Genotypes and Phenotypes
Activity 4: Slides 24–26.
Using short- and long-haired dachshunds as examples, students learn about how a change in the nucleotide sequence affects phenotype. They predict the outcomes of crosses between short- and long-haired dachshunds.
Traditionally, biology books have begun the section on genetics with a discussion of the experimental findings of Gregor Mendel (1822–1884). Mendel was an Austrian friar, who studied variation in plants to answer questions about heredity. He created lines of pea plants that were pure-breeding for specific traits, such as flower color (purple or white) and seed surface texture (smooth or wrinkled). He tracked the inheritance of these traits through multiple generations of pea plants, and conducted many crosses of plants with different characteristics. Mendel concluded that hereditary information is transmitted from parents to offspring in the form of discreet “particles,” which we now refer to as genes and alleles.
Beginning with the discovery of DNA, biologists are learning that genetics is much more complex that the particulate model of inheritance proposed by Mendel. In fact, most characteristics (phenotypes) are shaped by the actions of multiple genes, rather than the “one gene—one trait” model that so often is taught to biology students. Traits that are influenced by more than one gene, and often also by behavioral and environmental factors, are referred to as complex traits. Many common human illnesses and conditions, such as cardiovascular disease, diabetes or asthma, are the result of many genes acting in concert with environmental conditions.
To identify genetic changes associated with a particular characteristic or disease, biologists survey large sections of DNA from many different individuals to find areas where changes sequence of bases occurs consistently in affected individuals. Because new technologies make it possible to determine the exact order of the nucleotide bases—adenine (A), thymine (T), guanine (G), and cytosine (C)—in a segment or multiple segments of DNA, sections of the genomes of many different individuals can be compared to find changes in the DNA sequence that might be related to particular traits. These traits may include genetic changes that promote or prevent diseases. This type of survey approach was used to find the mutation responsible for long hair in dogs that students explored in the previous activity.
DNA sequence changes can be simple substitutions at a single location. Other kinds of changes include the insertion of extra bases, the deletion of sections of DNA, and the repetition of one or more sequences. Changes in the sequence of bases are called mutations, and they occur as mistakes when DNA is copied. However, mutations are not always detrimental. In fact, many mutations have no apparent effect on the organism, and in some cases, are beneficial.
As shown in the previous activity, changes in the DNA sequence can (but do not always) affect the proteins that are produced. Overall, the effects of these changes might be neutral, detrimental (leading to death, disease or reduced ability to survive and reproduce) or advantageous (such as conferring disease resistance or a competitive advantage over other individuals).
1. Remind students of the previous activity. Ask, What caused some dogs to grow long hair rather than short hair? [substitution of a single base pair, which changed the protein responsible for ending hair growth; the new protein is defective and does not stop growth of the hair shaft.] Next ask, Was this mutation (or substitution) harmful, neutral or beneficial? Give students opportunities to discuss the ways in which long hair might be helpful or detrimental to different kinds of dogs, depending on where they live or their activities.
2. Now, ask students, Do you think we could use genetic information to predict which puppies might end up with long or short hair? Allow students time to discuss the idea. Clarify for students the differences between phenotype and genotype. Genotype is an individual’s set of genes; phenotype is all of its observable characteristics. These two concepts are related: The genotype is expressed when the information encoded in the genes' DNA is used to RNA and protein molecules. The expression of the genotype contributes to the individual's observable traits, called the phenotype. Phenotype includes body characteristics, developmental patterns, biochemical properties and even behaviors. Environment (all of the external factors that influence an organism) also contributes to phenotype.
3. Prompt student thinking by asking the following questions.
What was the nucleotide at location 20 that resulted in long hair? [T]
How many copies of the nucleotide substitution were necessary for a dog to have long hair? [two]
Thus, what is genotype at location 20 of a dog with long hair? [TT; tell students that when both chromosomes have the same information, the condition is called “homozygous.”]
4. Follow by asking, Can you determine the genotype of a dog with short hair? [In this case, the answer is no, because a short-haired dog could be homozygous for the original form of the gene that codes for short hair; or it could carry one copy of the substitution. Tell students that when an individual carries two different versions (alleles) of a gene, the condition is described as “heterozygous.”]
- Tibetan Terrier © Bonzami Emmanuelle. Licensed for use.
- Helix graphic courtesy of the U.S. National Library of Medicine. Public domain.
- Moreno, N. (2017) Complex Traits: Using Dogs as a Model for Modern Genetics. Baylor College of Medicine: Houston. ISBN: 978-1-994035-08-2.
- Chromosomes and karotype © Matthew Breen, Ph.D. College of Veterinary Medicine, North Carolina State University. Used with permission.
Your slide tray is being processed.