Question

Triploid \((3 n)\) watermelons are produced by crossing a tetraploid \((4 n)\) strain with a diploid ( \(2 n\) ) plant. Explain why this mating produces a triploid individual.

Short answer

Crossing a tetraploid plant (4n) with a diploid plant (2n) results in a triploid offspring (3n) because the offspring inherits two sets of chromosomes from the tetraploid parent (through its 2n haploid gamete) and one set of chromosomes from the diploid parent (through its haploid n gamete). This leads to a total of three sets of chromosomes in the offspring, making it a triploid (3n) individual.

Step-by-step solution

Understanding Basic Genetics Terms

First, let's define some basic genetic terms: - Ploidy: The number of complete sets of chromosomes in a cell. Common ploidy types include diploid (2n), triploid (3n), and tetraploid (4n). - Chromosome: The thread-like structure found in the nucleus of a cell, carrying the genetic information in the form of genes. Now, let's look at the ploidy of the different strains involved in this exercise: - Diploid (2n) plant: Contains two complete sets of chromosomes. - Tetraploid (4n) plant: Contains four complete sets of chromosomes. - Triploid (3n) offspring: Contains three complete sets of chromosomes. This is what we want to explain.

Reproduction Mechanics in Plants

During reproduction, plants go through a process called meiosis where the number of chromosomes is reduced by half (from diploid to haploid). Haploid cells, or gametes, are the cells that will combine during fertilization to form a zygote, which will develop into a new organism. In this exercise, we have: - Tetraploid plant (4n): Its haploid gamete will have half the number of chromosomes, i.e., \( \frac{4n}{2} = 2n \) - Diploid plant (2n): Its haploid gamete will have half the number of chromosomes, i.e., \( \frac{2n}{2} = n \)

Crossing Tetraploid and Diploid Plants

When a tetraploid (4n) plant crosses with a diploid (2n) plant, their respective haploid gametes will combine during fertilization: - Tetraploid plant's haploid gamete: \(2n\) - Diploid plant's haploid gamete: \(n\) So, the resulting offspring (zygote) will inherit both sets of chromosomes: Offspring's chromosome count = Tetraploid plant's haploid gamete + Diploid plant's haploid gamete = \( (2n) + (n) = 3n \)

Conclusion

Crossing a tetraploid plant (4n) with a diploid plant (2n) results in a triploid offspring (3n) because the offspring inherits two sets of chromosomes from the tetraploid parent (through its 2n haploid gamete) and one set of chromosomes from the diploid parent (through its haploid n gamete). This leads to a total of three sets of chromosomes in the offspring, making it a triploid (3n) individual.

Key concepts

Ploidy in Plants

Ploidy refers to the number of sets of chromosomes present in the nucleus of a cell, and it plays a crucial role in plant diversity and reproduction. In plants, ploidy levels can vary widely, leading to different genetic configurations. Most commonly, plants are diploid, meaning they have two sets of chromosomes - one from each parent.

Diploidy ensures genetic diversity because the offspring inherits a mix of genetic traits. However, some plants, including certain crops, can be polyploid, meaning they have more than two sets of chromosomes. Polyploids include triploids, with three sets of chromosomes, and tetraploids with four. These varieties are often created by plant breeders to enhance certain traits, such as fruit size or crop yield.

For instance, triploid watermelons are bred for their seedlessness, which is a desirable characteristic for consumption. This type of plant manipulation is an example of how understanding ploidy can have practical applications in agriculture.

Chromosome Number

Chromosomes are the structures within cells that carry genetic information. The number of chromosomes is a fundamental feature of an organism's biology and affects how reproduction occurs. Each species has a characteristic number of chromosomes, which can be found in pairs in diploid organisms or in sets of three or more in polyploids.

In the production of gametes (sperm and egg cells), the chromosome number is halved to ensure that when fertilization occurs, the resulting offspring has the correct number of chromosomes. This halving is done through a process called meiosis. If there is a mismatch in the number of chromosomes contributed by the gametes, this can lead to variations in ploidy, such as triploidy in the crossed watermelons in the exercise.

Plant Reproduction

Plant reproduction can occur either sexually or asexually. Sexual reproduction involves the production of gametes through meiosis, whereas asexual reproduction does not involve gametes and leads to offspring that is genetically identical to the parent.

In sexual reproduction, plants can create diversity through the combination of different gametes. The mating of a tetraploid (4n) plant with a diploid (2n) plant exemplifies how sexual reproduction can result in varied ploidy among offspring. The fusion of their respective gametes, which contain half the parental chromosome number, leads to a new organism with a unique combination of chromosomes. The resulting triploid watermelons are one such novel outcome.

Meiosis and Gametogenesis

Meiosis is the specialized form of cell division that results in gametes with half the normal chromosome number. During meiosis, a diploid cell undergoes two rounds of cell division but only one round of DNA replication. This process ensures genetic diversity through recombination and reduces the chromosome number to produce haploid cells.

Gametogenesis is the process of gamete formation which, in plants, involves the development of sperm in pollen grains and eggs in the ovules. The halving of the chromosome number during meiosis is vital to maintain the species' chromosome number across generations. When gametes from different ploidy parents unite during fertilization, a triploid organism can be created, exemplified by the triploid watermelons that arise from crossing tetraploid and diploid plants.