A married couple adopted a male child. A few years later, twin boys were born to them. The blood group of the couple is AB positive and O negative. The blood group of the three sons is A positive, B positive, and O positive. The blood group of the adopted son is
Contents8
- AO positive
- BA positive
- CB positive
- DCannot be determined on the basis of the given data
Show answer
Answer: (A) O positive
The adopted son has blood group O positive.
Here's the genetics:
- Parent 1 blood group = AB → has alleles I^A and I^B (one A gene, one B gene).
- Parent 2 blood group = O → has alleles i and i (two recessive genes).
Possible biological children from AB × O cross:
- I^A + i = Blood group A,
- I^B + i = Blood group B.
IMPOSSIBLE biological child: Blood group O (would need i + i, but the AB parent cannot give an 'i' allele — they only have I^A or I^B).
So from an AB × O couple, children can ONLY be A or B — NEVER O or AB.
The three sons have:
- A positive,
- B positive,
- O positive.
Since O positive is IMPOSSIBLE from this couple's genetics, the O positive child MUST be the adopted one.
The A positive and B positive sons are the biological twin boys.
This is a classic genetics/Punnett square problem — remember: AB parent can never produce an O child.
An AB parent can only pass either A or B alleles to children, never the recessive O allele, making O blood group impossible in their biological offspring.
This tests basic Mendelian genetics through ABO blood group inheritance - a fundamental concept where dominant and recessive allele combinations determine phenotype.
The question checks if students can apply Punnett square logic to eliminate impossible genetic outcomes and identify the adopted child.
ABO Blood Group System
Science And Technology AB positive O negative A positive B positive O positive blood group
ABO Blood Group System: Genetics & Inheritance Patterns
ABO system has 4 blood types: A, B, AB, O based on antigens on red blood cells
I^A and I^B are codominant alleles, i is recessive to both
AB parent can only pass I^A or I^B — never produces O child
O parent can only pass i allele — all children get one recessive gene
The ABO blood group system is controlled by three alleles at a single gene locus. Understanding which combinations are possible from specific parent crosses is crucial for solving genetics problems in UPSC.
ABO Alleles & Blood Types
Blood Type | Genotype Combinations | Antigens Present | Can Donate To |
|---|---|---|---|
A | I^A I^A or I^A i | A antigen | A, AB |
B | I^B I^B or I^B i | B antigen | B, AB |
AB | I^A I^B | Both A & B antigens | AB only |
O | i i | No antigens | All blood types |
Common Parent Crosses
Parent Cross | Possible Children | Impossible Children | Key Rule |
|---|---|---|---|
AB × O | A or B only | AB or O | AB parent cannot give 'i' allele |
AB × AB | A, B, or AB | O | No 'i' alleles in either parent |
O × O | O only | A, B, or AB | Only 'i' alleles available |
A × B | A, B, AB, or O | None | Depends on whether A, B are homozygous |
In this question: AB × O cross can only produce A or B children. Since one son is O positive, he cannot be biological — he must be adopted. The A positive and B positive sons are the biological twins.
Trap: Ignoring that AB parent has no 'i' allele to give — AB × O can NEVER produce O child
Trap: Confusing Rh factor (positive/negative) with ABO genetics — Rh is inherited separately
Trap: Assuming all three sons could be biological without checking genetic possibility
Trap: Not recognizing codominance — AB blood type shows both antigens simultaneously
Rh Factor Inheritance
Science And Technology positive negative
Rh Factor: The Plus/Minus in Blood Groups
Rh positive (Rh+) is dominant over Rh negative (Rh-)
Rh factor is inherited independently of ABO blood type
Rh- × Rh- cross can only produce Rh- children
The Rh factor is a separate genetic system from ABO. A person is Rh positive if they have the Rh antigen (D antigen) on red blood cells, Rh negative if absent.
Rh Factor Genetics
Rh Type | Genotype | Parent Cross | Possible Children |
|---|---|---|---|
Rh positive | RR or Rr | Rh+ × Rh+ | All Rh+ (if both RR) or mix |
Rh negative | rr | Rh+ × Rh- | All Rh+ (if Rh+ is RR) or 50:50 |
Rh negative | rr | Rh- × Rh- | All Rh- only |
Clinical Significance
Rh incompatibility during pregnancy: Rh- mother carrying Rh+ baby
Hemolytic disease of newborn can occur in subsequent Rh+ pregnancies
Anti-D injection prevents sensitization in Rh- mothers
Punnett Square Analysis
Science And Technology
Punnett Square Method for Genetic Crosses
Punnett square predicts all possible offspring from a genetic cross
Each parent contributes one allele per trait to each offspring
Results show probability ratios of different phenotypes
A Punnett square is the standard method for solving genetics problems. It systematically shows all possible allele combinations from a cross between two parents.
Punnett Square Steps
%%{init: {"flowchart": {"wrappingWidth": 460}}}%%
flowchart TD
s1["`**Identify Parent Genotypes**
Write the alleles each parent can contribute`"]
s2["`**Set Up Grid**
Parent 1 alleles across top, Parent 2 down the side`"]
s3["`**Fill Combinations**
Each cell shows one possible offspring genotype`"]
s4["`**Count Ratios**
Calculate probability of each phenotype`"]
s1 --> s2
s2 --> s3
s3 --> s4Worked example from the question: AB (I^A I^B) × O (ii)
AB parent can give: I^A or I^B
O parent can give: i or i (both same)
Possible children: I^A i (A blood) or I^B i (B blood)
Impossible: ii (O blood) because AB parent has no 'i' to give
Trap: Forgetting that each parent contributes exactly one allele per trait
Trap: Mixing up which parent has which genotype when setting up the square
Trap: Not checking if a claimed offspring genotype is actually possible from the cross