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 0 negative. The blood group of the three sons is A positive, B positive, and 0 positive. The blood group of the adopted son is
O positive
Understanding Blood Group Inheritance in Families
This question asks us to determine the blood group of an adopted son based on the blood groups of his adoptive parents and his two biological twin brothers. To solve this, we need to understand how blood groups are inherited.
Analyzing the Parents' Blood Groups
We are given the blood groups of the adoptive parents:
- Parent 1: AB positive
- Parent 2: O negative
Let's break down the genotypes for both the ABO blood group system and the Rh factor.
ABO Blood Group System
The ABO blood group is determined by alleles $I^A$, $I^B$, and $i$.
- Blood group A corresponds to genotypes $I^A I^A$ or $I^A i$.
- Blood group B corresponds to genotypes $I^B I^B$ or $I^B i$.
- Blood group AB corresponds to genotype $I^A I^B$.
- Blood group O corresponds to genotype $ii$.
So, the genotypes of the parents for the ABO system are:
- Parent 1 (AB): $\text{Genotype } I^A I^B$
- Parent 2 (O): $\text{Genotype } ii$
Rh Factor
The Rh factor is largely determined by a gene with two common alleles, $Rh^+$ and $Rh^-$.
- Rh positive corresponds to genotypes $Rh^+ Rh^+$ or $Rh^+ Rh^-$.
- Rh negative corresponds to genotype $Rh^- Rh^-$.
So, the genotypes of the parents for the Rh factor are:
- Parent 1 (Positive): $\text{Genotype } Rh^+ Rh^+ \text{ or } Rh^+ Rh^-$
- Parent 2 (Negative): $\text{Genotype } Rh^- Rh^-$
Predicting Biological Children's Blood Groups
Biological children inherit one allele from each parent for both the ABO group and the Rh factor.
ABO Inheritance from Parents ($I^A I^B \times ii$)
Let's see the possible combinations of alleles the children can inherit:
- Parent 1 can pass either $I^A$ or $I^B$.
- Parent 2 can only pass $i$.
The possible genotypes for the biological children are:
- $I^A$ (from Parent 1) + $i$ (from Parent 2) = $\text{Genotype } I^A i$ (Blood group A)
- $I^B$ (from Parent 1) + $i$ (from Parent 2) = $\text{Genotype } I^B i$ (Blood group B)
Therefore, any biological child born to this couple can only have blood group A or blood group B. They cannot have blood group O or AB.
Rh Factor Inheritance
Parent 2 is $Rh^- Rh^-$, so they always pass $Rh^-$.
Parent 1 is Rh positive, meaning their genotype is either $Rh^+ Rh^+$ or $Rh^+ Rh^-$.
- If Parent 1 is $Rh^+ Rh^+$, they pass $Rh^+$. Children will be $Rh^+ Rh^-$ (Rh positive).
- If Parent 1 is $Rh^+ Rh^-$, they can pass $Rh^+$ or $Rh^-$. Children can be $Rh^+ Rh^-$ (Rh positive) or $Rh^- Rh^-$ (Rh negative).
Since the biological twins are stated to be positive (A positive and B positive), this confirms that inheriting Rh positive is possible, which is true if the AB positive parent is at least $Rh^+ Rh^-$.
Identifying the Sons' Blood Groups
The question states that the three sons (one adopted, two biological twins) have the following blood groups: A positive, B positive, and O positive.
We determined that biological children of this couple can only have blood group A or blood group B. The two biological twin sons must therefore have blood groups A and B.
Given the three listed blood groups are A positive, B positive, and O positive, and the biological twins account for A positive and B positive, the remaining blood group must belong to the adopted son.
- Three sons' blood groups: A positive, B positive, O positive.
- Biological twins' possible blood groups: A or B.
- Given the list, the biological twins' blood groups are A positive and B positive.
- The adopted son's blood group is the one remaining in the list.
The remaining blood group is O positive.
Conclusion on Adopted Son's Blood Group
Based on the genetic inheritance patterns, the biological children of a couple with blood groups AB and O can only have blood groups A or B. Since the list of the three sons' blood groups includes A positive, B positive, and O positive, the two biological sons must be A positive and B positive. This leaves the O positive blood group for the adopted son, as he is not biologically related to the couple.
| Parent 1 (AB positive) | Parent 2 (O negative) | Possible Biological Child Genotypes (ABO) | Possible Biological Child Phenotypes (ABO) |
|---|---|---|---|
| $I^A I^B$ | $ii$ | $I^A i$, $I^B i$ | A, B |
| Son | Known Information | Conclusion | Blood Group |
|---|---|---|---|
| Biological Twin 1 | Biological son | Must be A or B | A positive (from list) |
| Biological Twin 2 | Biological son | Must be A or B | B positive (from list) |
| Adopted Son | Not biological | Blood group is remaining one from list | O positive (from list) |
Revision Table: Key Genetics Concepts
| Term | Explanation |
|---|---|
| Allele | Different forms of a gene (e.g., $I^A$, $I^B$, $i$ for blood type). |
| Genotype | The set of alleles an individual has (e.g., $I^A I^B$, $ii$). |
| Phenotype | The observable trait, determined by the genotype (e.g., Blood group AB, Blood group O). |
| Dominance | When one allele masks the effect of another (e.g., $I^A$ and $I^B$ are dominant over $i$). |
| Codominance | When both alleles are expressed equally (e.g., $I^A$ and $I^B$ in AB blood type). |
| Punnett Square | A diagram used to predict the possible genotypes of offspring from a genetic cross (not explicitly used here, but the principle of combining parental alleles is applied). |
Additional Information on Blood Group Inheritance
Understanding blood group genetics is crucial for various reasons, including blood transfusions, organ transplants, and determining paternity.
- The ABO system involves multiple alleles and codominance.
- The Rh system is also important; Rh incompatibility between mother and fetus can cause health problems.
- Genetic crosses can be complex when considering multiple genes or alleles.
- While blood groups can exclude paternity, they cannot definitively prove it because many individuals share the same blood group.
- Adopted individuals do not inherit genes from their adoptive parents. Their blood group is determined by their biological parents.
Important Questions from Human Blood
The normal systolic pressure in human beings is:
- Which one of the following blood cells type secrete large number of antibodies during an immune response of the body?
- In humans, which chamber of the heart receives oxygen-rich blood from the lungs first ?
- What are the four human blood types in ABO blood group typing?
