Twins and Adoption in Pedigree Charts: The Special Marks
How to draw and read twins (monozygotic vs dizygotic) and adoption (adopted-in vs adopted-out) on a pedigree, with the standard bars and brackets explained.
- notation
- twins
- adoption
Most of a pedigree is squares, circles, and lines. But two situations need their own marks: twins and adoption. Both matter for genetics, and both are easy to get wrong.
Twins
Twins are shown by how they’re joined, not by shading:
- Monozygotic (identical) — a single horizontal bar on top, joining the two symbols. Sharing one bar says “one egg, one genotype.”
- Dizygotic (fraternal) — an inverted V, meeting at a point, between the two symbols. Two eggs, two genotypes.
Why the distinction matters: if one identical twin has a genetic condition, the other has essentially the same genotype — so affected status of the co-twin changes the odds dramatically. For fraternal twins it’s the same as any sibling pair.
Common mistakes:
- Using identical-twin bars for all twins. Check with the family: same placenta, same zygosity assumptions — but only a note, not proof.
- Forgetting that twins still sit on the sibling bar in birth order with their other siblings.
The twins template shows both kinds in one chart — open it and look at how the bars differ.
Adoption
Adoption is drawn with dashed brackets around the adopted person:
- Adopted in — the brackets face the family (the child is part of this family by adoption, not by blood).
- Adopted out — the brackets face away (this family is the biological family; the child belongs elsewhere by adoption).
The genetics takeaway is the biological connections: an adopted-in child’s medical history comes from the birth family, not the raising family. A thorough pedigree notes both families — which is why the adoption template draws birth family and adoptive family in one chart.
Common mistakes:
- Using dashed descent lines for everything. Descent dashes mean uncertain connection; adoption brackets mean a legal relationship.
- Omitting the birth family when the history is known — that’s the part that carries the genetics.
Adding these to your own chart
In the maker, twins and adoption are set on the child link, not the person: select the child and choose twin pairing (with zygosity) or relationship kind (biological / adopted-in / adopted-out). The chart draws the correct marks automatically — no manual line drawing. If you’re practicing notation for an exam, the symbols guide has the full set.