How to Track Genes in Your Family

Everyone has differences in the details of their genetic makeup. The unique combination of genetic information inherited from your parents makes you a unique individual. This page shows how one normal trait can be used to track genes in your family.

Clasp your hands. Which thumb is on top?

Most people have a natural tendency to clasp hands the same way each time. Although not proven, this trait is thought to be determined by a single gene that comes in two different forms:
r = right thumb on top
L = left thumb on top

As shown elsewhere on this site, a person has two copies of each gene. People who clasp their hands with the right thumb on top have two copies of the r form (rr) - one copy of r from mother and one copy of r from father. In the examples below, a circle represents a female, and a square represents a male. People who clasp hands with the right thumb on top are shown with a black background and labeled rr.

People who clasp their hands with the left thumb on top have at least one copy of the L form – they can be either LL or Lr. They have inherited a copy of L from one parent. The other parent passed along either L or r. In the examples below, people with the left thumb on top are shown with a blue background. As the examples show, clues from other family members may allow a specific prediction, or it may be impossible to decide between LL and Lr.

Example 1: In this family, the mother and daughter have their right thumbs on top. Both mother and daughter are rr. Since the father has his left thumb on top, he could be either LL or Lr. However, he had to pass one copy of r along to his rr daughter, so he must be Lr.

Example 2: In this family, most family members clasp hands with the left thumb on top (LL or Lr). Since the middle child clasps with right thumb on top (rr), she must have gotten r from both parents. Then, it is reasonable to conclude that both parents are Lr. The other two children could be LL (L from both mom and dad) or Lr (L from one parent and r from the other). It is impossible to say for sure.

Draw your family tree, and write down how each person clasps their hands. Next, try to make predictions about the gene forms in your family members. Don't worry if you run into problems. As Example 2 shows, a trait may move through a family with a pattern that makes it impossible to predict the genes. In some individuals, the trait may not be apparent. For example, some people may show no preference in how they clasp their hands.

Click here to see other examples of normal traits that have been used to illustrate how genes move through families.

Most family trees are more complicated than the example shown above. Help
with drawing your family tree can be found in Booklet 1, "A Guide to Family Health
History" located on our Family Health History page.

For additional help, here is an example showing how to draw a family tree with divorce and remarriage. The woman in the top row was married and had one daughter. After divorcing her first husband, she remarried and had a son and a daughter. Remember that each child’s genes come from the birth mother and father, even when new family relationships are made later.

What else runs in your family? A specific family resemblance? A special talent? Most of the traits that run in your family will not follow an easily predictable pattern. Unlike the examples given above, most genetic traits are influenced by more than one gene. Although each gene travels from parent to child in a predictable way, genes interact in complex ways to determine inherited traits. Following multiple genes and understanding their combined effects remains a scientific challenge!