From the 1984 lecture programme:
Everybody looks different, apart from identical twins, and many of the differences are inherited. Eye colour, hair colour, taste, smell, blood types and, of course, sex all vary and are inherited. The simplest patterns of inheritance in families gave Mendel his insight into the underlying mechanism.
The body is made up of many, many cells, all originally coming from one cell, the fertilised egg, which is the result of a fusion between a single egg and sperm. In its centre each cell has a nucleus which contains the chromosomes that carry the determinants of inheritance, the genes.
One set of chromosomes comes from the father, the other from the mother and if the father's set in the sperm includes the Y rather than the X chromosome, then you are a male.
Variation in the genes can cause many diseases as well as give rise to the normal differences which characterise individuals and populations, and which can be used to detect false paternity or, sometimes, even a murderer. The variety of mankind is extraordinary, the challenge is to comprehend it.