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Classical Genetics of Chromosomes
We should not proceed to a detailed discussion of molecular genetics without a brief review of Mendelian genetics. Chromosomes in eukary-otes consist mainly of DNA and histones. During some stages of the cell’s division cycle in plants and animals, chromosomes can be observed with light microscopes, and they display beautiful and fascinating patterns. Careful microscopic study of such chromosomes sets the stage for subsequent molecular experiments that have revealed the exact chemi-cal nature of heredity. We are now approaching a similar level of understanding of genetic recombination.
The basis of many of the classical studies is that most types of eukaryotic cells are diploid. This means that each cell contains pairs of identical or almost identical homologous chromosomes, one chromo-some of each pair deriving from each of the parents. There are excep-tions. Some types of plants are tetraploid or even octaploid, and some variants of other species possess alternate numbers of one or more of the chromosomes.
During normal cell growth and division, the pairs of chromosomes in each dividing cell are duplicated and distributed to the two daughter cells in a process called mitosis. As a result, each daughter cell receives the same genetic information as the parent cell contained. The situation, however, must be altered for sexual reproduction. During this process, special cells derived from each of the parents fuse and give rise to the new progeny. To maintain a constant amount of DNA per cell from one generation to the next, the special cells, which are often called gametes, are generated. These contain only one copy of each chromosome instead of the two copies contained by most cells. The normal chromosome number of two is called diploid, and a chromosome number of half this is called haploid. The cell divisions giving rise to the haploid gametes in animals and haploid spores in plants is called meiosis.
Figure 8.3 The classical view of meiosis.
During the process of meiosis, a pair of chromosomes doubles, genetic recombination may occur between homologous chromosomes, and the cell then divides (Fig. 8.3). Each of the daughter cells then divides again without duplication of the chromosomes. The net result is four cells, each containing only one copy of each chromosome. Subsequent fusion of a sperm and egg cell from different individuals yields a diploid called a zygote that grows and divides to yield an organism containing one member of each chromosome pair from each parent.
The chromosomes from each of the parents may contain mutations that produce recognizable traits or phenotypes in the offspring. Let us consider just one chromosome pair of a hypothetical organism. Let gene A produce trait A, and, if it is mutant, let it be denoted as gene a and itstrait be a. In genetic terminology, A and a are alleles. We can describe the genetic state of an individual by giving its genetic composition or genotype. For example, both copies of the chromosome in question could contain the A allele. For convenience, denote this as (A/A). Such a cell is called homozygous for gene A. A mating between organisms containing diploid cells of type (A/A) and (a/a) must produce offspring of the type (a/A), which is, of course, identical to (A/a). That is, the chromosomes in the offspring are copies of each of the parental chro - mosomes. These offspring are said to be heterozygous for gene A.
Figure 8.4 The haploids produced from diploid parent cells, the combinationsof haploids possible upon their fusion, and the apparent phenotypes if A is totally or partially dominant to a.
The interesting results come when two heterozygous individuals mate and produce offspring. A gamete can inherit one or the other of each of the homologous chromosomes from each chromosome pair. This generates a variety of gamete types. When large numbers of offspring are considered, many representatives are found of every possible combination of assortment of the chromosomes, and the re-sults become predictable. It is easiest to systematize the possibilities in a square matrix (Fig. 8.4). For evaluation of experimental results, however, the appearance of heterozygotes must be known or deduced. The appearance of a heterozygote (a/A) is that of trait A if A is dominant, which means automatically that a is recessive. Strict dominance need not be seen, and a heterozygote may combine the traits displayed by the two alleles. For example, if the trait of gene A were the production of red pigment in flowers and the trait of gene a were the absence of production of the pigment, the heterozygote (a/A) might produce half the normal amount of red pigment and yield pink flowers.
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