How to Calculate Allele Frequency Ratios for ABO Blood Groups: Understanding O, A, and B Distribution

The ABO blood group system is an excellent example of codominant genetic inheritance, involving three alleles: IAIA, IBIB, and $i$ (O). Understanding the frequency of these alleles in a population is crucial for studies in genetics, medicine, and anthropology. Here, we explore how to calculate the ratio of allele frequencies for O, A, and B when given specific population data.

The Problem

• Frequency of blood group O in the population = 25% (or 0.25)

• Alleles for blood group A and B are equally frequent

What is the ratio of allele frequencies between blood group O, A, and B?

Step 1: Understanding the Genetics

• Blood group O phenotype results from the genotype $ii$.

• Blood group A: IAIAIAIA or IAiIAi

• Blood group B: IBIBIBIB or IBiIBi

• Blood group AB: IAIBIAIB

Let:

• $p$ = frequency of IAIA (A allele)

• $q$ = frequency of IBIB (B allele)

• $r$ = frequency of $i$ (O allele)

Given: $p=q$ and $p+q+r=1$

Step 2: Hardy-Weinberg for Three Alleles

The frequency of blood group O (genotype $ii$) is r2r2:

r2=0.25  ⟹  r=0.25=0.5r2=0.25⟹r=0.25=0.5

So, $r=0.5$.

Now, $p+q=1-r=0.5$.

Since $p=q$:

p=q=0.52=0.25p=q=20.5=0.25

Step 3: Ratio Calculation

• O allele ($r$): 0.5

• A allele ($p$): 0.25

• B allele ($q$): 0.25

The ratio of O : A : B is

$$0.5:0.25:0.25=2:1:1$$

Step 4: Correct Answer

From the options:

• (1) 1:1:1

• (2) 2:1:1

• (3) 1:2:2

• (4) 3:1:1

The correct answer is (2) 2:1:1.

Why This Calculation Matters

• Genetic Counseling: Accurate allele frequency estimation helps in predicting blood group inheritance in families.

• Population Genetics: Understanding allele distributions aids in tracing human migration and evolution.

• Medical Applications: Blood group compatibility for transfusions depends on these frequencies.

Conclusion

When the frequency of blood group O is 25% and the alleles for A and B are equally frequent, the allele frequency ratio of O : A : B is 2 : 1 : 1.

Correct answer: (2) 2:1:1