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?

Verified step by step guidance

1. Identify the basic chromosome number (x) for chrysanthemums, which is given as 9. This represents the number of chromosomes in a single complete set.
2. Recognize that the varieties with 18, 36, 54, 72, and 90 chromosomes have chromosome numbers that are multiples of 9, indicating they are polyploids. Specifically, determine the ploidy level (number of sets) by dividing the total chromosome number by the basic set number: \(\text{Ploidy level} = \frac{\text{Total chromosomes}}{9}\).
3. Describe these varieties as diploid (2x = 18), tetraploid (4x = 36), hexaploid (6x = 54), octoploid (8x = 72), and decaploid (10x = 90), respectively, based on their chromosome counts.
4. Explain that the shared feature of the karyotypes is that all chromosomes are organized in complete sets of nine, meaning each variety has an integral number of these basic sets without any partial sets.
5. For the variety with 27 chromosomes, note that it is a triploid (3x = 27), which often leads to sterility because during meiosis, the odd number of chromosome sets cannot pair evenly, resulting in unbalanced gametes and thus infertility.

Key Concepts

Polyploidy

Polyploidy refers to the condition where an organism has more than two complete sets of chromosomes. In chrysanthemums, chromosome numbers like 18, 36, 54, 72, and 90 indicate multiples of a basic set of nine chromosomes, showing different levels of polyploidy (diploid, tetraploid, etc.). This genetic feature often affects traits like size and fertility.

Basic Chromosome Number (Monoploid Number)

The basic chromosome number is the number of distinct chromosomes in a single complete set (denoted as 'x'). For chrysanthemums, x = 9, meaning all varieties have chromosome counts that are multiples of nine. This concept helps classify the genetic makeup and understand the relationship between different varieties.

Sterility in Odd-Numbered Polyploids

Organisms with an odd number of chromosome sets, like the 27-chromosome chrysanthemum (3x9), often experience sterility due to irregular chromosome pairing during meiosis. This leads to unbalanced gametes, preventing successful reproduction, which explains why the 27-chromosome variety is sterile.