Which of these children will always be homozygous for both the blood group alleles and the Rh blood group system?

Options

A. O Rh + ve child from O Rh+ve mother and O Rh-ve father
B. A Rh-ve child from A Rh-ve mother and O Rh-ve father
C. AB Rh+ve child from A Rh+ve mother and B Rh-ve father
D. O Rh-ve child from O Rh-ve mother and O Rh-ve father

Answer

To determine which child will always be homozygous for both the blood group alleles and the Rh blood group system, we need to analyze the options based on the genetic principles of blood groups.

### Step-by-Step Solution:

1. **Understand Homozygosity**:
- Homozygous means having two identical alleles for a particular gene. For blood groups, this means that both alleles inherited from the parents are the same.

2. **Blood Group Alleles**:
- The ABO blood group system has three alleles: IA (A), IB (B), and i (O).
- The combinations of these alleles determine the blood group:
- IAIA or IAi = Blood group A
- IBIB or IBi = Blood group B
- IAIB = Blood group AB
- ii = Blood group O (homozygous)

3. **Rh Factor**:
- The Rh blood group system has two types: Rh positive (presence of Rh antigen) and Rh negative (absence of Rh antigen).
- Rh positive can be homozygous (Rh+/Rh+) or heterozygous (Rh+/Rh-), while Rh negative is always homozygous (Rh-/Rh-).

4. **Evaluate Each Option**:
- **Option A**: O Rh positive child from O Rh positive mother and O Rh negative father.
- The child can be O (ii) but Rh positive indicates heterozygosity (Rh+/Rh-). Thus, this option is incorrect.

- **Option B**: A Rh negative child from A Rh negative mother and O Rh negative father.
- The mother can be either IAi or IAIA (but since she is Rh negative, she is Rh-/Rh-). The father is ii (homozygous for O). Thus, the child could be IAi (A) or ii (O) but Rh negative is homozygous. This option is incorrect as the blood group is not guaranteed to be homozygous.

- **Option C**: A Rh positive child from A Rh positive mother and B Rh negative father.
- The mother could be IAIA or IAi (Rh+/Rh-), and the father could be IBi (Rh-/Rh-). This results in heterozygous combinations for both blood groups and Rh factor. This option is incorrect.

- **Option D**: O Rh negative child from O Rh negative mother and O Rh negative father.
- Both parents are ii (homozygous for O) and Rh-/Rh-. Therefore, the child will also be ii (homozygous for O) and Rh-/Rh- (homozygous for Rh negative). This option is correct.

5. **Conclusion**:
- The child that will always be homozygous for both the blood group alleles and the Rh blood group system is from **Option D**: O Rh negative child from O Rh negative mother and O Rh negative father.