Littorina Shells
Genetics Question. Hypothesis and Chi Test?
The shells of a species of marine snail, Littorina sitkana, can be either tall or short. Tall (T) is dominant to short (t). In addition, the shell colour brown (B) is dominant to the shell colour yellow (b).
A tall brown male is mated to a short yellow female and the resulting F1 offspring are. allowed to interbreed. The resulting 320 F2 offspring had the following phenotypes:
188 – Tall brown
51 – Tall yellow
66 – Short brown
15 – Short yellow
therefore….
a) Propose a hypothesis to explain the observed frequency of each phenotype, include the expected phenotypic ratio based on this hypothesis.
b) Compute the X2-value to test your hypothesis.
c) Explain whether the X2-value supports or refutes your hypothesis?
any help is appreciated!! =)
a. The F2 phenotypic ratio is very close to 9:3:3:1, which is what you expect from a cross between two indviduals that are heterozygous at both loci. Therefore, the F1s were most likely TtBb and that means the parents were TTBB and ttbb.
b. Chi-square is the sum of the ((obs-exp)^2)/exp for all phenotype classes. You have your observed values. Your expected values are:
Tall, Brown = (9/16)*320 = 180
Tall, Yellow = (3/16)*320 = 60
Short, Brown = (3/16)*320 = 60
Short, Yellow = (1/16)*320 = 20
So, Chi-square = ((188-180)^2)/180 + ((51-60)^2)/60 + ((66-60)^2)/60 + ((15-20)^2)/20 = 3.56
c. Then, you take your chi-square value and go to the chart. Find the row with 3 degrees of freedom (because d.f. = # of classes-1) and read across until you get to your Chi-square value, then go up to the top to see your p-value. With your value, 0.5>p>0.3, so since your probability is much more than 0.05, your data support your hypothesis.
Littoraria angulifera
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