Familiarization with the common terms in genetics is important in understanding the Mendelian laws or principles, and the role of genetic factors in plant growth and development. Mendel’s parental “factor” is now referred to by the common term gene. Gene is probably the most commonly used term in genetics and so deserves more elaboration.
The genes consist of the deoxyribonucleic acid or DNA, so called the chemical basis of heredity. They are carried in the chromosomes, the physical basis of heredity, within the cell. The chromosomes are those that appear under magnification as coiled, contracted, threadlike bodies in the nucleus at certain stage of cell division in diploid somatic cells.
In diploid organisms like humans the chromosomes, and the genes, normally occur in two forms: haploid or singly (1N) in the sexual cells or gametes, and diploid or in pair of two (2N) in the somatic cells. Somatic cells refer to the body cells and are distinct from the sexual cells.
In Homo sapiens, the diploid number of chromosomes is 46 or in pair of two sets in which each set has 23 chromosomes. This diploid chromosome number is maintained in the cells throughout the human, animal, and plant body through mitosis, the type of cell division (nuclear division) that is responsible for growth and development.
But meiosis (reduction division), which is responsible for the production of sexual gametes, reduces this number to one-half of the diploid number, that is, from 46 to 23 in humans. The same is true to plants which, like humans and other animals, also have sexes and sexual gametes. The garden pea has a diploid number of 14 chromosomes or a haploid number of 7.
The genes occur in pair in the somatic cells of diploid organisms in which case they are called alleles. These alleles are found on the same locus (pl. loci) or location in homologous or paired chromosomes. The two may be identical, for example a true breeding smooth-seeded garden pea may have identical alleles S and S (the genotype is denoted by SS) or the alleles s and s (with genotype ss) in the wrinkled-seeded plants. In hybrid plants produced by crossing two plants that differ in a set of characters, the gene pair may consist of contrasting genes or alleles, for example capital S for smooth and small s for wrinkled seed in garden pea (with genotype Ss).
To distinguish further between these two terms in genetics that are used interchangeably (genes and alleles), the term allele is intended to refer to a particular gene pair. The gene S for the smooth character is an allele of s, the gene for wrinkled seed, but neither S or s is an allele of the gene T or t for tall and short characters, respectively.
It is likewise important to distinguish the term genotype from phenotype. These are terms in genetics of common usage but tend to confuse the beginner. The former (genotype) refers to the gene combination or genetic constitution of an organism, for example SS, Ss or ss.
The manifestation of the genotype, for example smooth seeds for both the genotypes SS and Ss, and wrinkled for ss, is called phenotype. The phenotype may be descriptive of a single or combination of characters or of the entire organism like human vs. garden pea. However, it does not only refer to traits that can be perceived through the sense of sight.
It can also refer to other traits that can be perceived, or measured, through the other senses. It can be an external or internal trait. It can also be measured using chemical analysis, either organic or biochemical, or other methods of laboratory analysis.
The genotype that consists of one allele, such as SS and ss, is termed homozygous and the organism having such gene pair is called homozygote while that in which the two contrasting alleles are present, i.e. Ss, is called heterozygous with the organism having such gene pair correspondingly called heterozygote.
In addition, there are basic terms in genetics that are widely used in describing the behavior of genes in relation to the intraallelic interaction (between genes belonging to same gene pair) called complete dominance. The term dominant refers to the allele or to the character which expresses itself even in the presence of a contrasting one while that which is hidden or masked by another is called recessive. In Mendel’s garden pea, capital S or smooth seeds is dominant over small s or wrinkled seeds so that plants having a heterozygous genotype (Ss) exhibits only the character that is carried by capital S or smooth seeds while the small s or wrinkled character is hidden.Further, the individuals belonging to the first selfed and successive generations are denoted by various symbols. The two original parents that are cross-pollinated (and cross-fertilized) are denoted by the symbols P1 and P2 while the first and succeeding selfed generations are denoted by the symbols F1 or first filial generation, F2 or second filial generation, and so on (Fn).
Mendel’s experiments involving the inheritance of characters belonging to single gene pair, or monohybrid inheritance, led to the formulation of the law of segregation. The parents (P1 and P2) that he used in his experiments are purelines or true-breeding lines with constant characteristics, meaning that they exhibit the same trait from generation to generation. True-breeding plants when selfed or allowed to self fertilize produce only progeny like themselves.
Gregor Mendel went further to investigate the process of inheritance involving two gene pairs, or dihybrid inheritance, which in turn led to the law of independent assortment.
The above terms in genetics largely apply to all forms of life.
(Ben G. Bareja Jan. 2012)