Problem 1

Question: In a plant species where 2n = 24, calculate the chromosome number in the following: (a) pollen grain, (b) egg cell, (c) zygote, (d) endosperm, (e) seed coat.

Step-by-Step Solution:

Given: 2n = 24, so n = 12

(a) Pollen grain:

• PMC (2n = 24) undergoes meiosis → microspore (haploid, n)
• Pollen grain = haploid (n) = 12 chromosomes

(b) Egg cell:

• Derived from the functional megaspore (n) by mitosis
• Egg cell = haploid (n) = 12 chromosomes

(c) Zygote (Syngamy):

• Sperm (n = 12) + Egg (n = 12) = 2n = 24 chromosomes

(d) Endosperm (Triple fusion):

• Sperm (n = 12) + Polar nucleus 1 (n = 12) + Polar nucleus 2 (n = 12) = 3n = 36 chromosomes

(e) Seed coat:

• Derived from integuments (maternal sporophytic tissue, 2n)
• Seed coat = 24 chromosomes (same as parent plant)

Summary Answer: (a) 12, (b) 12, (c) 24, (d) 36, (e) 24