ABSTRACT
Humans have approximately 75,000 different genes, encoding the structure and hence function of the tens of thousands of different proteins found in the body. Each gene consists of a unique sequence of bases somewhere along one of the enormously long DNA molecules that are condensed into the chromosomes found inside the nucleus. A DNA sequence coding for a functional protein is referred to as a ‘normal’ gene; a coding for a dysfunctional protein is a ‘mutant’. A comparison of the base sequence of the gene responsible for the assembly of haemoglobin from an individual suffering from sickle cell anaemia with the same gene segment from an unaffected individual reveals a very minor change in the DNA sequence. If, however, an individual inherits similarly mutated genes from both parents, the complete lack of normal protein usually precipitates a pathological condition. Conditions which come to light only when defective genes are inherited from both parents are recessive genetic traits.
