In sum, the genetic factors affecting plant growth and development and crop productivity are all those which exclude the environmental factors. They refer to the overall gene constitution of the plant or the smallest unit of this entirety which dictates the expression of specific traits.
They are also referred to as internal factors because they determine plant characteristics from within the plant, specifically from within the cell. In other words, a plant displays a unique trait because there is this genetically dictated blueprint that makes such a trait inherent in the plant. Without such a genetic blueprint, any manipulation of the external factors (called environmental) will fail in attempting to make the plant display such a trait.
Genetic Control: Overview
The gene (Gregor Mendel’s “factor”) is the carrier of information that determines a biological characteristic of an organism and is transmitted from parents to progeny upon reproduction. It is composed of deoxyribonucleic acid (DNA), referred to as the chemical basis of heredity (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 reactions which leads to the formation of certain products. In order for a biochemical reaction to proceed, a specific enzyme must be available.
In the presence of favorable environmental conditions, the entire combination of genes dictates the final characteristics of an organism. It determines whether an organism should be a plant, an animal, a fungus, a protist, or a moneran. This genetic 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. It controls adaptation, resistance to pests and diseases, stress tolerance, and crop yield.
The credit for pioneering the study of the basics of genetic factors should go to Gregor Mendel. He discovered that the genes are transmitted between generations in a 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 other gene pairs. 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 constant whether the plant is short or tall. This has been called Mendel’sLaw of Independent Assortment.
Crop yield is controlled by multiple genes, each of which determines the expression of certain characters but contributes to the final yield through additive and/or interactive effects. Such characters, such as the number of seeds per plant and average seed weight, are referred to as yield parameters.
But then, the reality is that gene expression is not always in accordance with the rule of independent assortment. There are many cases in which two or more gene pairs interact to form a new phenotype, an interallelic interaction called epistasis. Other interactions and genetic phenomena affect plant characteristics such as multiple genes, polyploidy, and mutation.
The Chromosome as Genetic Factor
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 a certain stage of cell division called mitosis. The number as well as the size and shape of chromosomes, called a karyotype, varies from species to species.
The chromosomes are considered the physical basis of heredity. They occur singly in haploid (1N) sexual 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.
By way of example, rice and corn (maize) are two distinctly different organisms with the former having 24 and the latter with 20 diploid chromosome numbers. However, it is not the number of chromosomes alone that is responsible for diversity or identicality. Even if two organisms have the same chromosomal number, they may still differ because of the unidentical shapes and sizes of individual chromosomes. Further, they may vary in the number of genes and the gene-to-gene distances in each chromosome, as well as the chemical and structural composition of these genes. Finally, genomes differ from organism to organism.
Advances Applying the Concept of Genetic Factors Affecting Plant Growth and Development
Although the control for the expression of phenotypes by genetic factors 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.
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 a mutation, as exemplified by the appearance of white variegation or albinism, the absence of pigmentation, in several plants for example in the snake plant (Sansevieria trifasciata). 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 in the control of plant expression by genetic factors at the chemical and molecular level has also paved the way for accelerated advances in the field of genetic engineering. Through recombinant DNA technology, genes can now be transferred from one organism to another.
However, recent reports revealed the possibility of producing genetically edited organisms (GEOs). This technique of genome editing offers wide possibilities in the development of improved cultivars without introducing foreign genes as in transgenic crops or genetically modified organisms or GMOs.
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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.