Biology confusion. Please explain, 10 points.?

[Rose L]

The fish in a population of parrot fish may have scales of one or two colors. Some fish have blue scales, others green. Twenty-Five percent of the population is homozygous recessive (bb) for green scales. The remaining 75% of the population has blue scales, but you don't know if they are heterozygous (Bb) or homozygous dominant (BB). Assuming the population is in Hardy Weinberg equilibrium, use p+q=1 and the Hardy Weinberd equation (p2 + 2pq + q2 =1) to figure out the genotype frequencies for BB and Bb. Please show the steps, I am completely lost.

[Lucas C]

Let p = the frequency of the dominant allele and q = the frequency of the recessive allele. The following statement should therefore be true:

p + q = 1

In other words, the fraction of dominant alleles plus the fraction of recessive alleles should account for ALL of the alleles (1 = 100%).

Now if we assume the population is in genetic equilibrium, then the fraction of individuals who have the BB genotype should be p², and the fraction of individuals who have the bb genotype is q². The fraction of individuals who are heterozygous (Bb) is given by the expression 2pq. So:

p² + 2pq + q² = 1

In other words, the homozygous dominant individuals plus the heterozygous dominant individuals plus the homozygous recessive individuals should account for ALL invidivuals.

Now we know that 25% of the population is bb. The other 75% is BB + Bb. So:

p² + 2pq = 0.75

q² = 0.25

We can't solve the first equation yet, but we can solve for q:

q = square root of 0.25 = 0.5

So one-half of the population's alleles is recessive.

The other half, then, must be dominant. p = 0.5 as well.

Now that we know the values of p and q, we can solve for p² (the frequency of homo. dominant individuals) and 2pq (the hetero. dominant individuals)

p² = (0.5)² = 0.25

2pq = 2(0.5)(0.5) = 0.5

There you have it: one quarter of the population is BB, one half is Bb, and one quarter is bb. A perfect 1:2:1 distribution...

I hope that helps. Good luck!

[Missy R]

Put them into a punnett square and you will have your genotypes

[patricia c]

Always start with the homozygous recessive. In this case green scales (bb)
The homozygous recessive will be q2, so
q2 = 0.25
You can work out what q is from square root of 0.25
Remember that p+q = 1.
Put the value of q into that equation and you can work out what p is and hence what p2, and 2pq !!
Easy, get the calculator out!