I used to teach a lad called David. Both of his parents were tall, slim, highly intelligent and articulate. David was also tall, slim, highly intelligent and articulate. Anyone looking at him could quickly see a resemblance to both his parents.
His younger brother Simon, however, was short, chubby and 'one of life's plodders'. He looked nothing like either of his parents.
Yet Simon was their natural son, whereas David had been adopted by them!