Problem Type 1: Basic Ploidy in Life Cycle
Problem: In Funaria (moss), if the diploid cell has 20 chromosomes, determine the chromosome number in: (a) Spore, (b) Protonema, (c) Egg cell, (d) Zygote, (e) Capsule cells
Solution: Given: 2n = 20, therefore n = 10
| Cell/Structure | Ploidy | Chromosome Count | Reasoning |
|---|---|---|---|
| Spore | n (haploid) | 10 | Produced by meiosis in capsule (2n → n) |
| Protonema | n (haploid) | 10 | Develops from spore by mitosis; stays haploid |
| Egg cell | n (haploid) | 10 | Produced by mitosis in archegonium of gametophyte |
| Zygote | 2n (diploid) | 20 | Egg (n=10) + Antherozoid (n=10) fuse → 2n |
| Capsule cells | 2n (diploid) | 20 | Part of sporophyte (develops from zygote) |
Problem Type 2: Angiosperm Chromosome Numbers
Problem: In a mango tree (2n = 40), find: (a) Pollen grain, (b) Egg cell, (c) Embryo, (d) Endosperm
Solution: 2n = 40, n = 20
| Structure | Ploidy | Chromosomes | Formation |
|---|---|---|---|
| Pollen grain | n | 20 | Microspore → pollen (male gametophyte) |
| Egg cell | n | 20 | Part of embryo sac (female gametophyte) |
| Embryo | 2n | 40 | Sperm (n=20) + Egg (n=20) = Zygote (2n=40) |
| Endosperm | 3n | 60 | Sperm (n=20) + 2 Polar nuclei (n+n=40) = 3n=60 |
Key Rule: Endosperm = 3n = (n + 2n) = n + (n+n) = sperm + two polar nuclei