A mother with blood group B gives birth to a child with blood group O. An alleged father i… — JAMB UTME

A mother with blood group B gives birth to a child with blood group O. An alleged father involved in a paternity case has blood group AB. Based on the genetic principles of codominance and multiple alleles governing the ABO blood group system, which of the following conclusions regarding paternity is correct?

The alleged father is excluded because he lacks the recessive $i$ allele required to produce a blood group O child.Answer
B
The alleged father can be the biological father if the $I^A$ allele behaves as a recessive gene when inherited alongside allele $i$ .
C
The alleged father has a 25% probability of being the father because the $I^A$ and $I^B$ alleles mask the recessive $i$ allele in his genotype.
D
The alleged father is confirmed as the biological father because codominance allows blood group AB parents to produce children of any ABO phenotype.

Answer

The alleged father is excluded because he lacks the recessive

i

allele required to produce a child with blood group O.

Blood group O is a recessive phenotype resulting from the homozygous genotype

ii

. For a child to have blood group O, both biological parents must pass on a recessive

i

allele. A person with blood group AB has the genotype

I^A I^B

due to codominance between the

I^A

and

I^B

alleles. Since this individual carries no

i

allele, he can only pass either

I^A

I^B

to his offspring. Therefore, he is genetically excluded from fathering a child with blood group O.

Step-by-Step Solution

Determine the genotype of the child with blood group O.

The child has the genotype

ii

, requiring one recessive

i

allele from each biological parent.

Blood group O is an autosomal recessive phenotype in the ABO system.

Determine the genotype and possible gametes produced by the alleged father.

The alleged father has genotype

I^A I^B

and produces gametes containing either the

I^A

allele or the

I^B

allele.

The

I^A

and

I^B

alleles are codominant, resulting in blood group AB.

Evaluate whether the alleged father can contribute to the child's genotype.

The alleged father cannot contribute an

i

allele to the child.

An individual with genotype

I^A I^B

completely lacks the

i

allele.

Formulate the final conclusion regarding paternity.

The alleged father is definitively excluded as the biological father.

A child cannot inherit an allele from a biological parent who does not possess that allele.

Key Concept

Codominance and Multiple Alleles in ABO Blood Group Inheritance

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