Used to describe a characteristic or condition that appears only in individuals who have received two copies of a gene mutation, one copy from each parent. People with two copies of the altered gene are called homozygotes. Their parents, each with a single copy of the altered gene, appear normal and are called heterozygotes, or carriers of the altered gene.
Also used to describe one of the types of single gene inheritance - autosomal recessive inheritance.
Each parent who is a carrier for an autosomal recessive condition can pass on either the usual form of the gene or the gene containing the change for the condition.
At conception, therefore, there are are four possibilities. The first is that a child will inherit the usual gene from both parents and will not have the condition nor will be a carrier. The next possibility is that the child can inherit the usual gene from the first parent, and the gene for the condition from the second parent. This child will be a carrier. The third possibility is that the child will inherit the altered gene from the first parent and the usual gene from the second parent - this child will also be a carrier. The final possibility is that the child will inherit the altered gene from both parents and therefore will have the condition.
At conception therefore, a child of parents who are both carriers has a 1 in 4 chance of inheriting two usual forms of the gene, a 1 in 2 probability of being a carrier and a 1 in 4 probability of inheriting the condition.
Sometimes health professionals are asked about the probability of a healthy brother or sister of someone with an autosomal recessive condition being a carrier for the condition. At conception this is 1 in 2. However, if a brother or sister is healthy, this excludes the possibility of their having inherited one of the combinations of genes - they cannot have inherited two copies of the altered gene. This leaves three possible combinations - two of which are carriers and one with two copies of the usual gene. Thus the probability that a healthy brother or sister of someone with an autosomal recessive condition is a carrier is 2 out of 3. This information can be important sometimes in working out the probabilities of having a child with a recessive condition.
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