Mouse Genetics

Learning Target

• Develop a model to show how sexual reproduction results in offspring with genetic variation.

Success Criteria

• Define gene, trait, and allele.
• Discover how genes are passed down from parents to offspring.
• Distinguish between dominant and recessive traits.
• Predict genetic outcomes using a Punnett Square.

First, you need a little Scientific Vocabulary

• Allele – one of two or more forms that a gene could take.
• Dominant allele – an allele that is always expressed when it is present.
  • Dominant alleles are usually represented by capital letters, such as F.
• Recessive allele – an allele that is not expressed when the dominant allele is present.
  • Recessive alleles are usually represented by lowercase letters, such as f
• Gene – a segment of DNA that determines or helps to determine a trait.
  • Most genes give instructions for building a particular protein.
  • Many familiar traits are determined by several genes.
• Genotype – the genetic makeup of an organism.
• Phenotype – the physical appearance of an organism.
  • Organisms with different genotypes can have the same phenotype. For example, an FF mouse and an Ff mouse both have black fur.
• Heterozygous – having two alleles that are different.
• Homozygous – having two alleles that are the same.
• Hybrid – the offspring of genetically different parents.
  • For example, the offspring of pure FF and ff parents is an Ff hybrid.
• Punnett square – a diagram that shows the possible offspring of two parents.
  • Punnett squares allow you to determine the probability of each offspring genotype.

Activity A: Patterns of inheritance

• Click Clear before you begin
• Drag a black mouse and a white mouse into the parent boxes, but don’t click Breed yet.

Question - What patterns are shown by offspring traits?

1. Predict: What do you think the offspring of a black mouse and a white mouse will look like?
2. Observe: Click Breed several times. What do you see?
3. Observe: Drag two offspring into the Holding Cages. These mice are called hybrids because their parents had different traits. Click Clear, and then breed the two hybrids. What do you see now?
4. Experiment: Turn on Show statistics. Click Breed until there are 100 offspring. How many offspring were black? How many were white?
5. Explore: Try other combinations of mouse parents. Write the results of each experiment in your notes. When you have finished, answer the following questions. (Note: You can refer to the parents as “pure black,” “pure white,” or “hybrid.”)

• Which parent combination(s) yield only white offspring?
• Which parent combination(s) yield only black offspring?
• Which parent combination(s) yield a mixture of black and white offspring?

Back to Teams

• Based on experiments similar to these, Gregor Mendel devised a theory of inheritance. Use your own observations to come up with your own explanation of how a trait such as fur color is passed down from parents to offspring.
• Write your explanation down in your notebook.

Activity B: Genetics Basics

• Click Clear before you begin
• Drag a black mouse and a white mouse into the parent boxes, but don’t click Breed yet.

Introduction: Inherited traits are encoded on a molecule called DNA (deoxyribonucleic acid). Genes are segments of DNA that control a particular trait. Most genes have several different versions, or alleles. The genotype is the allele combination an organism has.

Question: How do alleles determine fur color?

1. Observe: Turn on Show genotype. Move your cursor over a mouse to see its genotype.
   • What is the genotype of the black parent? White parent? These mice are homozygous for fur color, meaning both alleles are the same.
   • Click Breed. What is the genotype of the offspring mice? These mice are heterozygous for fur color, meaning the alleles are different.
2. Analyze: Dominant alleles are always expressed when present. Recessive alleles are not expressed when the dominant allele is also present. Look at the two alleles for fur color.
   • Which allele is dominant, and which fur color does it produce?
   • Which allele is recessive, and which fur color does it produce?
3. Predict: Place two of the Ff offspring into the Holding Cages. Click Clear, and then place them into the parent boxes
   • Which allele(s) could the offspring inherit from parent 1?
   • Which allele(s) could the offspring inherit from parent 2?
   • What are the possible genotypes of the offspring?
4. Experiment: Click Breed several times, and look at the genotypes of the offspring. Did you find all the predicted genotypes? Explain.

Back to Teams

Fill in the remaining squares.

• What are the genotypes of the offspring?
• What percentage of the offspring will have black fur?
• What percentage of the offspring will have white fur?

2. Experiment: Click Breed several times. Were your predictions correct?

3. Model: Use the Punnett squares below to model each parent combination. After filling in each Punnett square, predict the percentages of black and white offspring.

Back to Teams

1. Draw conclusions: How well did the Punnett squares predict the offspring percentages for each parent pair?
2. Summarize: In your own words, describe what heredity is and how it works in mice.