An exception (deviation) to Medelian law of idenpendent assortment is

Options

A. (a) crossing over
B. (b) incomplete dominance
C. (c) segregation
D. (d) linkage

Explanation

**Step-by-Step Solution:**

1. **Understanding Mendelian Law of Independent Assortment**:
- Mendel's law states that the segregation of one pair of alleles is independent of the segregation of another pair of alleles during gamete formation. This means that traits are inherited independently when they are on different chromosomes.

2. **Identifying the Conditions for Independent Assortment**:
- For independent assortment to occur, the genes must be located on different chromosomes. If the genes are on the same chromosome, they may not assort independently.

3. **Recognizing the Deviation from the Law**:
- The deviation from the law of independent assortment occurs when genes are located on the same chromosome. This phenomenon is known as linkage.

4. **Explaining Linkage**:
- Linkage refers to the tendency of genes that are located close to each other on the same chromosome to be inherited together. This was demonstrated by Thomas Hunt Morgan in his experiments with fruit flies (Drosophila).

5. **Evaluating the Options Given**:
- A) Crossing over: This refers to the exchange of genetic material between homologous chromosomes and does not represent a deviation from independent assortment.
- B) Incomplete dominance: This is a form of inheritance where neither allele is completely dominant, which does not relate to the law of independent assortment.
- C) Segregation: This is a feature of the law itself and does not represent a deviation.
- D) Linkage: This is the correct answer as it describes the physical association of genes on the same chromosome, which leads to a deviation from independent assortment.

6. **Conclusion**:
- The exception to Mendelian law of independent assortment is **linkage**.