The ẞ-globin gene codes for the ẞ-globin polypeptide of haemoglobin. It has two alleles, HbA (normal) and Hbs (sickle cell). The sickle cell allele differs from the normal allele due to a base substitution mutation and this mutation results in a single amino acid change to the ẞ-globin polypeptide. There are three possible genotypes and phenotypes. • Hbs Hbs, sickle cell anaemia, a severe disease • HbA Hbs, sickle cell trait with mild or no symptoms of sickle cell anaemia • HbA HbA, normal (healthy) A man and woman who both have sickle cell trait may choose to have children by IVF. This allows the genotype of embryos to be determined by gene testing before the embryos are implanted. Embryos with the normal genotype can then be selected and implanted into the mother. One technique that can be used in gene testing an embryo for the Hbs allele is restriction fragment length polymorphism (RFLP) analysis. This involves digesting a DNA sample from an embryo with a restriction endonuclease and then separating the DNA fragments by gel electrophoresis. The position of the DNA fragments on the gel can show if the embryo has the Hbs allele.