Multiple Sets of Chromosomes

Chromosomal abnormalities can also occur if more than one sperm fertilizes an egg or if the developing embryo fails to divide its cells properly. These situations result in the addition of a complete set of chromosomes. Normal individuals with two sets of chromosomes are called diploid, while those with three sets are called triploid, and those with

Figure 7.1 Human Male Karyotype.

four are called tetraploid. These conditions in humans and other mammals are so deleterious that affected fetuses do not survive to birth.

Interestingly, however, extra sets of chromosomes are not deleterious to many other organisms, including all plants and some animals, including fish. Indeed, many crop plants we eat are not diploid. In order to reproduce, the reproductive cells must undergo meiosis, a process in which they double their chromosome numbers and then divide them equally so that the gametes contain half the number of chromosomes (see figure 2.3). In the case of diploid individuals, meiosis results in the presence of a single set of chromosomes, a hap-loid chromosome set, in all the gametes. Plants that have more than two sets of chromosomes, if the number of sets is even, can still reproduce because they can evenly divide their chromosomes. For example, tetraploid plants produce diploid gametes, and octaploid plants produce tetraploid gametes. Strawberries that we buy at the grocery store are octaploid. Similarly, wheat is hexaploid: it contains six sets of chromosomes, an even number of chromosome sets that can be evenly divided into triploid gametes.

What, however, is the effect of triploidy, the presence of three sets of chromosomes in an organism? An example of a triploid plant is the

Figure 7.1 Human Male Karyotype.

banana. All commercial bananas are triploid. In this case, there is no way to divide the three sets of chromosomes equally in halves. Remember that during meiosis, the chromosome sets are first doubled. In the banana, the number of sets would then be six. But then these six sets of chromosomes would have to be divided in half twice, or into four, since each pregametic cell gives rise to four gametes. The result is one and one-half sets of chromosomes, not a whole number, and thus meiosis cannot take place properly. Triploid commercial banana trees are sterile; their fruits do not make seeds. On the other hand, wild bananas, which are diploid, do have seeds (see figure 7.2). Thus, commercial bananas are reproduced by cuttings. Using similar logic, the fish and game departments in many states purposely make triploid fish, so that the fish they release for the fishing public are unable to breed and reproduce with wild fish. An additional bonus in triploid fish is that the energy a diploid fish invests into reproduction is used for growth in triploids, resulting in bigger fish.

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