Variation in Chromosome Number Help (page 2)
Variation in Chromosome Number
Each species has a characteristic number of chromosomes. Most higher organisms are diploid, with two sets of homologous chromosomes: one set donated by the father, the other set by the mother. Variation in the number of sets of chromosomes (ploidy) is commonly encountered in nature. It is estimated that one-third of the angiosperms (flowering plants) have more than two sets of chromosomes (polyploid). The term euploidy is applied to organisms with chromosomes that are multiples of some basic number (n),while aneuploidy refers to chromosome numbers that are not exact multiples of n.
- Monoploid. One set of chromosomes (n) is sometimes found in some less complex organisms such as fungi. Monoploids in complex multicellular organisms are usually smaller and less vigorous than the normal diploids. A notable exception exists in male bees and wasps. Monoploid plants are known but are usually sterile.
- Diploid. Two sets of chromosomes (2n) is typical of most animals and complex multicellular organisms. A diploid state arises from the union of two haploid gametes.
- Triploid. Three sets of chromosomes (3n) can originate from the union of amonoploid gamete (n) with a diploid gamete (2n). The extra set of chromosomes of the triploid is distributed in various combinations to the germ cells, resulting in genetically unbalanced gametes. Because of the sterility that characterizes triploids, they are not commonly found in natural populations.
- Tetraploid. Four sets of chromosomes (4n) can arise in body cells by the somatic doubling of the chromosome number. Doubling is accomplished either spontaneously or it can be induced in high frequency by exposure to chemicals such as the alkaloid colchicine. Tetraploids are also produced by the union of unreduced diploid (2n) gametes.
- Autotetraploid. The prefix "auto" indicates that the ploidy involves only homologous chromosome sets. Somatic doubling of a diploid produces four sets of homologous chromosomes (autotetraploid). Union of unreduced diploid gametes from the same species would accomplish the same result. Meiotic chromosome pairing usually produces quadrivalents (four synapsing chromosomes) that can produce genetically balanced gametes if disjunction is by twos, i.e., two chromosomes of the quadrivalent going to one pole and the other two to the opposite pole. If disjunction is not stabilized in this fashion for all quadrivalents, the gametes will be genetically unbalanced. Sterility will be expressed in proportion to the production of unbalanced gametes.
- Allotetraploid. The prefix "allo" indicates that nonhomologous sets of chromosomes are involved. The union of unreduced (2n) gametes from different diploid species could produce, in one step, an allotetraploid that appears and behaves like a new species. Alternatively, two diploid plant species may hybridize to produce a sterile diploid F1. The sterility results from the failure of each set of chromosomes to provide sufficient genetic homology to affect pairing. The sterile diploid can become fertile if it undergoes doubling of the chromosome number. The allotetraploid thus produced has two matched sets of chromosomes that can pair just as effectively as in the diploid. Double diploids of this kind, found only in plants, are called amphidiploids.
- Polyploid. This term can be applied to any cell with more than 2n chromosomes. Ploidy levels higher than tetraploid are not commonly encountered in natural populations, but some of our most important crops are polyploid. For example, common bread wheat is hexaploid (6n), some strawberries are octaploid (8n), etc. Some triploids as well as tetraploids exhibit a more robust phenotype than their diploid counterparts, often having larger leaves, flowers, and fruits (gigantism). Many commercial fruits and ornamentals are polyploids. Sometimes, a specialized tissue within a diploid organism will be polyploid. For example, some liver cells of humans are polyploid. The triploid endosperm tissue of corn and other grains is a common polyploid. Polyploids offer an opportunity for studying dosage effects; i.e., how two or more alleles of one locus behave in the presence of a single dose of an alternative allele. Because of double fertilization in angiosperms (Fig. 1-10), the genotype of sperm nuclei in pollen grains can influence not only the traits of the plant embryo, but also those of the seed endosperm. This phenomenon is known as xenia.
EXAMPLE 7.4 In corn, starchy endosperm is governed by a gene S that shows xenia with respect to its allele for sugary endosperm (s). Four genotypes are possible for these triploid cells: starchy = . If pollen contains sperm, the resulting endosperm will be starchy regardless of the genetic contribution of the seed parent (xenia).
The term haploid, strictly applied, refers to the gametic chromosome number. For diploids (2n), the haploid number is n; for an allotetraploid (4n), the haploid number is 2n; for an allohexaploid (6n) the haploid number is 3n; etc. Organisms such as bacteria and viruses are called haploids because they have a single set of genes. However, since they do not form gametes comparable to those of higher organisms, the term "monoploid" is more appropriate.
Variations in chromosome number may occur that do not involve whole sets of chromosomes, but only parts of a set. The term aneuploidy is given to variations of this nature, and the suffix "-somic" generally refers to a particular organism and its chromosome number (which may be an abnormal situation).
- Monosomic. Diploid organisms that are missing one chromosome of a single pair are monosomics with the genomic formula 2n – 1. The single chromosome without a pairing partner may go to either pole during meiosis, but more frequently will lag at anaphase and fails to be included in either nucleus. Monosomics can thus form two kinds of gametes, (n) and (n – 1). In plants, the n – 1 gametes seldom function. In animals, loss of one whole chromosome often results in genetic unbalance, which is manifested by high mortality or reduced fertility.
- Trisomic. Diploids that have one extra chromosomeare represented by the chromosomal formula 2n + 1.One of the pairs of chromosomes has an extra member, so that a trivalent structure may be formed during meiotic prophase. If two chromosomes of the trivalent go to one pole and the third goes to the opposite pole, then gameteswill be (n + 1) and (n), respectively. Trisomy can produce different phenotypes, depending upon which chromosome of the complement is present in triplicate. In humans, the presence of one small extra chromosome (autosome 21) results in Down syndrome, which is a condition characterized by mental retardation and a variety of physical features and malformations.
- Tetrasomic. When one chromosome of an otherwise diploid organism is present in quadruplicate, this is expressed as 2n + 2. A quadrivalent may form for this particular chromosome during meiosis that then has the same problem as that discussed for autotetraploids.
- Double Trisomic. If two different chromosomes are each represented in triplicate, the double trisomic can be symbolized as 2n + 1 + 1.
- Nullosomic. An organism that has lost a chromosome pair is a nullosomic. The result is usually lethal to diploids (2n – 2). Some polyploids, however, can lose two homologues of a set and still survive. For example, several nullosomics of hexaploid wheat (6n – 2Þ exhibit reduced vigor and fertility but can survive to maturity because of the genetic redundancy in polyploids.
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