In the presence of favorable environmental conditions, the entire combination of genes dictates the final characteristics of the plant such as plant type, resistance to pests and diseases, stress tolerance, and crop yield. Thanks to Gregor Mendel, the “Father of Genetics,” the concept of the genetic factor affecting plant growth and development is now well understood.
The basis of the genetic control of plant expression is the gene, a carrier of information that determines a biological characteristic of an organism and is transmitted from parents to progeny upon reproduction.
The gene is composed of the deoxyribonucleic acid (DNA), the chemical basis of heredity or transmission of traits from parents to progeny. The DNA directs the synthesis of proteins, particularly enzymes, by the plant. Each enzyme in turn catalyzes specific biochemical reaction which leads to the formation of certain products. In order for a biochemical reaction to proceed, a specific enzyme must be available.
The genetic control of plant growth and development also has reference to the chromosome. The genes are located at specific loci in the chromosome, those cellular bodies within the nucleus which, under the microscope, appear as coiled contracted threads or rod-like bodies at certain stage of mitosis. The number as well as the size and shape of chromosomes, called karyotype, varies from species to species.
The chromosomes are considered the physical basis of heredity. They occur singly in haploid (1N) spores and gametes; in pairs (2N) in the diploid body (somatic) cells, mother cells and the fertilized egg; in triplicates (3N) in the triploid endosperm cells; and in multiple sets in the polyploid cells.
The diploid (2N) number of chromosomes in the body cells in humans is 46, 24 in rice and tomato, 20 in corn, and 14 in garden pea. In 2005, it was reported in the journal Nature (436:793-800, Aug. 11, 2005) that 37,544 genes have been identified in the genome of rice. Genome refers to all the genes present in one complete haploid set of chromosomes of an organism.
In diploid organisms such as garden pea, the gene exists in two alternative forms called alleles that are always found at the same point or locus on homologous chromosomes (paired chromosomes). These alternative genes are always denoted by letter symbols. For example, the experiment conducted by Gregor Mendel on garden pea is illustrated by assigning the capital letter “S” for the smooth character of seeds and the small letter “s” for the wrinkled character.
The credit for the formulation of the basics of genetic control goes to Gregor Mendel. He discovered that the genes are transmitted between generations in uniform predictable fashion. He crossed strains of true-breeding pea that differ in seven pairs of alternative characters. It is now known that in pea the inheritance of these observable characters, or phenotype, is controlled by a genetic constitution, or genotype, consisting of a single gene pair.
Gregor Mendel likewise demonstrated the concept that alleles belonging to separate gene pairs assert their effects independently from each other. Stated otherwise, each gene pair acts independently of each other so that in garden pea, the phenotypic frequency of 3/4 smooth against 1/4 wrinkled seeds in the second filial generation or F2 remains as it is whether the plant is short or tall. This has been called Mendel’s Law of Independent Assortment.
But then, the reality is that genetic control does not always follow the principle of independent assortment. There are many cases in which two or more gene pairs interact to form a new phenotype, notably crop yield. Yield is controlled by multiple number of genes, each of which determines the expression of certain character but contributes to the final yield through additive and/or interactive effect. Such characters, such as number of seeds per plant and average seed weight, are referred to as yield parameters.
Although the genetic control of plant expression mainly emanates from the nucleus of the cell, there are instances of cytoplasmic inheritance in which transmission of traits to the offspring is through the maternal cytoplasm. Certain cytoplasmic organelles, such as the plastids and mitochondria, contain DNA. This has been exploited in the hybridization of corn and rice whereby male sterile lines are used. This technique has reduced the cost of detasseling, the manual removal of corn tassel, and emasculation, the removal of the immature anther from a bud or flower.
The entire genetic make-up determines whether an organism should be a plant, an animal, a fungus, a protist, or a moneran. This factor also determines whether a plant should be a tree, a shrub, a herb, a vine, a liana, either a vascular or a non-vascular plant, a gymnosperm or an angiosperm, and down to the smallest classification of a species, a variety, a line, or a strain.
But there are cases also in which the gene or genotype is modified in nature leading to the formation of a new character. This is called mutation, as exemplified by the appearance of white variegation or albinism, the absence of pigmentation, in several plants. Mutation can be induced, a phenomenon that has been exploited in the production of new cultivars through irradiation or mutation breeding.
The hurdling of the complexities of genetic control at the chemical and molecular level has also paved the way for accelerated advances in the field of genetic engineering. Through the recombinant DNA technology, genes can now be transferred from one organism to another.
HARTL DL, FREIFELDER D, SNYDER LA. 1988. Basic Genetics. Portola Valley, CA: Jones and Bartlett Publishers, Inc. 505 p.
POEHLMAN JM. 1977. Breeding Field Crops. Connecticut: AVI Publishing Co., Inc. 427 p.
NATURE. 2005. The map-based sequence of the rice genome. Nature. 436:793-800. Retrieved April 6, 2011 from http://www.nature.com/nature/journal/v436/n7052/full/nature03895.html.
(Ben G. Bareja April 2011)