why some illnesses and diseases “skip” a generation.
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It depends on the disease.
If it is entirely genetic, there are a few reasons
Autosomal dominant means you only need one faulty gene and will affect both men and woman. These are conditions where you often see each generation affected.
Most of the common autosomal dominant conditions have something called high penetrance or complete penetrance. That means that if you have the faulty gene, you get the disease. For example, huntingtons disease is a 100% of people who have the faulty gene will get Huntington’s disease. Likewise for achondroplasia (most common form of dwarfism) and osteogenesis imperfects (the inherited brittle bone disease).
However there are some conditions which have reduced penetrance. An easy example is the BRCA1 gene. 80% of women with a faulty version of this get breast cancer. That means 1 in 5 don’t. Men with it have a 1 in 100 chance of getting breast cancer (men have breast tissue too!). So there are many unaffected.
Reasons why people maybe unaffected in different conditions are:
– they’ve inherited other genes that can be protective
– things about their environment or lifestyle reduce the chances. These can be obvious things, such as not smoking in those with a cancer predisposing conditions, or subtle things we have no control over
– very mild disease: the “missed generation” person may have very mild disease so it isn’t recognised
– if it not something that begins at birth or childhood, the person may die before they develop symptoms, so it looks like it skipped a generation
For other conditions that are only partially genetic (eg asthma) it is because there are a collection of genes that make it it more likely that they will develop the disease than someone who does not have a family history, but because of environmental factors and probably other inherited genes, it’s not a guarantee. But at the same time, these conditions can happen without a family history and occur anyway. Examples are things like asthma, type 1 diabetes, eczema. So you might see an affected family member somewhere else in the family, but it won’t be an every generation type line through the family tree
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