Question

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?

Short answer

Answer: The shared feature in the karyotypes of these chrysanthemum varieties is that they all have a multiple of the basic set of 9 chromosomes. The variety with 27 chromosomes is sterile because it is triploid, having an odd number of sets of chromosomes, which leads to difficulties in creating balanced gametes during meiosis and results in infertility.

Step-by-step solution

Identify the pattern in the given number of chromosomes

The given number of chromosomes in different varieties are 18, 36, 54, 72, and 90. We can notice that all of these numbers are multiples of 9. So, we can say that the basic set of chromosomes in these varieties is 9.

Describe the varieties genetically

Since all the varieties have a multiple of the basic set of 9 chromosomes, we can describe them genetically as: - Variety with 18 chromosomes: 2x (where x is the basic set of 9 chromosomes) - Variety with 36 chromosomes: 4x - Variety with 54 chromosomes: 6x - Variety with 72 chromosomes: 8x - Variety with 90 chromosomes: 10x

Identify the shared feature in karyotypes

The karyotypes of all these varieties share the common feature that they have a multiple of the basic set of 9 chromosomes. This suggests that these varieties can undergo normal cell division and produce fertile offspring since they have a balanced number of chromosomes.

Analyze the infertility of the variety with 27 chromosomes

The variety with 27 chromosomes can be described as 3x. Although it also has a multiple of the basic set of 9 chromosomes, the number 27 is an odd number, which means the variety is triploid (having three sets of chromosomes instead of the usual two sets). Triploid organisms generally have difficulties during cell division, specifically during meiosis, to produce balanced gametes and hence cannot produce fertile offspring. Therefore, the variety with 27 chromosomes is sterile.

Key concepts

Karyotype Analysis

Karyotype analysis is a technique used to visualize an organism's complete set of chromosomes. It involves pairing and ordering all the chromosomes of an organism to provide a genetic snapshot that helps in identifying chromosomal anomalies. In plants, this can reveal whether the species is diploid, with two sets of chromosomes, or polyploid, with multiple sets.

In the case of chrysanthemums with varying chromosome numbers, karyotype analysis would show the multiples of the basic set of nine chromosomes, reflecting the different varieties as diploids (2x), tetraploids (4x), hexaploids (6x), octaploids (8x), and decaploids (10x). The sterile variety with 27 chromosomes, which is a triploid (3x), would display an unbalanced set, leading to sterility due to issues during meiosis. Karyotype analysis serves as a critical tool to understand the genetic makeup and reproductive potential of these plant varieties.

Polyploidy in Genetics

Understanding Polyploidy

Polyploidy refers to the condition of having more than two complete sets of chromosomes in the cells of an organism. It is a common phenomenon in plant genetics and plays a significant role in the variation and evolution of species. Polyploid plants, such as many varieties of chrysanthemums, can exhibit increased vigor, improved adaptation, and a greater range of attributes.

The product of chromosome multiplication, polyploids can be categorized as autopolyploids or allopolyploids. Autopolyploids have multiple sets of chromosomes from the same species while allopolyploids result from hybridization between different species. In polyploid plants, karyotype analysis is essential for genetic studies as it helps breeders to select for desirable traits and to understand the genetic architecture of polyploidy.

Triploid Sterility

Issues with Triploid Organisms

Triploid sterility is the condition where an organism has three sets of chromosomes and is usually sterile. This occurs because during meiosis, the process that produces gametes, having an odd number of chromosome sets results in an unequal division. Consequently, the gametes produced are not balanced, leading to infertility.

In the example of the sterile chrysanthemum variety with 27 chromosomes, the triploid nature prevents it from producing fertile offspring. Therefore, while diploids and even-numbered polyploids can undergo normal meiosis, triploids face genetic challenges affecting their reproduction. This principle applies not only to chrysanthemums but to many other plant and animal species where triploidy can be observed.