Key concepts
Physical traits

Has anyone ever told you that you look just like one of your parents or grandparents? Some characteristics, such as 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 dad's side of the family, and for this Father's Day you can thank him 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 botanist Gregor Mendel discovered that in pea plants some physical features, such as flower color, are passed down in clear and predictable patterns. Today we know that offspring inherit their DNA from both parents—half from each. This results in two copies of every gene, which is composed of a variety of pairs of DNA. Many genes come in different versions, called alleles. Alleles are differences in the DNA sequence of a 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 crossbred them. When crossing homozygous purple flowered plants to homozygous white flowered plants, the offspring (which were heterozygous) had purple flowers. The purple allele was dominant and the white one was recessive. When the heterozygous purple-flowered offspring were crossed with one another, some of their offspring would wind up with two copies of the recessive allele, giving them white flowers.

• Paper and a pencil or pen
• Four sheets of paper
• 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 from whom you will be getting data. 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” (pdf) 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 or detached earlobes. (Attached earlobes are clearly attached to the side of the head at the bottom of the earlobe whereas detached ones hang free.) 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 on your "Widow's Peak" family tree.
• Determine whether each family member has hair on their mid-digits (the middle joints of the fingers) or does not have it. 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 on your "Mid-Digit Hair" family tree.
• Have each family member make a fist with their thumb sticking up. Determine whether their thumb when extended 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 be mostly recessive and which might be mostly 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 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 do 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?

Although the presence or absence of attached earlobes, a widow's peak, mid-digit hair and 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 or heterozygous for the allele. 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 (because 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.

More to explore
The Royal Family: Physical Traits , from The House of Windsor
Your Family Health History , from the National Human Genome Research Institute
Dominant versus Recessive , from The Tech Museum of Innovation, Stanford at The Tech
Pedigree Analysis: A Family Tree of Traits , from Science Buddies

This activity brought to you in partnership with Science Buddies