All different species of life, whether they are plants or animals, have a number of very unique characteristics that make them what they are. These characteristics are carried from generation to generation in a code, a set of very clear instructions, that allow for the reproduction and continuation of that species. This code is carried in molecules of DNA and is extremely complex. The code inside one human cell contains between 50,000 and 100,000 separate instructions called genes and each gene controls a different characteristic. Sometimes a single gene controls a characteristic, such as hair colour, but more usually several genes are involved. Often a pair of genes, one from the mother and one from the father, control a characteristic. Normally, one of the pair is dominant and the other is recessive and the dominant one will mask the effects of the recessive one so that the dominant gene will determine the particular characteristic. These characteristics are known as phenotypes and the genetic make up of an individual is known as the genotype.
Knowledge of genes allows us to explain how characteristics are passed from one generation to the next and also to predict characteristics in that next generation. We can, therefore, determine the eye colour of a baby by studying the genes of the parents (in genetics, a dominant gene is represented by a capital letter and a lower case letter represents a recessive gene). If a male with brown eyes (genotype BB) were to father a child to a female with blue eyes (genotype bb) then the baby’s genotype would be Bb since the male’s genes are, in this case, the dominant ones. However there are three possible genotypes for eye colour: BB, Bb and bb. The genotypes BB and Bb will both give a brown-eyed baby whereas genotype bb would give a blue-eyed baby. Using this representation of genes, it is possible to predict what will happen when parents with other eye colour genotypes have children. Knowledge of genetics is also helpful in the diagnosis and treatment of hereditary diseases such as cystic fibrosis and haemophilia.
