Q.Which one of the following involves the transfer of genetically different pollen grains to the stigma?

Imagine you are a plant. You cannot walk over to a neighbour and exchange pollen. Instead, you rely on wind, bees, or butterflies to carry your pollen to another flower. But here is the problem: if that pollen lands on your own stigma (the female part of the same flower, or a flower on the same plant), you might end up fertilising yourself. That would be like marrying your sibling — it leads to weak, inbred offspring. Plants have evolved a clever way to prevent this: self-incompatibility.

At its core, self-incompatibility is a plant's built-in "no-self-marriage" rule. It is a genetic mechanism that allows the pistil (the female organ) to recognise and reject pollen from the same plant (or a genetically identical plant). The pollen grain may land on the stigma, but the plant actively blocks it — the pollen tube fails to grow, or the ovule is not fertilised. The result is that only pollen from a different, genetically distinct plant can successfully fertilise the ovules.

This is not a random failure. It is a precise, genetically controlled rejection system. The NCERT textbook (Class 12 Biology, Chapter 2) describes it as "the inability of a plant with a functional male and female reproductive system to produce seeds when self-pollinated." In other words, the plant is perfectly fertile — it just refuses to mate with itself.

Why does this matter? Because it forces cross-pollination — pollen from one plant to another. This ensures genetic diversity, which is the raw material for evolution. A diverse population is more resilient to diseases, pests, and changing climates. For farmers and plant breeders, self-incompatibility is both a challenge and a tool. It makes it harder to produce pure inbred lines (since the plant refuses to self-pollinate), but it also makes hybrid seed production easier — you can plant two varieties together and let nature enforce cross-pollination.

The genetics behind it can be simple or complex, but the core idea is this:

• There is a single gene (called the S-locus) with many different versions (alleles) in the population.
• If the pollen carries an S-allele that matches either of the two S-alleles in the pistil, the pollen is rejected.
• If the pollen carries a different S-allele (one not present in the pistil), it is accepted and fertilisation proceeds.

So, a plant with S1 and S2 alleles will reject pollen carrying S1 or S2, but accept pollen carrying S3, S4, S5, etc. This is called gametophytic self-incompatibility (common in many fruits like apples, pears, and tomatoes). There is also sporophytic self-incompatibility (seen in cabbage, mustard, and sunflowers), where the rejection is determined by the pollen parent's genetics, not the pollen grain's own S-allele. …