Free response · Unit 7, Skills: Statistics and math, Skills: Experimental design and argumentation Beta

Allele frequencies against Hardy-Weinberg expectations

Interpreting and Evaluating Experimental Results · 9 points

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The question

Name: ______________________   Date: ____________   Allele frequencies against Hardy-Weinberg expectations (9 points)

Data table

Armor genotypes of young and adult sticklebacks

Threespine sticklebacks are small fish. A gene called Eda has two codominant alleles: C, which gives a full row of bony armor plates along the side, and L, which gives only a few plates. A DNA test of a tiny fin clip shows each fish's genotype. In a large lake with no inflow or outflow streams, trout that eat sticklebacks were introduced ten years ago. In June, researchers caught 200 one-month-old juveniles at random from all parts of the lake, took fin clips and released them. The following June they caught 200 one-year-old fish of the same year class, also at random, and genotyped them. Sticklebacks in this lake first breed at two years old.

Number of fish of each Eda genotype (n = 200 per sample). Critical values of χ² at p = 0.05: 3.84 (1 degree of freedom), 5.99 (2 degrees of freedom).
SampleCC (full armor)CL (partial armor)LL (low armor)
Juveniles, age 1 month2010080
Same year class, age 1 year4011050

  1. (a) (i) Calculate the frequency of allele C among the juveniles. (ii) Calculate the number of CL juveniles expected if the juveniles are in Hardy-Weinberg equilibrium. [2 points]

  2. (b) The researchers compare the one-year-olds with the genotype counts expected if every genotype survived equally well from one month to one year. (i) State the null hypothesis for this comparison. (ii) Justify the decision to sample the same year class twice, rather than comparing juveniles and adults from different years. [2 points]

  3. (c) Using the juveniles' observed genotype frequencies to calculate the expected counts among the 200 one-year-olds, calculate the chi-square value, and determine whether the null hypothesis should be rejected at p = 0.05. [2 points]

  4. (d) (i) Identify the Hardy-Weinberg condition most likely violated between one month and one year, and justify your answer using the data. (ii) Explain why the juveniles can fit Hardy-Weinberg expectations even though this condition is violated in the lake. (iii) Predict how the frequency of allele C among the juveniles this year class produces when it first breeds will compare with 0.35, and justify your prediction. [3 points]