The short answer
Yes, some do. A condition can seem to skip a generation in three main ways. X-linked conditions such as hemophilia and colour blindness pass from an affected grandfather to all of his daughters, who usually have no symptoms, and then to some of their sons. Recessive conditions travel silently through healthy carriers until two carriers have a child together. And some dominant genes cause no symptoms in some of the people who carry them, or only late in life. A few conditions, such as Huntington's disease, essentially never skip. When one seems to, the usual explanation is a relative who was never diagnosed, or who died before it would have started.
Three ways a condition skips a generation
1. Through women who carry it: X-linked conditions
This is the classic “skipped generation”, and the reason for the old saying that some conditions pass from grandfather to grandson.
A man has one X chromosome, and he gives it to every daughter and to none of his sons. So when a man has an X-linked recessive condition:
- all of his daughters are carriers, usually with no symptoms or only mild ones;
- none of his sons are affected, and they cannot pass it on;
- each son of a carrier daughter has a 50% chance of being affected, and each daughter a 50% chance of being a carrier like her mother.
The middle generation looks healthy, and the condition “comes back” in the grandsons. Examples include hemophilia, Duchenne muscular dystrophy, red-green colour blindness and G6PD deficiency.
2. Through healthy carriers: recessive conditions
A recessive condition needs two changed copies of a gene, one from each parent. People with one copy are carriers and are healthy — and most carriers never know.
The gene can therefore pass quietly down a family for generations, until two carriers of the same condition have children. Each child of that couple then has a 25% chance of being affected. To the family it looks as if the condition appeared out of nowhere, or skipped several generations at once.
This is more likely when the parents are related, because they are more likely to have inherited the same carrier gene from a shared ancestor — see first cousins having a baby. Cystic fibrosis, sickle cell disease, thalassemia, spinal muscular atrophy and many forms of inherited deafness work this way.
3. Through people who carry it but never become ill
With a dominant condition, one changed copy is normally enough to cause it, and each child of a carrier has a 50% chance of inheriting it. But for some conditions, not everyone who carries the gene develops the disease. Geneticists call this reduced penetrance.
That produces a skipped generation too: a grandfather who was affected, a father who carried the gene all his life without symptoms, and a grandchild who is affected. The father’s children still had the full 50% chance.
Three other things make a dominant condition look as if it skipped someone:
- It started late. A relative who died at 50 may simply not have reached the age it usually begins.
- It was mild. Some conditions vary so much within a family that one person’s version is missed entirely, while the next generation’s is obvious.
- It is different in men and women. BRCA changes, for example, raise breast and ovarian cancer risk far more in women, so they often travel unnoticed through a father.
Conditions that do not really skip
Some conditions show up in every generation that carries them, almost without exception. Huntington’s disease is the best-known example: nearly everyone who inherits a full-length repeat develops it, if they live long enough.
Mitochondrial conditions pass from a mother to all of her children, but how severe they are can vary so much that a mildly affected mother may not look affected at all.
When “skipped” really means something else
Before concluding that a condition skipped someone, rule out the ordinary explanations:
- A relative who had it but was never diagnosed, was diagnosed with something else, or whose illness was never talked about.
- A relative who died young, before the age the condition usually starts.
- A change that is new in the affected person, and not inherited at all — see de novo mutations.
- A family tree that is not quite the biological one — adoption, donor conception, or a different biological parent. It is more common than people expect, and it matters for the numbers.
What it means for the next generation
| What is in your family | Your own chance | What it means for your children |
|---|---|---|
| X-linked: your father is affected, and you are a woman | You are a carrier | Each son 50% affected; each daughter 50% carrier |
| X-linked: your mother is a carrier, and you are a woman | 50% that you are a carrier | If you are: each son 50% affected |
| X-linked: you are a man and unaffected | You do not carry it | No risk passed on from you |
| Recessive: your brother or sister is affected, and you are well | 2 in 3 that you are a carrier | Only at risk if your partner is a carrier too |
| Dominant, reduced penetrance: your parent is a known carrier | 50% that you carry it | If you do, 50% for each child |
Common conditions: no fixed pattern
“Does it skip a generation?” is also asked about diabetes, heart disease, bipolar disorder, autism and schizophrenia. These do not follow a single-gene pattern at all. The risk comes from many genes plus environment, so a parent can inherit part of the risk and never become ill, and a grandchild can inherit more of it.
For these, the useful question is not whether it skips but how much a relative’s diagnosis raises your own chance. See type 2 diabetes, Alzheimer’s disease, cancer and schizophrenia.
Sources and further reading
Common questions
- What diseases skip a generation?
- The X-linked recessive ones most clearly — hemophilia, Duchenne muscular dystrophy, red-green colour blindness and G6PD deficiency, which pass from a grandfather through his daughter to her son. Recessive conditions such as cystic fibrosis, sickle cell disease, thalassemia and spinal muscular atrophy can stay hidden for many generations. So can dominant conditions with reduced penetrance, including the BRCA genes and some inherited heart conditions.
- Does colour blindness skip a generation?
- Often, yes. Red-green colour blindness is X-linked recessive: a colour-blind man passes the gene to all of his daughters, who usually see normally, and each of their sons then has a 50% chance of being colour blind. "Grandfather, then grandson" is the textbook picture.
- Can a disease skip two or more generations?
- Yes. A recessive gene can pass silently from carrier to carrier for centuries, until two carriers have a child. An X-linked one can pass through a chain of carrier women if, by chance, none of the boys in between inherited it.
- Why does my son have a genetic condition when neither my husband nor I do?
- There are three common answers. You may be a carrier of an X-linked condition; you and your husband may both carry the same recessive one; or the change may be new in your son. A genetics clinic can usually tell which by testing your son first.
- Do diabetes, autism or bipolar disorder skip a generation?
- Not in the strict sense. Common conditions are shaped by many genes and by environment, so they follow no fixed pattern. They can appear in a grandparent and a grandchild with healthy parents in between, simply because the parents inherited some of the risk without becoming ill.
- Does Huntington's disease skip a generation?
- Essentially no. Almost everyone who inherits a full-length repeat develops it if they live long enough. An apparent skip usually means a parent who died young of something else, a borderline repeat length, or a diagnosis that was never made or never talked about.