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 each option provided in the question.

### Step-by-Step Solution:

1. **Understanding Blood Group Genetics**:
- Blood groups are determined by alleles A, B, and O.
- The Rh factor is determined by the presence (+) or absence (-) of the Rh antigen.
- Homozygous means having two identical alleles for a trait, while heterozygous means having two different alleles.

2. **Analyzing Each Option**:
- **Option 1**: An Rh positive child from an Rh positive mother and an Rh negative father.
- The mother can be either homozygous (Rh+/Rh+) or heterozygous (Rh+/Rh-). The child can inherit either Rh+ or Rh- from the mother, making it possible for the child to be heterozygous. Thus, this option does not guarantee homozygosity.

- **Option 2**: An Rh negative child from an Rh negative mother and an Rh negative father.
- Both parents are Rh negative (genotype Rh-/Rh-). The child must inherit Rh- from both parents, making the child homozygous Rh negative (Rh-/Rh-). This option guarantees homozygosity for the Rh factor, but we need to check the blood group alleles as well.
- The mother could be O (genotype O/O) and the father could also be O (genotype O/O), making the child O (genotype O/O). Thus, this option guarantees homozygosity for both blood group alleles and Rh factor.

- **Option 3**: An AB Rh positive child from an A Rh positive mother and a B Rh negative father.
- The mother can be A (genotype A/A or A/O) and the father can be B (genotype B/B or B/O). The child can inherit A from the mother and B from the father, making the child heterozygous (AB). Therefore, this option does not guarantee homozygosity.

- **Option 4**: An O Rh negative child from an O Rh negative mother and an O Rh negative father.
- Both parents are O (genotype O/O) and Rh negative (genotype Rh-/Rh-). The child must inherit O from both parents (genotype O/O) and Rh- from both parents (genotype Rh-/Rh-). Thus, this option guarantees homozygosity for both blood group alleles and Rh factor.

3. **Conclusion**:
- The only option that guarantees homozygosity for both the blood group alleles and the Rh blood group system is **Option 2** (Rh negative child from Rh negative mother and Rh negative father) and **Option 4** (O Rh negative child from O Rh negative mother and O Rh negative father). However, since the question asks for a child that will always be homozygous, the best answer is **Option 4**.

### Final Answer:
**Option 4: O Rh negative child from O Rh negative mother and O Rh negative father.**