A man with blood group A, whose mother had blood group O, marries a woman with blood group… — JAMB UTME

A man with blood group A, whose mother had blood group O, marries a woman with blood group B, whose father had blood group O. What is the probability that their first child will have blood group O?

25%Answer
B
0%
C
50%
D
75%

Answer

The probability that their first child will have blood group O is 25%.

Both parents inherited a recessive allele

i

from their respective blood group O parents, giving them heterozygous genotypes (

I^A i

and

I^B i

). Crossing these genotypes produces four offspring genotypes (

I^A I^B

I^A i

I^B i

ii

) in equal proportions, resulting in a 25% (1 in 4) chance for a child with blood group O (

ii

Step-by-Step Solution

Determine parental genotypes from their phenotypes and family histories.

The man has blood group A (

I^A I^A

I^A i

). Because his mother was blood group O (

ii

), he inherited an

i

allele, giving him genotype

I^A i

. The woman has blood group B (

I^B I^B

I^B i

) and her father was blood group O (

ii

), giving her genotype

I^B i

An individual with blood group O has genotype

ii

and must pass a recessive

i

allele to all offspring.

Perform a genetic cross between the two parents (

I^A i \times I^B i

The offspring genotypes formed are

I^A I^B

(blood group AB),

I^A i

(blood group A),

I^B i

(blood group B), and

ii

(blood group O) in a 1:1:1:1 ratio.

Punnett square analysis of two heterozygous individuals yields four equally likely genotype combinations.

Calculate the probability of obtaining genotype

ii

(blood group O).

1 out of 4 possible outcomes is genotype

ii

, which equals 25% or

\frac{1}{4}

Blood group O is expressed only when the homozygous recessive genotype

ii

is present.

Key Concept

Multiple Alleles and Codominance in ABO Blood Group Inheritance

Estimated Time:1m 30s

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