The twin symbol in genograms shows two siblings connected to their parents by lines that converge at a single point, rather than descending from separate points on the marriage line. Identical (monozygotic) twins add a horizontal bar between the converging lines, while fraternal (dizygotic) twins have no bar. This notation captures the unique bond and clinical dynamics of twin relationships.
Identical Twins
Converging lines + bar
Fraternal Twins
Converging lines, no bar
Twins: lines converge at one point | Identical: horizontal bar | Fraternal: no bar
How Twins Are Shown in a Genogram
In standard genogram notation, twins are distinguished from regular siblings by the way their connecting lines descend from the parents' marriage line. Regular siblings each have their own separate vertical line dropping from the horizontal marriage line. Twins, however, share a single point of origin: both lines descend from the same point and diverge downward to reach each twin's symbol.
The critical visual difference between identical and fraternal twins is the presence or absence of a horizontal bar. Identical (monozygotic) twins have a short horizontal bar connecting the two diverging lines, indicating they developed from a single fertilized egg. Fraternal (dizygotic) twins have no bar, indicating two separate eggs were fertilized simultaneously.
Standard Notation
According to McGoldrick, Gerson, and Petry: "Twins are shown by lines from the family line converging to a common point. Identical twins are indicated by a bar connecting the twin lines; fraternal twins have no connecting bar." The convergent point and optional bar are the defining features of twin notation.
Identical vs. Fraternal Notation
The distinction between identical and fraternal twins is not just biological but clinically significant. The genogram captures this with simple but effective notation:
Identical (Monozygotic)
Converging lines with a horizontal bar. These twins share nearly 100% of their DNA, are always the same sex, and often have particularly intense psychological bonds.
Fraternal (Dizygotic)
Converging lines without a bar. These twins share approximately 50% of their DNA (like any siblings), may be different sexes, and often have more differentiated identities.
Triplets and Multiples
Three or more lines converge at the same point. Bars connect identical multiples within the group. For example, identical triplets have bars connecting all three; two identical plus one fraternal has a bar only between the identical pair.
Twin Loss
When one twin has died, the deceased twin's symbol receives an X while the twin notation (converging lines and bar) remains intact.
Triplets and Higher-Order Multiples
For triplets, quadruplets, or higher-order multiples, the same convergence principle applies: all lines descend from a single point on the marriage line. The number of lines equals the number of children. Bars connect any subset that is identical. For example, with triplets where two are identical and one is fraternal, a bar connects only the two identical children, while the third line has no bar.
Higher-order multiples (quadruplets and beyond) are increasingly common due to fertility treatments. The genogram should note if conception involved assisted reproductive technology (IVF, fertility drugs), as this information is medically and sometimes psychologically relevant.
Clinical Significance
Twin relationships carry unique clinical dynamics that the genogram helps to map:
Twin bond intensity: Identical twins often share an unusually close emotional bond that can affect their ability to form independent relationships
Identity and individuation: Twins face unique developmental challenges around establishing individual identities, particularly identical twins who may struggle with differentiation
Favoritism and comparison: Parents may unconsciously favor one twin or assign complementary roles ("the smart one" and "the athletic one"), creating lifelong identity patterns
Twin loss grief: The death of a twin is often described as losing a part of oneself; surviving twins may experience complicated grief that differs from other sibling loss
Separation anxiety: Twins who have never been separated may experience intense anxiety when separated (college, marriage, relocation)
Generational patterns: Twinning can run in families (particularly fraternal twins); noting this pattern across generations provides genetic context
Pattern Recognition
When mapping twins, explore the family narrative around their twinship: Are they treated as a unit or as individuals? Were they dressed alike as children? Do they have distinct roles in the family? Has separation (school, marriage, geographic distance) been difficult? These patterns reveal how the family system has managed the unique challenge of raising children who share a birthday and, in the case of identical twins, a face.
How to Add Twins in GenogramAI
Steps to Add Twin Siblings:
1Select the parents' marriage line on the canvas
2Click "Add Child" and create the first twin
3Add the second twin as another child of the same parents
4Select both children and choose "Mark as Twins"
5Specify whether they are identical or fraternal
6The converging lines (and bar for identical) appear automatically
Twin Loss
The death of one twin presents unique clinical challenges. On the genogram, the deceased twin is shown with the standard X through their symbol while the twin notation (converging lines) remains. If the loss occurred at birth (vanishing twin syndrome or stillbirth of one twin), it is shown in the same way but with the appropriate loss notation.
The surviving twin's grief is qualitatively different from other sibling losses. Research by Woodward (1998) and others indicates that surviving twins report a persistent sense of incompleteness, difficulty forming close relationships that match the intensity of the twin bond, and identity confusion when part of what defined them (being a twin) is lost.
Special Cases
Conjoined Twins
Conjoined twins are shown with the identical twin notation (converging lines plus bar) with an additional notation indicating they are conjoined. If surgically separated, note the separation date. If one did not survive separation, use the deceased X for that twin while preserving the twin notation.
Unknown Zygosity
When it is unknown whether twins are identical or fraternal—common with same-sex fraternal twins who look similar or with historical family members—use the converging lines without a bar and add a question mark. Some practitioners use a dashed bar to indicate uncertain zygosity.
Twins from Assisted Reproduction
Twins conceived through IVF, fertility drugs, or other assisted reproductive technology may be noted with a specific annotation near the convergence point. This is medically relevant (IVF twins are almost always fraternal unless an embryo splits) and can be psychologically relevant for the family narrative around conception and identity.
How do I tell the difference between identical and fraternal twins on a genogram?
Identical twins have a horizontal bar connecting their two converging lines; fraternal twins do not. Both types share a single convergence point on the parents' marriage line, which distinguishes them from regular siblings who each have separate connection points.
How do I show triplets?
Three lines converge at a single point. If all three are identical, bars connect all three lines. If two are identical and one is fraternal, a bar connects only the identical pair. If all three are fraternal, no bars are used. The same logic extends to quadruplets and higher-order multiples.
What if one twin died at birth?
Show both twins with the converging line notation. The deceased twin receives an X through their symbol, and the death date (or "stillborn") is noted. The twin notation remains intact because the surviving child is still a twin, even though their co-twin has died. This is clinically important.
How do I show twins who were adopted separately?
Show both twins connected to the biological parents with converging solid lines (and bar if identical). Then show each twin connected to their respective adoptive families with dashed lines. This dual representation captures the biological reality of twinship alongside the social reality of separate adoptive families.
Does the twin notation change the birth order display?
Twins occupy the same birth-order position since they were born at the same time. On the genogram, they appear side by side in the sibling row. If there is a birth order relevance (Twin A born first), this can be noted, though for fraternal twins the first-born twin is typically placed to the left.
Can fraternal twins be different sexes?
Yes, fraternal twins can be different sexes (one male, one female). On the genogram, one would be shown as a square and the other as a circle, connected by converging lines without a bar. Different-sex twins are always fraternal, as identical twins are always the same sex.
How do I show the "vanishing twin" phenomenon?
Vanishing twin syndrome, where one twin is absorbed early in pregnancy, can be noted with a small symbol with an X at the convergence point. Include the approximate gestational age at which the twin was lost. While the surviving twin may not be aware of their twin status, this information can be relevant for medical history and, in some therapeutic frameworks, for understanding prenatal loss.