Chromosomal Variations

Section 2: Chromosomal Variations

We already know that meiosis and fertilization ensure the stability of chromosome numbers across generations in organisms. However, why do the chromosome numbers of potatoes and bananas differ significantly from their wild ancestors? In fact, there are many such examples in nature, which fully illustrates the complexity of the biological world. From another perspective, if there were only stability and no variation in the biological world, where would diversity and evolution come from?

Changes in the number or structure of chromosomes in somatic or germ cells of an organism are referred to as chromosomal variations.

Variations in Chromosome Number

Variations in chromosome number can be divided into two categories: one is the increase or decrease of individual chromosomes within a cell, and the other is the multiplication or reduction of the entire set of chromosomes as a unit.

Diploids and Polyploids
In the somatic cells of most organisms, chromosomes exist in pairs, meaning they contain two sets of non-homologous chromosomes. Each set of non-homologous chromosomes is called a chromosome set. For example, the somatic cells of wild potatoes contain two chromosome sets (Figure 5-5), each consisting of 12 different non-homologous chromosomes. Organisms with two chromosome sets in their somatic cells are called diploids.

Under normal circumstances, gametes formed by diploids through meiosis contain only one chromosome set. If an error occurs during meiosis in a diploid organism, resulting in a gamete with two chromosome sets, the combination of such a gamete with a gamete containing one chromosome set will develop into an individual whose somatic cells contain three chromosome sets, known as a triploid. If two gametes each containing two chromosome sets combine, the resulting individual will have four chromosome sets in its somatic cells, known as a tetraploid. If a diploid organism is affected by certain factors during its embryonic or seedling stage, leading to a failure in chromosome separation during mitosis, a tetraploid can be formed. Organisms with three or more chromosome sets in their somatic cells are collectively referred to as polyploids.

Tetraploids can form gametes with two chromosome sets through meiosis. Triploids, due to the presence of three sets of non-homologous chromosomes in their primordial germ cells, experience irregular synapsis during meiosis, preventing them from forming viable gametes. The absence of seeds in bananas and triploid seedless watermelons is due to this phenomenon.

In nature, almost all animals and more than half of higher plants are diploid. Polyploids are common in plants but rare in animals.

Compared to diploid plants, polyploid plants often have thicker stems, larger leaves, fruits, and seeds, and higher concentrations of nutrients such as sugars and proteins. For example, the fruits of tetraploid grapes are much larger than those of diploids, and the vitamin C content in tetraploid tomatoes is nearly doubled compared to diploids. Therefore, artificial induction of polyploids is often used to obtain polyploid plants for breeding new varieties.

There are many methods to artificially induce polyploids, such as cold treatment or the use of colchicine. The most common and effective method currently is to treat germinating seeds or seedlings with colchicine. When colchicine acts on dividing cells, it inhibits the formation of the spindle apparatus, preventing chromosomes from moving to the poles, leading to a doubling of chromosome numbers. Cells with doubled chromosome numbers that continue to undergo mitosis may develop into polyploid plants. Many new varieties, such as high-sugar-content beets and triploid seedless watermelons, have been cultivated worldwide using artificial induction of polyploids.

Haploids
In addition to the multiplication of chromosome numbers in the somatic cells of organisms, they can also be reduced as a unit. For example, the queen bee and worker bees have 32 chromosomes in their somatic cells, while drones have only 16 chromosomes (Figure 5-6). Organisms like male bees, whose somatic cells have the same number of chromosomes as the gametes of their species, are called haploids.

In nature, haploid plants occasionally appear in diploid species like corn, sorghum, rice, and tomatoes. Compared to normal plants, haploid plants are weak and highly infertile.

Variations in Chromosome Structure

Many human genetic diseases are caused by changes in chromosome structure. For example, Cri-du-chat syndrome is a genetic disorder caused by the partial deletion of the short arm of chromosome 5. The disorder is named for the affected infants' high-pitched cry, which sounds like a cat meowing. Patients with Cri-du-chat syndrome exhibit slow growth and severe intellectual disabilities.

Under natural conditions or due to human influence, structural variations in chromosomes can occur in four main types (Figure 5-7).

Changes in chromosome structure can alter the number or arrangement of genes on the chromosomes, leading to variations in traits. Most structural variations in chromosomes are harmful to organisms, and some can even be fatal.