# Tips for A2 Biology, Unit 4?

- 0 votes

Advice for Unit 4 Biology at A2 level, particularly with the Hardy-Weinberg Equation and Energy :)

## 2 Answers

- 2 votes

Get as many of the mark schemes as possible for all the topics and learn them, nearly all the questions are similar. Read around the subject slightly, AQA could throw a question in that might be unexpected.

For the Hardy-Weinberg equation read through the example in AQA Nelson Thornes book, do the summary questions and application, and then attempt past papers. Just have an expectation that it could be on the paper.

Remember: As there are two alleles the frequency of one plus the other is equal to 1 (100%)

P (dominant allele) + q (recessive allele)=1.0

remember the full equation of p² + 2pq + q² =1.0 (AA [homozygous dominant] + Aa [heterozygous] + aA [heterozygous] + aa [homozygous recessive]

They could give you information on those who posess the recessive allele or dominant allele.

If they give you recessive, the frequency of the homozygous recessive (aa/q²) =The number who posess it or express it in their phenotype(divided by)/ the total population.

Remember that q² is equal to two (homozygous) recessive alleles, similar to p² being two dominant alleles. So q=(square root)√frequency of homozygous recessive (q²)

After doing this we have the value of q. Reverse the p+q =1.0 equation

p=1-q.

Now you have both values of q and p so we can isolate 2pq from p²+2pq+q²=1 (two different alleles) if we need to calculate the heterozygotes. as we have calculated q from q² and p by subtraction. Just simply input the values into 2pq, 2xpxq=frequency of heterozygotes. This can be expressed as a percentage (x100) or you can work out the number in a population who are heterozygous etc. total of populationxfrequency of heterozygotes expresed as percentage.

For Energy did you mean Energy & ATP or energy and ecosystems? :)

- 0 votes

Ah thankyou so much!! For Energy I meant Energy in the Ecosystems, such as the Energy Transfer between Trophic levels and calculating them :)