Q.With the help of a neat and labelled diagram describe the development of female gametophyte of angiosperms. OR Explain "the law of independent assortment" with a suitable example.

This question offers a choice between describing how the embryo sac develops from the megaspore mother cell (with a labelled diagram) and explaining Mendel's law of independent assortment with a dihybrid cross. …

Part (a)

1. Megasporogenesis. One cell of the nucellus, near the micropylar end, becomes the diploid megaspore mother cell. It divides by meiosis to give a linear tetrad of four haploid megaspores.
2. Functional megaspore. The three megaspores nearer the micropyle degenerate; the lowest one, toward the centre of the nucellus, survives as the functional megaspore, the first cell of the female gametophyte, and grows, nourished by the nucellus.
3. Three free-nuclear mitoses. Its nucleus divides three times without wall formation, giving two, then four, then eight nuclei, four at each pole.
4. Organisation. One nucleus from each pole migrates to the centre (the two polar nuclei). The three at the micropylar end form the egg apparatus: a large egg cell and two synergids, whose filiform apparatus later guides the pollen tube. The three at the chalazal end become the antipodal cells.
5. Secondary nucleus. Just before fertilization the two polar nuclei fuse into the diploid secondary (definitive) nucleus, so the mature sac is seven-celled and eight-nucleate (six haploid nuclei and one diploid). Because it arises from a single megaspore the development is monosporic, and because it stays inside the megaspore it is endosporous.

This is the Polygonum type of embryo sac described in NCERT Class 12 Biology as well, so the same diagram serves CBSE and NEET.

Part (b)

The law of independent assortment states that when two pairs of contrasting characters are followed together, each pair segregates into the gametes independently of the other, so every combination of alleles is formed in equal proportion. …