To best define what “autosomal recessive inheritance” is, let’s break down its parts.

What is autosomal?

The genetic material of human beings comes in 46 total packets of DNA called chromosomes. Two of these chromosomes help determine biological sex: the X and Y chromosomes. The other 44 chromosomes are called “autosomes”.

So “autosomal” refers to genes that reside on non-sex chromosomes.

What is recessive?

Genes on each chromosome have variations called “alleles”. For example, sickle cell anemia is caused by a specific allele of a gene that leads to the production of abnormal hemoglobin.

Autosomes come in pairs, with one gene in each pair coming from the sperm and the other from the egg. Recessive alleles require two copies of the gene to be inherited. Thus, in order for a child to have sickle cell anemia, they must get the disease-causing gene from both the sperm and the egg. If they only receive one copy, the child will not have sickle cell anemia. In the case of sickle cell anemia, this is because the normal allele of the gene for hemoglobin received from one parent compensates for the abnormal one received from the other parent.

So “recessive” refers to an allele that is only expressed by the offspring when two copies of the recessive allele are inherited, one from each parent.

What is autosomal recessive inheritance?

Now we’re ready to put these concepts together to describe “autosomal recessive inheritance”: a pattern of genetic inheritance that emerges when a gene that’s on a non-sex chromosome (“autosomal”) requires two copies for the offspring to have a trait (“recessive”) that is passed down from parents to offspring (“inheritance”).

In autosomal recessive inheritance, children who get one recessive copy of the same gene from each parent will have the recessive trait. Someone who has one recessive copy of the gene and one that is not recessive (known as “dominant”, since it dominates over recessive alleles) is a “carrier”. A carrier does not have the condition but can pass the recessive gene to his or her children.

If a trait or condition is passed down through autosomal recessive inheritance, the chance of a child actually having the trait depends on whether the parents are affected by the trait or just carriers of the recessive allele.

The probabilities of a child being a carrier, affected, or being completely unaffected by the autosomal recessive trait when both parents are carriers is shown in Figure 1. In the image, both parents have one allele of a gene that is recessive. Thus, neither parent has the trait or disease; however, they are carriers, and thus can pass the disease down to their child. Since each parent has a roughly 50/50 chance of passing down the affected or unaffected allele to each child, you can estimate the probability of a child’s status by simply counting the number of ways each parent can pass down the gene. The possibilities are the following:

• Both parents pass down an recessive allele, so the child is affected by the trait or disease
• Parent 1 passes down the recessive allele, but parent 2 does not, so the child is a carrier
• Parent 2 passes down the recessive allele, but parent 1 does not, so the child is a carrier
• Both parents pass down an unaffected allele, so the child is unaffected

Thus, there are four total inheritance patterns, all of equal chance. Since there is only 1 of 4 ways the child could be affected by the trait (namely, by getting the recessive allele from both parents), the probability the child will be affected is 1 ÷ 4 = 25%. By similar reasoning, the chance the child is unaffected is also 25%. Note that there are two out of four ways the child could be a carrier, though; thus, the probability of the child being a carrier if both parents are carriers is 2 ÷ 4 = 50%.