An example of a mutation that is damaging or fatal for many is the inherited disease sickle cell anaemia. Although the gene is normally recessive, sickle cell anaemia occurs in individuals who have received it via both sperm and egg - from both parents.
Sickle cell anaemia affects the haemoglobin gene by encoding the wrong protein in the RNA protein synthesis that normally builds and maintains haemoglobin. A mutation - in this case a change of shape of the protein molecule - prevents it working properly.
But if this is such a harmful mutation, why is the gene so common in people of African origin? People who have inherited a single copy of the abnormal haemoglobin gene are more resistant to malaria. This means that they have a survival advantage compared with people with two copies of the 'normal' haemoglobin gene. This ensures that carrier families of the sickle cell gene do not simply die out. It also explains why this is a disease that predominantly affects people of African origin - the fact that they are at risk of malaria ensures that the gene survives in this population.
Individuals who suffer mutations may pass them on to their offspring. Other inherited genes can mitigate the effects, and some may be lost in the process of combination of two sets of genes at conception. However, if all the breeding members of a species share the same mutated DNA in common, these changes will pass to all their offspring.
In the activity that follows you can choose a genetic disease and 'infect' the family with it. You can choose to do this by making the original mother, at the top of the tree, a carrier; or the original father, or both. The inheritance of the disease then appears in the generations that follow.
In addition, you can either have very little environmental 'stress' on the family or increase this 'stress' to see if it has any effect on them. This 'stress' differs with each disease e.g. for sickle cell anaemia it is the prevalence of malaria.