Lesson 2 — Mendel's Laws and Monohybrid Crosses


Lab: Mouse Genetics (One Trait)

Introduction

© Getty Images

Many traits have two clear phenotypes, such as tall or short pea plants. Sometimes these traits are controlled by a single gene with two different alleles. One is dominant and the other recessive. When an organism has two copies of the dominant allele, or one dominant allele and one recessive allele, the dominant trait is expressed, such as green pea colour. Only when the organism has two copies of the recessive allele does the recessive phenotype come through, such as yellow pea colour.

In this simulation, you will explore mouse coat colour as a single gene trait. You will conduct various breeding cycles until you understand how dominant and recessive alleles perform and until you understand how to determine a mouse's genotype from test crosses and from observation of phenotypes.


Scientific Background
The patterns of inheritance shown by the Mouse Genetics (One Trait) Gizmo were discovered originally by Gregor Mendel. After attending the University of Vienna, Mendel became interested in pea plants to study the mode of inheritance. Pea plants have many phenotypic traits that are easy to observe and are determined by a single gene. After years of experiments, Mendel discovered two remarkable patterns. The heterozygous offspring (F1) of two pure breeding homozygous parents all resemble one parent. If two of the F1 heterozygous offspring are bred, 75% of their offspring (F2) show the dominant trait while 25% shows the recessive trait. These patterns helped Mendel formulate his theory of inheritance and begin the modern science of genetics.


Problem

Manipulate the P1 breeding pair and observe the resulting offspring to determine how dominance and recessive alleles produce a recognizable inheritance pattern. How can understanding this pattern allow you to predict the outcome of various new crosses? How can the results from a cross determine the genotype of the parents?


Materials

For this simulation, you require access to the Internet and a word processing program to record your results.


Procedure
  1. Open the link to Mouse Genetics (One Trait) Gizmo.

  2. Follow the instructions listed in Exploration Guide for Activity A: Patterns of Inheritance.

    1. The top two spots in the mouse house are the P1, or parental generation. Whichever two mice you place here will breed to produce the other five spots you see in the mouse house (the F1 generation).

    2. To start, you can place either two black or two white mice as parents. What kind of offspring result? Does it matter how many times you click "breed"? 

    3. The next pair of parents should consist of one white and one black mouse. What offspring result now?

    4. Now, keep two F1 mice by dragging them into the cages at the bottom. Then, clear the house with the "clear" button and drag the two from the cages into the top breeding spot. Click "breed" to generate the next group of offspring (the F2 generation). What kind of offspring may result from this cross? Breed this group until you get a few mice of both colours among the offspring, or F2.

  3. Follow the instructions for Activity B: Genetics Basics and Activity C: Modeling Inheritance. As you read and follow the instructions, be sure you can answer the questions listed.


Analysis
  1. If two parents of the same colour are bred together, do all their offspring always have that colour?

  2. How do you know that the trait for white fur is still present in a black fur heterozygous mouse?

  3. Why is it impossible to see a heterozygous white mouse?

  4. A black mouse is bred to a white mouse, producing 2 black and 1 white offspring. What is the genotype of the black mouse?

  5. A black mouse is bred to a white mouse. They produce 3 black offspring. What is the genotype of the black parent?

  6. An Ff mouse is bred repeatedly to an ff mouse, producing 500 total offspring. In theory, 250 offspring should be black and 250 should be white, but the actual numbers are 237 black and 263 white. Why does this happen?


  1. If two white mice are bred together, all their offspring will be white. If two pure-breeding black mice are bred together, all  their offspring will be black. If both black mice are heterozygous for black colour, 25% of their offspring will be white.

  2. A test cross can be completed by breeding the suspected black fur heterozygous mouse with a white mouse. If any white mice appear in their offspring, the black fur heterozygous mouse has a trait for white fur.

  3. The allele for white fur is recessive to black fur and a heterozygous mouse for fur colour always expresses the dominant fur colour, black.

  4. The black mouse must have an allele for the white fur to produce at least one white offspring. Therefore, the genotype of the black mouse is heterozygous, Ff.

  5. The genotype of the black mouse cannot be confirmed. Although all three offspring had black fur, further test crosses must be completed to ensure that the black fur colour of the offspring did not occur by chance.

  6. The theoretical value is 50:50 for black and white. The actual value always deviates from the theoretical or experimental value. This can be seen in the male to female ratio in human populations. Theoretically, any given population should be 50% female and 50% male. However, the actual number deviates slightly; for example, the female population in both Calgary and Edmonton at 49% is slightly below the theoretical value.

Assessment Questions
Complete the assessment questions in the simulation and check your answers.

Biology 30 © 2008  Alberta Education & its Collaborative Partners ~ Updated by ADLC 2019