CHAPTER 14Genetics and Propagation

BASIC GENETIC CONCEPTS IN PLANT SCIENCE

• The plants we cultivate for our survival and pleasure all originated from wild plants. However, most of our domesticated plants appear very different from their wild relatives.

• These differences have come about mainly through

The recombination of genes and The redistribution of heritable traits through the generations separating the wild types from the domestic plants.

• Understanding basic genetic principles is crucial to understanding the principles of the traditional methods of plant breeding and plant propagation and the most recent tool for crop improvement: Biotechnology and Genetic Engineering.

Chromosomes

• A chromosome is a long, threadlike structure consisting of deoxyribonucleic acid (DNA) and various proteins. It carries most of the genetic information and transmits that information from one generation of cells to the next.

• The number of chromosomes is the same in all vegetative cells of an entire plant species. This number is usually the 2n, or diploid, chromosome number.

• In the sex cells—the egg and sperm—the number is reduced by half and is termed the haploid, or 1n, chromosome number.

Chromosomes

• Although usually constant for a given species, the number, size, and appearance of chromosomes vary considerably between different plant species.

• The chromosome numbers are known for most plant species. For example, the diploid chromosome number for – Alfalfa (Medicago saliva) is 32– Barley (Hordeum vulgare) is 14– Corn (Zea mays) is 20– Sugar beet (Beta vulgaris) is 18

DNA

• DNA is a polymer—a very large molecule made up of many repeating units called nucleotides. DNA composed of two spiral strands (Fig. 14-1).

• One strand is connected with the other by two bases that are linked to each other by hydrogen bonds (Fig. 14-1).

• These bases are cytosine (C), guanine (G), adenine (A), and thymine (T). – Adenine and thymine are held together by two hydrogen bonds

(A = T). – Cytosine and guanine are held together by three hydrogen bonds

(C ≡ G).

• The nucleotides in each strand are held tightly together by phosphodiester bond, but the two spiral strands are bound together more loosely by the hydrogen bonds.

DNA Replication• When the chromosome divides during cell

division, the two spiral strands of the DNA molecule unravel and separate at the position of hydrogen bonds.

• Every base attached to each strand attracts its complementary base, so that each single strand immediately becomes a new double strand exactly the same as the original double strand (see Fig. 14-1).

• DNA replication makes it possible for the chromosomes to transmit genetic informationfrom one cell generation to the next

DNA/RNA

• DNA is double-stranded, whereas RNA is a single strand.

• RNA sugars have one more oxygen atom than DNA sugars.

• RNA has uracil (U) as a base in place of thymine (T).

• The DNA molecule acts as a template from which a complementary strand of RNA is formed.

• The form of RNA that carries the genetic instructions as a complementary copy of the DNA series of bases is called messenger RNA.

• Another form of RNA— transfer RNA—brings the amino acids to the ribosomes to construct the proteins.

• For all organisms, the genetic code is the same, meaning that a DNA sequence is translated into the same protein in every organism. (This universal concept is what enables genetic engineers to take the DNA from one species and put it into another and get the same protein in the second organism).

Genes

• Structurally, a gene is a specific sequence of triplet nucleotide along a DNA molecule.

• The gene is the ultimate hereditary unitthat functions as a certain part of a chromosome determining the development of a particular characteristic in an organism.

• One gene (or several interacting genes) may determine plant height, leaf shape, flower color, or fruit size.

• Any individual gene may have – a large effect or – a small effect.

• Genes act – independently– in conjunction with other genes.

• Because genes are arranged along the chromosome, genes on the same chromosome are linked—that is, genes on the same chromosome move from one cell generation to the next as a unit.

• Linkage is not perfect; sometimes during meiosis (reduction cell division), chromosomes break and exchange parts.

Homologous Chromosomes

• Homologous chromosomes have the same gene or genes affecting the same traits at corresponding positions.

• Genes are termed alleles to each other if they occupy the same position on homologous chromosomes and affect the same trait.

Allelic genes can be dominant or recessive to each other:

• Dominant gene, (A), is one that causes a certain characteristic to be expressed whether the plant is homozygous, (AA)—both alleles the same—or heterozygous, (Aa)—the two alleles different.

• Recessive gene, (a) causes the character it controls to be expressed only if both alleles are recessive, (aa).

Mitosis

• Cell division in vegetative cells (2n) is called mitosis.

• During cell division, the replicated chromosomes split longitudinally, replicating to produce two chromosomes that are identical to each other. One of each pair goes to on daughter cell, and one to the other.

• So each daughter cell has a genotype identical to that of the mother cell.

Meiosis and Fertilization

• Meiosis refers to the type of cell division that occurs in the flower—in the angiosperms—to form the cells from which the pollen grains and the embryo sac (which contains the egg) develop (see Figs. 14-2 and 14-3).

• In this type of cell division, the homologous chromosomes separate from each other without replicating, one going to one daughter cell and one to the other, thus reducing the number of chromosomes—the 1n or haploid number.

• During pairing of the two sets of homologous chromosomes, crossing over can occur. The chromosomes may break at the same locus on each so that they may rejoin after exchanging segments.

• This gene alteration would be expressed in altered characteristics of the new plants.

• In fertilization in angiosperms, one male gamete (1n) from the pollen grain unites with a female gamete—the egg (1n)—to form the zygote (2n), which develops into the em-bryo and finally the new plant.

• Also during fertilization in the angiosperms, one male gamete (1n) unites with the two polar nuclei (1n each) in the embryo sac to form a food storage tissue, the endosperm (3n), that can serve as a nutritive material for the developing embryo.

Mutations• Errors can and do occur during replication. When they

do, they are called mutations, and the altered genes may possibly result in changes in the characteristics of the plant.

• Most mutations have such slight effects that they go unnoticed.

• The great majority of noticeable mutations are deleterious, but some are not and they provide a source of variability that aids the plant breeder in developing new cultivars.

• Mutation rates can be vastly increased by treatment with– Ionizing radiation – Certain chemicals

Polyploidy• Polyploidy is a condition in which

individual plants have more than two sets of homologous chromosomes in their somatic (vegetative) cells.

• Plants may be – Triploid (3n), – Tetraploid (4n), – Pentaploid (5n), – Hexaploid (6n), and so forth.

• Polyploid plants may arise by – Duplication of the chromosome sets from a

single species—Autoploidy or – Combination of chromosome sets from two or

more species—Alloploidy. • The latter is the more common type of

polyploidy in nature.

• Many of the cultivated crop species evolved in nature as polyploids, as shown in Table 14-1 for oats, wheat, and tobacco.

48Nicotiana tabacumCultivated tobacco

24Nicotiana sylvestrisWild tobacco

42Triticum aestivumCommon wheat

28Triticum dicoccumEmmer

14Triticum monococcumEinkorn

42Avena sativaCultivated oats

28Avena barbataSlender wild oats

14Avena strigosaSand oats

Somatic Chromosome NumberSpeciesCommon Name

Polyploidism in Oats, Wheat, and Tobacco

TABLE 14-1

Cytoplasmic Inheritance

• Certain characteristics in some herbaceous plants can be controlled by cytoplasmic factors, which are contributed only by the female parent.

• Example: Male sterility in corn is due, in part, to a cytoplasmic factor and is used to produce hybrid seed without the laborious procedure of hand detasseling.

Genotype and Phenotype

• Genotype refers to the genetic makeup of the plant—its genetic constitution—the kinds of genes it has on the chromosomes and the order in which they are situated.

• Phenotype refers to the plant's appearance, behavior, and chemical and physical properties.

• The phenotype expressed is also influenced by the environment. (A plant may have genes for very vigorous growth but when grown under a deficiency of soil nitrogen, for example, its inherent vigor is not expressed).