Practice Problem
In pea plants, green color (G) is dominant to albino (g). If we cross two pea plants that are
heterozygous for color, what would be the expected phenotype ratio of the offspring? Do the Punnett
square and write the phenotypic ratio in the space below:
Let’s say that we actually did cross two heterozygous pea plants and obtained the following data:
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Now we will calculate a Chi-Square value for this data and find out if any difference between what
we observe and what we expect can or cannot be explained by chance.
First, write out our standard null hypothesis:
Null Hypothesis: _____________________________________________________________________
Next, we will follow the same procedure that we did for the M&Ms.
Fill out the table below by figuring out the number of expected offspring among these phenotypes
and calculating the Chi-Square value. Then, determine the degrees of freedom.
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What is the number of degrees of freedom for this problem? _________
(Remember: degrees of freedom = number of categories –1)
Based on the data, should you ACCEPT or REJECT the null hypothesis? Why?
In dogs, black (B) is dominant to chestnut (b), and solid color (S) is dominant to spotted (s). A male dog
with the genotype bbSs is mated to a female whose genotype is BbSs.
a) What are the phenotypes of the two parents?
b) Show a Punnett Square for this cross, and list the expected phenotypic ratios for the offspring.
c) After several litters from this cross, the following phenotypes of puppies were totaled: 5 solid
chestnut, 8 solid black, 3 spotted black, 2 spotted chestnut. Are these data within the experimental error
of the ratio you predicted above? Show your chi-square calculations, as well as your p value and
determination of significance.