In immunologically non compatible marriage resulting in Erythroblastosis foetalis in `Rh^(+)` Foetus, the genotype of mother is always

Options

A. `Rh^(-)`
B. `Rh^(+)` (homozygous)
C. `Rh^(+)` (Dd)
D. DD

Answer

To determine the genotype of the mother in an immunologically non-compatible marriage that results in erythroblastosis foetalis in an Rh positive fetus, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Erythroblastosis Foetalis**: This condition occurs when there is an incompatibility between the blood types of the mother and the fetus, specifically concerning the Rh factor. The mother’s immune system can produce antibodies against the Rh factor present in the fetus's blood.

2. **Identifying the Blood Types**: In this scenario, the mother has a negative Rh factor (Rh-) and the fetus has a positive Rh factor (Rh+). This incompatibility arises when the mother is Rh- and the father is Rh+.

3. **Genotype of the Mother**: Since the mother is Rh-, her genotype must be homozygous for the negative Rh factor. The Rh factor is determined by the presence of the D antigen. The genotype for Rh- is denoted as "dd" (homozygous recessive), meaning she does not have the D antigen.

4. **Fetus's Genotype**: The fetus can be Rh+ due to inheriting the Rh+ gene from the father. The father could be either homozygous (DD) or heterozygous (Dd) for the Rh factor. However, the mother's genotype does not change based on the father's genotype.

5. **Conclusion**: Therefore, in an immunologically non-compatible marriage resulting in erythroblastosis foetalis in an Rh+ fetus, the genotype of the mother is always **Rh negative (dd)**.

### Final Answer:
The genotype of the mother is always **Rh negative (dd)**.

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