Inheritance of sickle cell disease and thalassaemia

What it is

Each of these conditions is named by a pair of genes, and the pair decides what the hemoglobin study shows. A pair of normal genes, AA, gives the normal pattern. One normal gene A with one other gene gives a trait: A with beta or beta+ is beta thalassemia trait, and A with S is sickle cell trait. A pair with no A gene in it gives disease: beta0/beta0 or beta0/beta+ is beta thalassemia major, and SS is homozygous sickle cell disease. The two need not be alike, and three such pairs are named: sickle with beta0 thalassemia, sickle with beta+ thalassemia, and HbSC disease. Beta thalassemia minor is written beta/beta+.

What the S gene is, is a single change: sickle cell disease is an autosomal recessive disease with an abnormal hemoglobin S molecule, with a valine substituted for glutamic acid on the beta chains. The heterozygous state SA, with less than 40% hemoglobin S, is called sickle cell trait; the homozygous state SS, with more than 65% hemoglobin S, is called sickle cell disease. Beta thalassemia minor is a defect in a single allele of the beta gene, heterozygous for one normal beta globin allele and one mutated thalassemic allele, and is usually asymptomatic.

The terms themselves are worth holding, since the exams are written in them: homozygous means two of the same alleles, heterozygous two different alleles; an allele is a variant at a particular site on a chromosome; the genotype is the types of genes present, at the genetic level, and the phenotype is what it looks like on a clinical level. A recessive trait is one that is covered up.