When a child is conceived, there is no way of predicting which sperm and which egg will unite to make the baby. At conception one egg from mum and one sperm from dad is brought together to develop the foetus.
It is within the nucleus of each cell of the egg and the sperm that information called the DNA is stored on strands called chromosomes. It is this information that predicts the colour of the child’s eyes, hair etc. and will also carry any information that relates to a genetic disease.
In each cell (except the egg and the sperm), there are two chromosomes one coming from dad and one from mum. Each information such as the colour of the eyes is provided within a small part of the DNA: a gene. There are thousands of genes in the human DNA. It is only when each chromosome carries the information of the disease on a specific gene manned MMUT that the child is affected. Usually, mum and dad have only one of their chromosomes that carry the information of the disease and there are not affected.