Name___________________________________________ Biology 1 Date_____________________
Earlier this unit we used a Chi-square test to see whether the number of male and female students taking different science courses is consistent with the hypothesis that biological sex does not influence what science course students choose. (You can look back in your packet for the results!)
Remember that Chi-square is used for categorical data, to compare the difference between
observed and expected frequencies of different categories. Here is the formula:
The number produced by this formula is the chi-squared value. When combined with the degrees of freedom
(# of categories – 1), the p-value can be determined. It is similar to a T-test, where 0.05 is the cutoff of significance. If p<0.05 it means the observed is different than expected, in other words, your hypothesis must be rejected. If p>0.05, then you can say the differences are just due to chance, and you can accept your hypothesis. You can use graphpad.com or your calculator to help with the calculations.
The chi-square test in genetics.
1.
There is a breed (type) of cat without fur called “sphynx”.
Hypothesis: The allele that causes lack of fur is believed to be recessive.
Use this pedigree to determine the genotypes of the F
1
parents:
P
F
1
F
2 generation. Show your work (Punnett Square):
?
Use your knowledge of genetics to predict what percentage of sphynx cats would be expected in the F
2
When this type of cross (mating) was repeated several times, there were a total of 59 kittens. 11 kittens were sphynx and the other 48 had fur. What are the expected outcomes according to this hypothesis?
Use a chi-square test to determine if the data are consistent with the “sphynx” allele being recessive:
# Observed # Expected Chi-square value:
Sphynx kittens
Normal fur kittens
Conclusion:
Degrees of freedom:
p-value:

2. Another type of cat has no tail. This type of cat is called “manx”.
A cross (mating) between two manx cats produces mostly manx kittens. A cross between a manx cat and a normal cat produces about half manx and half normal kittens (with tails).
II
I a) What pattern does this allele seem to show? Give evidence for your answer from the pedigree.
Your Hypothesis: b) After a series of matings between a heterozygous manx cat and a normal cat, 43 manx kittens and 55 normal kittens are born. Draw a Punnett Grid to determine the expected phenotypic ratio from your hypothesis.
Use a chi-square test to see if the data are consistent with your hypothesis:
# Observed # Expected Chi-square value:
Manx kittens Degrees of freedom:
Normal kittens p-value:
Conclusion: c) A series of matings between two heterozygous manx cats produces 65 manx kittens and 38 tailed kittens.
Use a chi-square test to see if the data are consistent with your hypothesis:
# Observed # Expected Chi-square value:
Manx kittens
Normal kittens
Conclusion:
Degrees of freedom:
p-value:

Biology 1: Genetics and Chi-Square Statistical Test, part 2
The Manx Mystery Solved!
There is another pattern of inheritance for alleles besides dominant, recessive, co-dominant, X-linked, etc. This pattern is called “lethal”. The “manx” allele is a lethal allele.
If an egg and a sperm both carry a “lethal” allele, then the embryo they form will not grow. These embryos will never be born. d) Make a Punnett grid for two manx cats and give the expected phenotypic ratio for the kittens: e) Apply the hypothesis that the manx allele is lethal to the two manx cat problems from page 1:
After a series of matings between a heterozygous manx cat and a normal cat, 43 manx kittens and 55 normal kittens are born.
H
0
:
# Observed # Expected Chi-square value:
Manx kittens Degrees of freedom:
Normal kittens p-value:
Conclusion:
A series of matings between two heterozygous manx cats produces 65 manx kittens and 38 tailed kittens.
H
0
:
# Observed # Expected Chi-square value:
Manx kittens Degrees of freedom:
Normal kittens p-value:
Conclusion: f) Why is it possible for two different hypotheses (2b & 2e) to both be consistent with the same data? g) Is it possible to have a pure-breeding line of manx cats? Explain.

3. There is a breed of cats called “American Curl”. These cats have ears that curl back. a) Matings between two heterozygous American curls produce 77 curl kittens and 28 normal kittens.
Consider what you have learned from this activity. What can you say about the pattern of inheritance for the “American curl” allele?
Record your ideas and calculations:
Hypotheses: b) Further matings between two heterozygous American curls produce a total of 770 curl kittens and
280 normal kittens.
Discuss whether these data have any effect on your conclusions. Record your ideas and calculations: c) Would it be possible to have a pure-breeding line of American curl cats? Explain.