To determine the frequency of the recessive allele in the given chicken population, we follow these steps:
1. Identify the total number of chickens:
- Homoyzgous dominant (RR): 45
- Heterozygous dominant (Rr): 30
- Recessive (rr): 25
Therefore, the total number of chickens is:
[tex]\[
45 + 30 + 25 = 100
\][/tex]
2. Calculate the total number of alleles:
Each chicken has 2 alleles. So, the total number of alleles in the population is:
[tex]\[
100 \times 2 = 200
\][/tex]
3. Determine the number of recessive alleles:
- Homoyzgous dominant (RR) chickens contribute 0 recessive alleles.
- Heterozygous dominant (Rr) chickens contribute 1 recessive allele each.
- Recessive (rr) chickens contribute 2 recessive alleles each.
Therefore, the total number of recessive alleles is:
[tex]\[
0 \times 45 + 1 \times 30 + 2 \times 25 = 0 + 30 + 50 = 80
\][/tex]
4. Calculate the frequency of the recessive allele:
The frequency of the recessive allele is the number of recessive alleles divided by the total number of alleles:
[tex]\[
\frac{80}{200} = 0.4
\][/tex]
Therefore, the correct expression to calculate the frequency of the recessive allele is:
[tex]\[
\boxed{B.\ \frac{80}{200}}
\][/tex]
