Certain varieties of chrysanthemums contain 18, 36, 54, 72, and 90 chromosomes; all are multiples of a basic set of nine chromosomes. How would you describe these varieties genetically? What feature do the karyotypes of each variety share? A variety with 27 chromosomes has been discovered, but it is sterile. Why?

Key Concepts

Polyploidy

Polyploidy refers to the condition in which a cell or organism has more than two complete sets of chromosomes. In the case of the chrysanthemums, the varieties with 18, 36, 54, 72, and 90 chromosomes are all polyploid, as they are multiples of the basic set of nine chromosomes. This condition is common in plants and can lead to increased size and vigor.

Karyotype

A karyotype is a visual representation of an organism's complete set of chromosomes, organized by size, shape, and number. The karyotypes of the chrysanthemum varieties share the characteristic of being based on the same basic set of nine chromosomes, which indicates their genetic relationship and helps in identifying their ploidy levels.

Sterility in Polyploids

Sterility in polyploid organisms, such as the variety with 27 chromosomes, often arises from an uneven number of chromosome sets, which can disrupt normal meiosis. In this case, the odd number of chromosomes (27) prevents proper pairing during gamete formation, leading to the production of non-viable gametes and resulting in sterility.