The process of copying DNA in human cells is very accurate, but mistakes do happen. Estimates of the mutation rate vary, but a 2011 study found that for every 85 million nucleotides assembled in DNA during human sperm or egg production, one will be a mistake--a mutation. The statistic concerns mutations in sperm and egg cell production because only mutations in these cells are passed on to the next generation.
Human body cells form two broad categories: germ cells and somatic cells. Germ cells produce sperm and eggs; all the other tissues of the body are somatic cells. A somatic cell mutation in an organism is passed on to daughter cells in the organism. But this type of mutation doesn't affect future generations because only genes carried by sperm or egg will affect offspring. A mutation in a germ cell, by contrast, will not affect a parent but will affect children.
Children typically inherit some mutations from their parents. The average mutation rate of 1 in 85 million nucleotides or genetic code letters during sperm or egg production may sound low. However, the human genetic code is 6 billion letters long. This mutation rate adds up to dozens of mutations per generation, although many of these mutations have no visible effect. In general, scientists think that sperm cell DNA carries more mutations than egg cell DNA because women are born with all their eggs, but men make new sperm continuously throughout their lifetimes.
Sometimes, a mutation is so severe it's lethal and a fetus carrying the mutation never reaches full term. Many miscarriages, for example, are caused by serious mutations or chromosomal rearrangements that prevent the fetus from developing normally. In these cases, although a mutation occurred in a germ cell, it is not passed to the offspring because the offspring was not born. In other cases, mutations cause birth defects that while not lethal, are serious and can wreak havoc on the offspring's quality of life.
The process of cell division that makes sperm and egg cells is complicated. It's wrong to assume that all mutations that occur in any germ cell will be inherited. There's no guarantee that the specific sperm or egg cell carrying a mutation will become part of a new organism. The mutation will be passed on only if it occurred on a chromosome in either the sperm cell or the egg cell that later unite to form a fertilized egg, or zygote.