USA Biology Olympiad (USABO) Practice Exam

How many chromosomes does an egg cell have in a plant with monosporic development if the endosperm has 72 chromosomes?

1. 72

2. 48

3. 24

4. 36

The correct answer is: 24

In plants that undergo monosporic development, the process of gamete formation (specifically, the development of an egg cell) is linked to the process of fertilization and the ploidy of the plant’s cells. In this type of development, the endosperm is typically triploid, meaning it contains three sets of chromosomes. When the endosperm has 72 chromosomes, this indicates that it is triploid, derived from a fusion of one sperm cell (which contributes one set of chromosomes) and two polar nuclei from the female gametophyte (which together contribute two sets of chromosomes). Therefore, the total number of chromosomes in the endosperm reflects the combination of contributions from both the egg cell and the sperm. To find the number of chromosomes in the egg cell, we can determine the diploid number found in the somatic cells of the plant. Since the endosperm is triploid with 72 chromosomes, the diploid number (2n) is a crucial piece of information. When we divide the number of chromosomes in the endosperm by three (because it is triploid), we find that the diploid number of the plant is \( \frac{72}{3} = 24 \). The