system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:If DNA copying errors are random occurences, why do some diseases that are known to be caused by gene mutations can reoccur in their offsprings at a random point in their life?
gpt
There are two types of mutations: germline and somatic.

> **Germline:** passed to the offspring at birth.

> **Somatic:** occurs *de novo* in the organism through mutation.

Only somatic mutations are "random".  Diseases arising from somatic mutations are almost exclusively cancers.  Germline mutations are inherited.  The manifestation of heritable disease from germline mutations is called the disease's **penetrance**.  Penetrance basically refers to the total fraction of people with the disease relative to the fraction with the mutation.

Many diseases are penetrant at birth (i.e. effects of the mutation are seen at birth).  Examples of diseases penetrant at birth include sickle cell, cystic fibrosis, congenital dwarfism, etc.  Other diseases' penetrance is **age dependent**.  Age-dependent penetrance means effects are only seen at a specific age.  The poster child of age-dependent penetrance is Huntington's disease.

So, to answer in brief, most mutations that cause disease are not copy errors they are inherited.  Despite this, just because a known detrimental mutation is present doesn't mean it is phenotypically expressed at birth, it may require a certain age or condition to appear.