Autosomal recessive is one of the possible ways that genetic traits can be inherited. In this case, the gene is located in one of the 22 autosomal chromosomes and two copies of the altered allele are needed to develop the altered phenotype (orange color in the figures) instead of the normal one (grey color in the figures). One copy of the normal allele is enough to hide the phenotype of the recessive trait. You can see in figure 1 how both the genotypes AA and Aa yield the same phenotype and all the children of an AA parent will have the normal phenotype.
Figure 1. Autosomal recessive aa x AA
If the altered gene version is responsible for a disease, the affected people have two altered alleles (aa). Heterozygous Aa are not affected by the disease, but they are carriers and could pass the altered gene on to their children. Affected children usually are born to couples where both parents are carriers but not affected by the disease. As can be seen in figure 2, their children have a 25% chance of being affected, 50% chance of being a carrier and 25% chance of not being affected and not being a carrier. It is transmitted equally to sons and daughters and the disease is usually not present in all the generations of the family. If one parent is affected (aa) and the other is a carrier (Aa) the likelihood of having an affected child goes up to 50% (figure 3).
Figure 2. Autosomal recessive Aa x Aa.
Figure 3. Autosomal recessive Aa x aa.
If you want to read about autosomal dominant inheritance pattern in Spanish, you can follow the link herencia autosomica dominante.