The concept of things “skipping a generation” is a popular belief that certain traits or conditions can pass over a generation before reappearing in the next one. This idea has spread through common wisdom and become accepted as fact for many families. But is there any scientific evidence that supports the notion that traits actually skip generations?
Common Examples of Things Said to Skip Generations
There are several traits that are commonly believed to skip a generation genetically:
- Physical features like eye color, hair color, dimples, etc.
- Diseases and medical conditions like asthma, diabetes, cancer, etc.
- Mental health conditions like depression, anxiety, bipolar disorder, etc.
- Personality traits like shyness, sense of humor, introversion/extroversion
- Talents and abilities like athleticism, musicality, artistic skills, etc.
In many families, people observe these kinds of traits seeming to skip generations. For example, a family might notice that a grandmother and grandson both have bright blue eyes while the mother’s eyes are brown. Or a family history of diabetes may seem to skip from a grandfather to a granddaughter while the father is unaffected. These patterns reinforce the idea that traits are skipping generations.
Scientific Explanations
While it’s easy to find examples that seem to support generational skipping, does science actually back up this phenomenon? Genetic experts have a few explanations for why traits can appear to skip generations:
1. Recessive Genes
Many genetic traits are carried on recessive genes. This means that the gene will only be expressed if a person inherits two copies of it, one from each parent. If a parent has one copy of the gene but does not express the trait, they can still pass that recessive gene copy on to a child. If that child also inherits a copy from their other parent, the trait will be expressed in the child while skipping the parent’s generation.
2. Coincidence and Small Sample Size
With only one or two examples, it may look like a trait skipped a generation by coincidence. For instance, if only 1 grandparent and 1 grandchild have blonde hair while 4 unaffected people are in between, it seems like blondeness skipped but it could have been random chance. The small sample of people observed makes generational skipping seem more pronounced.
3. Variable Gene Expression
Many genes are not simply “on” or “off.” There is a spectrum of how strongly a trait is expressed. So while a parent may carry a gene variant, it may not be expressed enough for them to show the trait. But in the next generation, random fluctuation may cause that gene to be expressed more strongly, making it seem like the trait skipped a generation.
4. Polygenic Inheritance
Most traits are influenced by multiple genes. This means that the combination of gene variants inherited can vary a lot between generations. One generation might get a set of genes that leads to higher expression of a trait while the next might not inherit that exact mix. This makes traits seem more capricious between generations.
5. Epigenetics
Epigenetics looks at how environmental factors can influence how genes are expressed. Things like diet, stress levels, or exposure to toxins can activate or deactivate certain genes. If one generation has different epigenetic factors than the next, traits controlled by those genes may appear to skip generations.
6. New Mutations
Random new gene mutations can always occur. If a mutation happens that leads to a trait not seen before in a family, it may seem like that trait skipped generations. In reality, it’s just arising for the first time in that lineage.
When Traits Do Seem to Skip Generations
While the “skipping generations” idea is oversimplified, there are a few scenarios where traits legitimately seem more likely to be expressed every other generation or to skip generations:
- Mitochondrial genes: Mitochondrial DNA is only passed from mother to child. So mutations in these genes truly only occur every other generation, leading to mitochondrial disorders seeming to skip generations.
- X-linked recessive disorders: These involve mutations on the X chromosome and disproportionately affect boys. Since boys always inherit their X from their mother, this makes X-linked disorders like hemophilia seem to skip generations.
- Codominant traits: With codominant inheritance, both alleles for a gene are partially expressed. This can obscure the typical dominant/recessive patterns and make traits seem to skip around.
- Multifactorial thresholds: For diseases and disorders influenced by multiple genes and environmental factors, there are underlying “liability thresholds” that make a diagnosis more or less likely. Small inherited variations in liability can make a condition seem to skip generations.
In summary, while generational skipping of most traits is an oversimplification, there are select examples where genetic effects do lead to sporadic generational expression.
Examples of Traits That Don’t Actually Skip Generations
While the “skipping generations” idea is compelling, science shows us that for most traits, it is inaccurate or an oversimplification. Here are some common examples of traits that do not truly skip generations:
Eye Color
Eye color is determined by multiple genes and dependent on how much melanin pigment is produced in the iris. The full spectrum of possible eye colors from brown to green to blue represents a range of melanin levels. While parents with brown eyes may have a child with bluer eyes due to recessive genes being expressed, the melanin levels are still inherited quantitatively based on the exact gene variants passed down.
Height
Height is influenced by many genes. While height can vary between generations, each child inherits a blend of the height genes from both parents. Extremely short or tall stature is usually due to distinct genetic conditions rather than random skipping of generations.
Mental Illness
Mental health conditions like depression or schizophrenia have complex underlying genetics that depend on many gene variants. Having a lower “dose” of risk genes may make someone unlikely to be affected, while a higher dose may exceed the threshold for illness. But the genes themselves are still being quantitatively inherited each generation.
Musical Ability
Musical talents are influenced by both genetic propensities and opportunities to develop skills through practice. While interests and training opportunities may vary between generations, intrinsic capacities like rhythm sense, pitch perception, and auditory processing are still genetically passed on in a continuous distribution.
Athleticism
Traits like speed, endurance, strength, and coordination have complex polygenic underpinnings that are influenced by many genes. Parents pass on some blend of their own athletic genetic potential. While one child may train hard enough to actualize their inherited abilities more than another, the inherent abilities themselves are still handed down gradually.
The Bottom Line
While it’s tempting to believe traits we don’t share with our parents represent some quirk of genetics skipping generations, the truth is more complex. Our genomes are not just a series of distinct traits that are either “on” or “off.” Most characteristics we inherit represent the cumulative effects of many genes blending together. Some gene variants may be passed silently for generations before their effects are expressed, while others blend seamlessly into quantitative ranges of traits. Environmental influences add further complexity. While a few singular traits do exhibit true generational skipping, for the vast majority of human characteristics, the notion that they skip generations is inaccurate.
Traits that Really Do Skip Generations
Here is a table summarizing examples of traits that can truly appear to skip generations due to genetic reasons:
| Trait Category | Example Trait | Reason for Skipping Generations |
|---|---|---|
| Mitochondrial inheritance | Leber’s hereditary optic neuropathy | Mitochondrial DNA only passed on from mother |
| X-linked inheritance | Red-green color blindness | X chromosome passed only from mother to son |
| Codominant inheritance | ABO blood type | Both alleles expressed in phenotype |
| Multifactorial threshold | Schizophrenia | Liability threshold must be exceeded for diagnosis |
While these examples show scenarios where traits can truly skip generations, they represent special cases. For the majority of human characteristics, genetic inheritance occurs incrementally rather than sporadically skipping generations.
Conclusion
In summary, while many common traits and conditions may seem to skip generations, true generational skipping is rare when all genetic factors are considered. While unusual patterns of inheritance do occur in narrow situations like mitochondrial genes or X-linked recessive disorders, most human characteristics are influenced by many genes blending together incrementally. Traits only appear to skip generations when looking at small sample sizes or singular genetic effects rather than the full genome interacting with the environment. So while it’s interesting to observe uncanny resemblances between grandparents and grandchildren, science shows that for most traits, generational skipping is more perceived than real.
