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Huntington's chorea is autosomal dominant; sickle-cell anaemia stems from an autosomal mutant (beta-globin) allele; agammaglobulinemia follows a recessive-gene pattern; and thalassemia is expressed in the homozygous recessive state.

Huntington's chorea is caused by a dominant mutant allele on an autosome, so even one copy causes the disease (autosomal dominant). Sickle-cell anaemia is caused by a single point (missense) mutation in the beta-globin gene — described here as an 'autosomal mutant allele' — and the disease is fully expressed when the mutant allele is inherited from both parents. Agammaglobulinemia (an antibody-deficiency disorder) follows a recessive pattern. Thalassemia results from mutations affecting globin-chain synthesis and produces the major (severe) form specifically when …