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regions of the chromosome between the genes. In bacteria, the genes are closely packed together, but in higher organisms such as plants and animals, the DNA between genes comprises up to 96% of the chromosome and the functional genes only make up around 4 to 5% of the length. [Viruses also contain genetic information and some have genes made of DNA. Other viruses have genes made of the related molecule, ribonucleic acid, RNA.]

In addition to the DNA, the genetic material itself, chromosomes carry a variety of proteins that are bound to the DNA. This is especially true of the larger chromosomes of higher organisms where histone proteins are important in maintaining chromosome structure (see Ch. 4). [Bacteria also have histone-like proteins. However, these differ significantly in both structure and function from the true histones of higher organisms—see Chapter 9.]

Different Organisms may Have Different Numbers of Chromosomes

The cells of higher organisms usually contain two copies of each chromosome. Each pair of identical chromosomes possesses copies of the same genes, arranged in the same linear order. In Figure 1.07, identical capital letters indicate sites where alleles of the same gene can be located on a pair of chromosomes. In fact, identical chromosomes are not usually truly identical, as the two members of the pair often carry different alleles of the same gene. The term homologous refers to chromosomes that carry the same set of genes in the same sequence, although they may not necessarily carry identical alleles of each gene.

A cell or organism that possesses two homologous copies of each of its chromosomes is said to be diploid (or "2n", where "n" refers to the number of chromosomes in one complete set).Those that possess only a single copy of each chromosome are haploid (or "n").Thus humans have 2 x 23 chromosomes (n = 23 and 2n = 46).Although the X and Y sex chromosomes of animals form a pair they are not actually identical (see below). Thus, strictly, a male mammal is not fully diploid. Even in a diploid organism, the reproductive cells, known as gametes, possess only a single copy of each chromosome and are thus haploid. Such a single, though complete, set of chromosomes carrying one copy of each gene from a normally diploid organism is known as its "haploid genome."

Bacteria possess only one copy of each chromosome and are therefore haploid. (In fact, most bacteria have only a single copy of a single chromosome, so that n = 1). If one of the genes of a haploid organism is defective, the organism may be seriously endangered since the damaged gene no longer contains the correct information that the cell needs. Higher organisms generally avoid this predicament by being diploid and having duplicate copies of each chromosome and therefore of each gene. If one copy of the gene is defective, the other copy may produce the correct product required by the cell. Another advantage of diploidy is that it allows recombination between two copies of the same gene (see Ch. 14). Recombination is important in promoting the genetic variation needed for evolution.

diploid Having two copies of each gene haploid Having one copy of each gene haploid genome A complete set containing a single copy of all the genes (generally used of organisms that have two or more sets of each gene) homologous Related in sequence to an extent that implies common genetic ancestry ribonucleic acid (RNA) Nucleic acid that differs from DNA in having ribose in place of deoxyribose and having uracil in place of thymine

FIGURE 1.08 Diploid, Tetraploid and Hexaploid Wheats

The origin of modern hexaploid bread wheat is illustrated. Einkorn wheat hybridized with goat grass to give tetraploid wheat. This in turn hybridized with the weed Triticum tauschii to give hexaploid bread wheat. The increase in grain yield is obvious. Courtesy of Dr. Wolfgang Schuchert Max-Planck Institute for Plant Breeding Research, Köln, Germany.

FIGURE 1.08 Diploid, Tetraploid and Hexaploid Wheats

The origin of modern hexaploid bread wheat is illustrated. Einkorn wheat hybridized with goat grass to give tetraploid wheat. This in turn hybridized with the weed Triticum tauschii to give hexaploid bread wheat. The increase in grain yield is obvious. Courtesy of Dr. Wolfgang Schuchert Max-Planck Institute for Plant Breeding Research, Köln, Germany.

Note that haploid cells may contain more than a single copy of certain genes. For example, the single chromosome of E. coli carries two copies of the gene for the elongation factor EF-Tu and seven copies of the genes for ribosomal RNA. In haploid cells of the yeast Saccharomyces cerevisiae as many as 40% of the genes are duplicate copies. Strictly speaking, duplicate copies of genes are only regarded as genuine alleles if they occupy the same location on homologous chromosomes. Thus these other duplicate copies do not count as true alleles.

Occasionally, living cells with more than two copies of each chromosome can be found. Triploid means possessing three copies, tetraploid means having four copies, and so on. Animal and plant geneticists refer to the "ploidy" of an organism, whereas bacterial geneticists tend to use the term "copy number." Many modern crop plants are polyploids, often derived from hybridization between multiple ancestors. Such poly-ploids are often larger and give better yields. The ancestral varieties of wheat originally grown in the ancient Middle East were diploid.These were then displaced by tetraploids, which in turn gave way to modern bread wheat (Triticum aestivum) which is hexaploid (6n = 42) (Fig 1.08). Hexaploid bread wheat is actually a hybrid that contains four sets of genes from emmer wheat and two sets from the wild weed, Triticum tauschii (= Aegilops squarrosa). Emmer wheat is a tetraploid (4n = 28) derived from two diploid ancestors—einkorn wheat (Triticum monococcum) and a weed similar to modern goat grass (Triticum speltoides = Aegilops speltoides). A small amount of tetraploid wheat (Triticum turgidum and relatives) is still grown for specialized uses, such as making pasta.

Cases are known where there are fewer or more copies of just a single chromosome. Cells that have irregular numbers of chromosomes are said to be aneuploid. In higher animals, aneuploidy is often lethal for the organism as a whole, although certain aneu-ploid cells may survive in culture under some conditions. Although aneuploidy is usually lethal in animals, it is tolerated to a greater extent in plants. Nonetheless, in rare cases, aneuploid animals may survive. Thus, partial triploidy is the cause of certain human conditions such as Down syndrome, where individuals have an extra copy of chromosome #21. The presence of three copies of one particular chromosome is known as trisomy.

Dominant and Recessive Alleles

Consider a diploid plant that has two copies of a gene involved in making red pigment for flowers. From a genetic viewpoint, there are four possible types of individual plant;

aneuploid Having irregular numbers of different chromosomes copy number The number of copies of a gene that are present homologous chromosomes Two chromosomes are homologous when they carry the same sequence of genes in the same linear order ploidy The number of sets of chromosomes possessed by an organism tetraploid Having four copies of each gene triploid Having three copies of each gene trisomy Having three copies of a particular chromosome

Partial Dominance, Co-Dominance, Penetrance and Modifier Genes 9

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