Autosomal Recessive Diseases

In autosomal recessive disorders, the alleles on both homologous chromosomes are mutated. There is therefore no normal allele to provide the missing function encoded by the mutated genes. Most inborn errors of metabolism are caused by autosomal recessive traits. Usually, the heterozygous carriers of autosomal recessive

Fig. 2. Autosomal recessive pattern of inheritance. Only a single generation shows affected individuals. Both parents of these individuals are carriers. Other family members may be carriers.

Fig. 3. Transmission of alleles in an autosomal recessive trait. The a allele is recessive to the A allele. The paternal alleles are designated by p and the maternal alleles by m. Both parents are carriers, each having a single a allele. The children show the possible genotypes for offspring of two carrier parents: homozygous affected, two carriers, and a homozygous normal.

Fig. 3. Transmission of alleles in an autosomal recessive trait. The a allele is recessive to the A allele. The paternal alleles are designated by p and the maternal alleles by m. Both parents are carriers, each having a single a allele. The children show the possible genotypes for offspring of two carrier parents: homozygous affected, two carriers, and a homozygous normal.

traits appear clinically normal, although closer laboratory testing may reveal a biochemical difference from individuals with two functional alleles. Autosomal recessive diseases affect males and females, and often there is no known family history of the disease before the first affected individual comes to medical attention (Fig. 2). Each offspring of the mating of two carriers of an autosomal recessive trait has a 25% risk of being affected, a 50% risk of being a carrier, and a 25% chance of inheriting a normal allele from each parent (Fig. 3). In many autosomal recessive disorders, multiple mutant alleles exist; frequently patients are compound heterozygotes for two distinct mutations.

Fig. 4. X-linked dominant pattern of inheritance. Individuals with the trait are shown in black. There is no male-to-male transmission. All the daughters of affected males have the trait, whereas one-half of the offspring of affected females have the trait, regardless of sex.

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