive, meaning the normally functioning gene in a pair will override the abnormal one, if a normal gene is present.

What does it mean to be a carrier?

A carrier is a healthy person who has one altered copy of a gene and one normally functioning copy. Because carriers do not become ill, many families pass recessive gene mutations down from generation to generation without ever knowing it.

Recessive gene mutations are known as "autosomal recessive" if they occur on one of the 22 pairs of non-sex chromosomes and "X-linked" if they occur on an X chromosome.

Autosomal recessive inheritance

Autosomal recessive diseases occur when both parents are carriers of the same disease and their child inherits an altered gene from each of them. Children of carrier parents have:

• A 25% chance of inheriting two altered genes and developing the disease.
• A 25% chance of inheriting two normally functioning genes.
• A 50% chance of inheriting only one altered gene and becoming a carrier.

X-linked recessive inheritance

X-linked conditions like Fragile X syndrome occur when there is a gene mutation on the X chromosome. An X-linked condition is usually inherited from a woman who is a carrier of the condition. Carrier females have a working copy of a gene on one X chromosome and an altered copy on the other.

Women pass on one of their X chromosomes in each egg. When the X chromosome with the mutation is passed on in an egg and a Y chromosome is passed on in a sperm, the resulting male fetus will have the X-linked condition.

Males with X-linked inheritance are generally affected and females are unaffected carriers. However, X-linked inheritance is complex. Depending on the specific disease and mutation, an affected boy may be symptom-free or a carrier girl may have mild symptoms.