BioEd Online

Teacher Background

From the tiny, long-haired Pekingese to the tall, short-haired Great Dane, domestic dogs show immense variety in their sizes, shapes, coat colors and textures, and behaviors. Selective breeding by humans for desirable genetic changes or mutations generated this variability.  Today, more than 400 distinct breeds of dogs are recognized.

Because of its wide range of characteristics, the domestic dog (Canis familiaris) has become an important model for modern genetic studies. However, most of this diversity is found only between dog breeds. The characteristics of individuals within a particular dog breed are very uniform—genetically and in appearance. In other words, dogs are homogeneous in their appearance (phenotype) and genetic makeup (genotype) within breeds, and are phenotypically and genetically heterogeneous among or between breeds. This pattern of variability, in which particular traits are accentuated within breeds, results from inbreeding (breeding of close relatives).

Dogs are members of the Canidae family of mammals. This group also includes coyotes, foxes, jackals, and wolves. The gray wolf is the closest relative of the dog. However, the gray wolf (Canis lupus) does not show the wide range of phenotypic variability found in the dog.

Even though dog breeds appear quite distinct, they still freely interbreed without physical or other barriers. In other words, all domestic dogs are members of the same species. Even with their wide range of appearances, dogs are more similar to one another genetically than they are to grey wolves, their closest relatives.

Dogs and wolves, which are separate species, also occasionally do interbreed. Normally, the two species maintain separate identities, have different physical characteristics and have different evolutionary histories. In addition, dogs and wolves usually are isolated reproductively (in natural circumstances, dogs and wolves rarely interact or have opportunities to breed).

Experts believe that the domestication and divergence of dogs from wolf populations began about 15,000 years ago. Most modern dog breeds originated within the past few hundred years. Domestication led to selection for and retention of certain favorable traits, such as herding or tracking behaviors, in addition to preferred physical characteristics.

As part of this activity, students examine a diagram that shows the relationships among species in the dog family (Canidae) based on molecular genetic information. It is important to note that diagrams of this type are not family trees or genealogies. Instead, they represent similarities or differences among living (extant) groups based on their genetic information. Groups that are found on the same branches (or clades) of the tree share unique genetic information and are descendants of the same founding or ancestral population.

In the accompanying diagram, species A and B are more closely related to each other more than to any other species or cluster of species on the diagram. Similarly, species C and D are more closely related to each other than to a member of any other group. Of the species on the diagram, species E is the least related to any of the other groups.

This type of diagram, in which groups are defined by shared inherited characteristics, usually is called a phylogeny or phylogenetic tree. The term phylogeny comes from the Greek words, phylon (tribe or clan) and genesis (origin).

Objectives and Standards

Materials and Setup

• Complex Traits PowerPoint^® slide set (slides 2–10)
• Computer and projector, or interactive whiteboard
• Copies of student page, "Dogs and Related Species" (one per student or one per group)

Procedure and Extensions

Download the PDF for an Answer Key, Extensions, Student Pages and additional information.

1. Share with students that they will be starting a unit on modern genetic information, using examples from dogs. Project Slide 2 as an introduction to the unit, then show Slide 3 and ask students, Would you have this animal as a pet? Why or why not? Allow a few minutes for students to respond to your questions. [Animal depicted is a grey wolf.]

2. Show the next slide to students, and ask, What about this animal? Is it suitable as a pet? Why or why not? Tell students that the slide depicts a common dog breed, an English springer spaniel.

3. After students have discussed characteristics of the dog, project Slide 5, which compares the gray wolf and dog, side by side.

4. Create a table at the front of the class and show Slide 6, which provides samples of characters and traits of the wolf and dog shown in the previous slides. Have students work in groups of two or four to create similar charts that list characters that differ between the wolf and dog shown. Have each group find at least five characters that are different for the wolf and the dog. OR conduct a discussion and create a class chart.
   
   NOTE: You may want to clarify for students that “character” refers to a feature that can vary from one individual or group to another. “Trait” refers to a specific variation or form of a given feature. For example, hair texture is a character. Curly hair and straight hair are two different traits.  

5. Show students Slide 7, and explain that scientists now can use genetic information to estimate the relationships among different groups of species. Species that are more closely related share more genetic information. In the slide, the numbers in red represent similar versions of genes. Ask, Which numbers are shared by all of the groups? [1,2,3]. Which numbers distinguish the branch containing Species A, B, C and D, from the branch with Species E? [4]. How are the branches with Species A and B, and C and D different?
   
   One also can say that Species A and B share a common ancestor, as do Species C and D. The entire group of Species shares a more distant common ancestor, which was characterized only by genes 1, 2 and 3.

6. Give each student or group of students a copy of the student page, “Dogs and Related Species.” This diagram also is provided as Slide 8. Explain to students that this diagram was created using real genetic information from the different species. Have students interpret the diagram to answer the questions at the bottom of the student page. (Optional viewing: Slides 9–10 contain photos of the dogs featured on the illustration.)

Handouts and Downloads