Carriers are the key
A recessive trait shows only when both copies of the gene are recessive. A parent who shows the dominant trait can still carry one hidden recessive copy, and two carriers can have a child with the recessive trait. If that copy came from a grandparent, the trait looks like it skipped a generation.
| Generation | Example (recessive trait a) |
|---|---|
| Grandparent | Grandma aa (shows the trait) |
| Parent | Dad Aa (does not show it, a carrier) |
| Grandchild | Dad Aa × Mom Aa → 1 in 4 chance of aa (trait shows again) |
Many facial traits involve several genes, so real resemblance cannot be explained by this model alone. It works best for traits like blood type.
Estimating from grandparents
- If one grandparent shows the recessive trait, a parent who shows the dominant trait must be a carrier.
- If all grandparents show the dominant trait, the parent’s carrier chance can only be estimated from population frequency.
- Work out the carrier chance on each side and combine them to approximate the grandchild’s chance.
Same parents, different siblings
Each parent passes on one of two copies at random every time. Siblings share about half their genes on average, so children of the same parents can differ in blood type, eye color and height. Apart from identical twins, siblings being quite different is normal.
Pedigree and siblings in the app
Genetics class → 3-generation pedigree
Lower in the Genetics class tab of the Android app Genetic Kids Lab, the 3-generation pedigree takes a trait and the four grandparents’ phenotypes, then shows each parent’s chance of being a carrier and the grandchild’s chance of showing the trait.
Lab → Make 4 siblings
In the Lab tab, Make 4 siblings creates four virtual children of the same parents with height, blood type, eye color and an average trait match rate. Save a child you like with Save favorite to find it later in the History tab.
FAQ
If a trait skips a generation, will it always come back in the grandchildren?
No. Even when both parents are carriers, each child has a 1 in 4 chance, and if only one parent is a carrier, the trait does not show.
Can blood type look like it skipped a generation?
Yes. If two type A parents are both AO, they can have a type O child who seems to take after a type O grandparent. See the blood type chart.
Genetic Kids Lab - Child Trait Simulator
Pick the parents’ traits and see trait ratios across 1,000 virtual children. An educational, just-for-fun simulator that works offline. Free on Android.