Seeing Pedigree Science: Making a Family Tree of Traits
Has anyone ever told you that you look just like one of your parents or grandparents? Some characteristics, like the shape of your hairline or whether your earlobes are attached or detached, are inherited. In this activity you’ll get to see how writing some characteristics onto a family tree can help you determine just how you inherited them. You will likely discover some characteristics that you got from your father, and for this Father’s Day you can thank your father for giving them to you.
When we look at members of a family it’s easy to see that some physical characteristics, or traits, are shared. In the 1860s, Gregor Mendel discovered that in pea plants some traits are passed down in clear and predictable patterns. Today we know that offspring inherit half of their DNA from each parent. This results in two copies of every gene. Many genes come in different versions, called alleles. Alleles are changes in the actual DNA sequence of the gene. If you have two identical alleles, you are said to be homozygous for that gene, whereas if you have two different alleles you’re heterozygous.
Some alleles are dominant, meaning that if you have one copy of that allele you will display that trait. Other alleles are recessive, meaning you need two copies of that allele to display the trait. For example, Mendel took pea plants that were homozygous for different traits and crossed them. When crossing homozygous purple flowered plants to homozygous white flowered plants, the offspring had purple flowers. The purple allele was dominant. When the heterozygous offspring were crossed to each other, they sometimes had white flowers. The white allele was recessive.
- Paper and a pencil or pen
- Access to a photocopier or four sheets of paper total
- At least three generations of people from the same family. The more members of the family that are available, the better the results will be.
- Draw a family tree, or pedigree, showing the different members of your family. Include all of the family members that you will be getting data from. You can designate the males by a square and the females by a circle. You can look at this resource on Your Family Health History for examples of family trees.
- Make three copies of your family tree (so you have four total).
- Label each family tree one of the following: “Earlobes,” “Widow’s Peak,” “Mid-digit Hair,” and “Hitchhiker’s Thumb.”
- Gather the family members together that you put on your family tree. Alternatively you can do this activity with some members separately.
- Determine whether each family member has attached earlobes or detached earlobes. (Attached earlobes are clearly attached to the side of the head at the bottom of the earlobe, whereas detached earlobes are not.) There can be a range of attachment – just do your best to determine how attached the earlobes are. Do many relatives have the same type of earlobe, or is there variation? Can you see how this trait was inherited? Write down your results in your “Earlobes” family tree.
- Determine whether each family member has a widow’s peak or not. (A widow’s peak is where the hairline comes to a V-shaped point above the person’s forehead.) Keep in mind that widow’s peaks can vary considerably – when determining if a person has a widow’s peak, count any sort of V-shaped hairline as a widow’s peak. Do many relatives either have a widow’s peak or don’t have one? Can you see how this trait was inherited? Write your results in your “Widow’s Peak” family tree.
- Determine whether each family member has hair on their mid-digits (the middle joints on your fingers) or does not have mid-digit hair. You may need to look closely at each person’s hands – if they have any hair on the mid-digit, even a tiny strand, then they have mid-digit hair. Do many relatives have hair on their mid-digits, or are they hairless? Can you see how this trait was inherited? Write your results in your “Mid-digit Hair” family tree.
- Have each family member make a fist with their thumb sticking up and determine whether their thumb is straight or curved. (A curved thumb is also called a hitchhiker’s thumb.) Keep in mind that thumbs come in a wide range of curvedness, from completely straight to completely curved – do your best to decide if a person’s thumb looks curved or straight. Do many relatives have a hitchhiker’s thumb, or are their thumbs mostly straight? Can you see how this trait was inherited? Write your results in your “Hitchhiker’s Thumb” family tree.
- Overall, can you see how different traits were passed down through your family? Are there traits that skipped a generation? Are there traits that were in every generation? Can you figure out which traits might mostly be recessive and which might mostly be dominant?
Extra: There are several other genetic traits that you could investigate using this activity. How does it look like other traits are inherited, such as freckles, cleft chins, toe lengths (whether the big toe is longest or the second toe is longest), and which thumb is on top when you interlace your fingers?
Extra: In this activity you looked at existing family members to investigate how different traits are inherited. Can you use your knowledge from this activity to predict what traits future offspring of different family members might have, if you know what traits their partner has?
Extra: People have long debated whether handedness is a genetic trait. One scientific study showed a correlation between handedness and hair whorl direction, specifically suggesting that a greater percentage of left-handed people have counterclockwise hair whorls than right-handed people. Can you find a correlation between handedness and the direction of peoples’ hair whorls in your family?
Observations and Results
Could you see how some traits were passed down? Did it look like having detached earlobes, a widow’s peak, mid-digit hair, and a straight thumb were all dominant traits?
While earlobe attachment, presence of a widow’s peak, mid-digit hair, and having a hitchhiker’s thumb are thought to be primarily genetically inherited traits, there is some controversy over exactly how they are inherited – in other words, they may be affected by more than one gene, more than two alleles, or factors other than genetics. That said, having detached earlobes, a widow’s peak, mid-digit hair, and a straight thumb are generally considered to be dominant traits, although clearly their inheritance is complex. A dominant trait is one that only needs one copy of the dominant allele to be displayed. For a person to show a recessive trait, they generally need two alleles for the recessive trait. Because a person only needs one copy of the dominant allele to show the trait, they could be homozygous for the allele or heterozygous. This means that a person who shows a dominant trait could have a child with a person who shows the recessive trait (or even another person who shows the dominant trait) and the child may show either trait (since the parent with the dominant trait may be heterozygous). Two people who both show a recessive trait are most likely to have a child that also shows the recessive trait, but inheritance is often affected by other factors.
Ask an Expert
- The Royal Family: Physical Traits, from The House of Windsor
- Your Family Health History, from the National Human Genome Research Institute
- Dominant vs. Recessive, from The Tech Museum of Innovation, Stanford at The Tech
- Pedigree Analysis: A Family Tree of Traits, from Science Buddies