Mendel postulated that genes (characteristics) are inherited as pairs of alleles (traits) that behave in a dominant and recessive pattern. ... A dihybrid cross demonstrates independent assortment when the genes in question are on different chromosomes or distant from each other on the same chromosome.

Option a is correct. because genes may or may not be present on same chromosomes.

Independent Inheritance in Pea Plants:

Independent inheritance refers to the phenomenon where the inheritance of one pair of characters does not affect the inheritance of another pair of characters. In pea plants, this is observed when two pairs of characters are crossed. The correct answer is option 'B' - "They are present on separate chromosomes." Let's understand why.

Explanation:

Pea plants have several pairs of contrasting characters, which are inherited from the parent plants. These characters are located on separate chromosomes, which are thread-like structures found in the nucleus of cells. Chromosomes carry genes, which are the units of hereditary information.

Chromosomes and Genes:
- Chromosomes are composed of DNA and protein molecules. They carry the genetic information that determines the traits of an organism.
- Genes are specific segments of DNA on chromosomes that contain the instructions for the development of particular traits or characteristics.

Law of Independent Assortment:
- The law of independent assortment states that the alleles (different forms of a gene) for different traits segregate independently during the formation of gametes.
- This means that the alleles for one trait separate and assort into gametes randomly and independently of the alleles for other traits.

Separate Chromosomes and Independent Inheritance:
- In pea plants, if two pairs of characters are located on separate chromosomes, they are more likely to show independent inheritance.
- When genes for different traits are located on separate chromosomes, they assort independently during meiosis, which is the process of gamete formation.
- During meiosis, homologous chromosomes line up randomly and segregate independently, resulting in the random assortment of alleles for different traits into gametes.
- This random assortment leads to the independent inheritance of different pairs of characters in pea plants.

Conclusion:
The independent inheritance observed when two pairs of characters are crossed in pea plants is due to the fact that the characters are present on separate chromosomes. This allows for the random assortment of alleles during meiosis and the independent inheritance of different traits.