Autosomal Recessive Inheritance

Inheritance refers to how genetic information is passed down from parent to child. Mendelian inheritance, or “classic” inheritance is one of the foundations of genetics. Gregor Mendel, who is often called the founder of modern genetics, performed many experiments on pea plants and was able to make several important observations. Those observations led to “rules” that are still followed to this day, and allowed Mendel to define the basic inheritance patterns.

Each of the inheritance patterns follows certain rules:

• Each child inherits half of their genetic information from their mother and half from their father. In the case of human beings, a child inherits 23 chromosomes from their mother and 23 from their father, for a total of 46 chromosomes (23 pairs).

• The chromosomes carry our genes. Genes come in pairs, just like chromosomes come in pairs. For the most part, everyone has two copies of every gene. Since a child inherits only one chromosome of each pair from a parent, the child also inherits only one copy of a gene from each parent. Each parent contributes one copy of every gene, so the child ends up with two copies of each gene.

• Genes for different traits are inherited separately from one another. For example, the “gene for eye color” is not linked to the “gene for height.” A child may have his mother's eyes, but not her height. For the most part, each trait is inherited separately.

The basic patterns of inheritance follow all of these rules, as well as some of their own. Autosomal recessive inheritance is one of the major basic patterns.

Autosomal recessive inheritance refers to a recessive allele being passed on from parent to child on any chromosome other than the X or the Y. An allele is a “version” of a gene. Everyone has two copies of each gene. Those copies may be the same, or they may be different versions of the gene. In order for an autosomal recessive condition to appear, both copies of the gene must have a mutation. In other words, two mutated recessive alleles must be present. Autosomal recessive conditions can only be passed on if both parents pass on an allele with a mutation.

One very important concept in autosomal recessive inheritance is that of a “carrier.” A carrier is a person who does not show signs or symptoms of a condition, but still has the ability to pass on an allele with a mutation. A carrier generally has one allele with a mutation and one allele without a mutation. In other words, a carrier is a heterozygote for that particular gene. Since an autosomal recessive condition only appears when both alleles of a gene pair have a mutation, the carrier is not affected. A carrier usually does not even know he or she carries a mutation. However, when a carrier has children, he or she may pass on either the allele with the mutation or the allele without.

If two heterozygous carriers have children together, it is possible for them to have a child affected with an autosomal recessive condition. This is one way in which a condition can appear in a family for the first time with no known family history. The diagram below depicts the likelihood of two carriers having a child with an autosomal recessive condition:

The orange and purple shapes represent chromosomes. The red markers represent an allele with a mutation. The blue markers represent an allele without a mutation.

The orange and purple shapes represent chromosomes. The red markers represent an allele with a mutation. The blue markers represent an allele without a mutation.

In this picture, both parents are carriers and there are four different possible results for a pregnancy. Both parents could pass on alleles without mutations, and the child would be neither affected nor a carrier. Dad could pass on an allele without a mutation and mom could pass on an allele with a mutation, and the child would be a carrier. Similarly, mom could pass on an allele without a mutation and dad could pass on an allele with a mutation; again, the end result would be a child who is a carrier. Finally, both parents could pass on an allele with a mutation. This would result in a child having two mutated alleles and thus being affected with the autosomal recessive condition. Therefore, if two carriers for an autosomal recessive condition have children, there is a 1 in 4 (25%) chance with every pregnancy a child will be neither affected nor a carrier, a 2 in 4 (50%) chance the child will be an unaffected carrier, and a 1 in 4 (25%) chance the child will be affected with the condition.

Everyone carries mutated genes in their genome, whether for a genetic condition of some kind or another type of trait. Most people go through life never knowing what could be passed down to the next generation. Since there are usually no harmful health effects to being a carrier, identifying carriers of an autosomal recessive condition is difficult. There are two main ways in which this could occur. One, a person could have specific carrier testing performed and receive a positive result. This is often done either before or during a pregnancy. For instance, some couples are offered carrier testing for cystic fibrosis, a relatively well-known autosomal recessive condition. The more likely method of identification, though, is through family history. If two carriers happen to have children together, and one or more of those children happens to be born with an autosomal recessive condition, the possibility of more family members being carriers naturally arises. These family members often seek out genetic testing for themselves once the mutations in the known carriers (the parents of the affected child) have been identified.

Thus, unless a family history of the condition is already present, it can be completely unexpected when a child with an autosomal recessive condition is born to two healthy parents. The possibility of an affected child is not as unexpected if one parent is already affected. How are the chances of having an affected child altered if a parent is affected? The answer to this is largely dependent on the other parent. Let us first examine the situation in which the second parent is neither affected nor a carrier:

In this situation, the affected parent always passes on an allele with a mutation. The unaffected parent always passes on an allele without a mutation. As such, the only possible combination of alleles for their offspring is one allele with a mutation and one allele without. So, if an affected individual and a non-carrier for an autosomal recessive condition have children, there is a 4 in 4 (100%) chance with every pregnancy the child will be a carrier, and a 0 in 4 (0%) chance the child will be affected. This can be surprising to a family who assumes an affected parent will always have affected children.

If this scenario is altered a bit and the unaffected parent is a carrier, the picture changes:

This change makes a big difference to the chances for an affected parent having an affected child. The affected parent can still pass on only an allele with a mutation. If the second parent is a carrier, though, she can pass on either an allele with a mutation or one without. There are two possible outcomes: a child could inherit one mutated allele and one non-mutated allele, or a child could inherit two mutated alleles. Therefore, if an affected individual and a carrier partner have children together, there is a 2 in 4 (50%) chance with every pregnancy the child will be a carrier for the autosomal recessive condition, and a 2 in 4 (50%) chance the child will be affected. If an individual affected with an autosomal recessive condition plans to have children, it is often recommended that their partner undergo carrier testing ( before becoming pregnant) in order to have the most accurate idea of the possibilities. The difference between a 0% chance of having an affected child and a 50% chance can seem very significant to some families.

There is also the potential situation in which two affected individuals have children together:

Each parent can pass on only an allele with a mutation. The only possible combination for a child to inherit is two mutated alleles. Therefore, if two affected parents have children together, there is a 4 in 4 (100%) chance with each pregnancy the child will be affected with the autosomal recessive condition.

Regardless of a parent's carrier status, it should be noted that no one has the ability to choose which chromosomes and which alleles are passed on . A parent is not able to examine their alleles, and choose one to be inherited over the other. If a child inherits an allele with a mutation from an affected parent, it is by random chance. If a child inherits an allele without a mutation, that is also by random chance. In either case , no one is to blame and it is no one's fault .

Examples of autosomal recessive inheritance include: phenylketonuria (PKU), cystic fibrosis, sickle cell disease, Tay-Sachs disease, galactosemia, and diastrophic dysplasia.

Genetic Resources Home Page | Return to Basics of Genetics