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How to Find Twins on Pedigree Charts

by Alyssa LaRenzie

Pedigree charts represent a family tree using various symbols. Each chart connects family members through marriage and generations born with the oldest family members at the top. Pedigree charts can also be used to track a certain genetic trait, such as eye color, by marking the dominant or recessive gene with a different symbols. Twins also have a special symbol in the chart.

Identify twins by locating diagonal lines on the pedigree chart. Horizontal lines indicate marriages and vertical lines indicate children in most cases. If two siblings have diagonal lines that meet at the horizontal line connecting the parents, these children are twins.

Determine whether the twins are identical or fraternal twins by whether or not the siblings are connected by an additional horizontal line. This extra line, which forms a triangle coming down from the parents' horizontal marriage line indicates that the twins are identical twins, also known as monozygotic twins. The absence of this line means the twins are fraternal or dizygotic twins.

Learn more information about the twins by decoding the symbols. A square represents a male and a circle represents a female. A shaded shape indicates that person has the genetic trait that's being focused on in the chart. A diagonal slash through a person means that person is deceased. These lines go over a shape rather than connecting them to another, like the diagonal lines that represent twins.

Tip

  • Don't confuse the horizontal line connecting identical twins as a marriage. If a twin is married, an additional horizontal line will be drawn from the other side. This person will not have a vertical line connecting to the parents of the twins.