Pedigree Analysis: A Family Tree of Traits

Abstract

Objective

The goal of this science project is to use pedigree analysis to determine how several human physical traits are inherited.

Credits

Sandra Slutz, PhD, Science Buddies

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Introduction

Do you have the same hair color or eye color as your mother? Do people say you look just like your grandfather when he was your age? When we look at members of a family it is easy to see that some physical characteristics or traits are shared, but what are the rules that govern the inheritance of these traits?

It was Gregor Mendel, an Augustinian monk and scientist, who first discovered in the 1860's that some traits are passed down from generation to generation, in very 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 several different versions, called alleles. Alleles are changes in the actual DNA sequence of the gene. When you have two identical alleles you're said to be homozygous for that gene. People with two different alleles are heterozygous for that gene. The actual trait, such as red, black, or blond hair, which results from a combination of alleles, is referred to as the phenotype.

Mendelian traits—the kind of characteristics Mendel first studied—are due to a single gene. Some of such a gene's alleles are dominant, meaning that if you have even one copy of that allele, you'll display that trait. Mendel took pea plants, which bred true, meaning for generation after generation they had the same phenotypes. (Today we know that they bred true because they were homozygous for the traits Mendel was looking at). He took a true-breeding purple flowered plant and crossed it to a true-breeding white flowered plant and saw that the offspring all had purple flowers. The purple allele was dominant—a single copy of that allele was sufficient. He then took these offspring and crossed them to one another. He found that their offspring occasionally had white flowers. That's because the white allele was recessive, which means you need two copies of that allele to display a certain trait. The offspring that inherited the white allele from both parents displayed the white phenotype.

Figure 1. Two heterozygous parents displaying the dominant phenotype can give rise to offspring with either the dominant or the recessive phenotype depending on which alleles the offspring inherit.

Just like Mendel observed generations of related pea plants to determine the mode of inheritance for flower color and other physical traits, scientists can examine generations within a family and discover the mode of inheritance for human traits. To do this, scientists create family trees, called pedigrees, showing as many generations of a family as they can and marking who had which phenotype. In order for scientists to understand each other's pedigrees, they use a standard set of symbols and notations. For example, males are always designated by a square and females are always designated by a circle.

Figure 2. Here are two pedigrees showing the same three generations of a family. The pedigree on the left is an example of a dominant trait in this family. The pedigree on the right depicts a recessive trait.

In this science project, you'll create pedigrees for four human physical traits to determine which phenotypes are dominant and which are recessive. All four are Mendelian traits:

• Do you have freckles or not?
• When you interlace your fingers, which thumb is on top—the right or the left?
• Do you have attached earlobes or detached earlobes?

  Figure 3. This diagram shows the difference between detached earlobes and attached earlobes.

• Some people's hairlines come to a V-shaped point above their forehead. This is called a widow's peak. Do you have one or not?

  Figure 4. Here is an example of a straight hairline versus one with a widow's peak.

  
  

Terms and Concepts

• DNA
• Gene
• Allele
• Homozygous
• Heterozygous
• Phenotype
• Mendelian trait
• Dominant
• Recessive
• Mode of inheritance
• Pedigree
• Attached earlobe
• Detached earlobe
• Widow's peak
• Genotype

Questions

• What is a Mendelian trait?
• What are the different modes of inheritance for a Mendelian trait?
• How do you draw a pedigree?

Bibliography

• For a fun and interactive way to learn about dominant and recessive traits try:
  EDC Inc. (2002). Weblab: Mendel's Peas. Education Development Center, Inc., and MathResources, Inc. Retrieved February 19, 2008 from http://www2.edc.org/weblabs/Mendel/MendelMenu.html
  

  Here are some resources for learning how to create pedigrees:

• Bennett, R.L et al. (1995). Recommendations for standardized human pedigree nomenclature. American Journal of Human Genetics. 56(3), 745-752. Can be found at: http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1801187
• Staff. (n.d.). Engle Laboratory, Children's Hospital Boston. How to draw your pedigree. Retrieved February 20, 2008 from http://www.childrenshospital.org/cfapps/research/data_admin/Site339/mainpageS339P2sublevel9Flevel11.html

Materials and Equipment

• Paper
• Pen
• Access to a photocopier (optional)

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Experimental Procedure

Before starting this science project, you should go through your reference material and familiarize yourself with the proper way to draw a human pedigree.

1. To start this project, you should draw a pedigree showing the different members of your family.
   1. Include as many family members as you can get data from. The more people and generations you include, the more likely it is that you'll have enough information to determine the mode of inheritance.
   2. You might need help from your parents to figure out all the relationships.
2. If you have access to a photocopier, make four copies of the pedigree—one for each trait you are going to evaluate. If photocopying isn't an option, manually copy the pedigree.
3. Determine the phenotype of each person on your pedigree for each of the four traits. Use a separate pedigree for each trait. Note: Widow's peak can vary considerably; score any sort of V-shaped hairline as positive.
4. From your pedigrees, can you deduce the mode of inheritance for each trait? For which traits is your pedigree informative? If you don't have enough information to determine the mode of inheritance of a particular trait, try making a pedigree for another family.

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Variations

• Through this science project, you've recorded the phenotypes of various family members for a variety of traits, and you've determined the mode of inheritance for these traits. Now, using this information, predict your own genotype for these traits. Depending on the genotype of your future mate, what possible phenotypes could your children have?
• Through your bibliographic research, did you discover any other Mendelian traits? What are they? Do the pedigree analysis for any of those that seem interesting.
• Not all traits are governed by a single gene. Most characteristics are multigenic, meaning that the observed phenotype is due to the action of several genes in combination. Eye color is an example of a multigenic trait. Do some bibliographic research to learn about how eye color is inherited. Once you know how it is inherited, draw your family pedigree for eye color. Can you use the pedigree and your knowledge about how eye color is inherited to determine the possible genotypes of the family members in your pedigree?
• Discovering the mode of inheritance of a trait can sometimes be very difficult. People have long debated whether handedness (i.e. being left-handed or right-handed) is a genetic trait. Read the latest scientific theory on handedness: http://www.americanscientist.org/issues/pub/2004/1/head-in-hand. If you analyze handedness and hair whorls in your pedigree, does your data support or refute this current theory?

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