MENDEL’S WORK: THE EYE OPENER TO GENETICAL STUDY
In a couple of decades, before Gregor Mendel was born in 1822, experimental hybridization was achieved and documented in 1760 by a researcher called Josef Kölreuter. Joseph Kölreuter crossed different strains of tobacco and generated fertile offspring which varied in appearance from the strain of the parents. Among these offspring are plants of the hybrid generation being their parents, while a few took strains of the original strains being their grandparents. The offspring of the second generation was observed to possess some variations contradicting the theory of direct transmission. Other investigators also investigated hybridization until Mendel’s experiment on plant hybridization which helped him put forward the major postulates of transmission genetics in 1866. During the mid-nineteenth century, researchers successfully made headway in the understanding of inheritance. For seven years (1856-1863), a hybridization experiment was carried out by Mendel on garden peas, and he proposed the laws of inheritance in living organisms. Mendel chose garden pea plant for experimentation because of some defined reasons.
REASONS FOR CHOOSING GARDEN PEA BY MENDEL
- It was used by earlier investigators wherein they generated hybrids.
- High availability of pure varieties.
- It has short life cycle, and many generations can be grown over short period of time.
- Garden pea is less difficult in growing and is small.
- When undisturbed, it can self-fertilize, but cross fertilization can also be induced successfully.
CONTRASTING TRAITS STUDIED BY MENDEL
- Flower colour (Violet/white)
- Seed shape (Round/wrinkled)
- Seed colour (Yellow/green)
- Pod colour (Green/yellow)
- Pod shape (Inflated/constricted)
- Flower position (Axial/terminal)
- Stem height (Tall/dwarf)
Mendel, in his study, crossed tall and dwarf pea plants to study the inheritance of one gene. He then assembled the produced seeds that resulted from this cross and grew them to generate plants of the first hybrid generation (Filial1 progeny or the F1). He observed that F1 progeny plants were phenotypically tall like one of its parents, and none were observed as a dwarf. Similar observations were made for the remaining pairs of traits studied. From his findings, all of the offspring showed traits that were similar to either male or female parent. Based on the observation, he inferred that the F1 always resembled either one of the parents and that the trait of the other parent was not seen in them. Mendel then inbred the tall F1 plants and surprisingly, he discovered that in the Filial2 generation, some of the offspring were dwarf. Which is the character that was not seen in the F1 generation but is now expressed. The proportion of the F2 plants that were dwarf was one-quarter while three-quarter of the F2 plants were tall. The phenotypical expressions of the offspring were identical to their parental type (tall and dwarf traits) and did not show any offspring with in-between or intermediate height. Similar results were obtained with the remaining six traits that he researched. This finding was only possible because the traits he studied were monogenic, this implies that they were controlled by a single gene. If they were polygenic, the plants would have produced intermediate traits.
Mendel proposed from his finding that material was being transferred from parents to offspring, unchanged, through their gametes. These materials move across successive generations, and they should be the reason for the resemblance noticed across generations. However, they could be altered due to their interaction with mutagen and other cellular activities. He called these materials ‘factors’. We presently address them as ‘genes’. Genes, therefore, are the units of inheritance. Multiple genes occur within a chromosome and they contain the needed information to express a particular trait in an organism. Examples of such traits are height, intelligence quotient, shape, etc. The process by which characters or traits (for example, blood group) are passed on from parent to progeny is called inheritance. Inheritance is the basis of heredity. Alleles are genes that code for a pair of contrasting characters, i.e., in simple form, they are slightly different forms of the same gene.
After gene discovery, a challenge in gene representation was faced and scientists adopted the use of alphabetical symbols for each gene coding of trait. For example, regarding the character, height, T represents the Tall trait while t represents dwarf, and T and t are alleles of each other. Hence, the pair of alleles in the plant for height would be TT, Tt, or tt, if the plant is diploid in the chromosome set. Traits with gene inscription TT and tt are described to be homozygous dominant and homozygous recessive respectively. For allele t, the gene can only be expressed if found in the homozygous state. At the heterozygous state, its effect is masked. Traits with Tt are described to be heterozygous since alleles express contrasting traits. The letter description TT and tt for traits refer to the genotype of the plant while the descriptive terms ‘tall’ (for TT and Tt) and ‘dwarf’ (for TT) are the phenotypes.
Mendel, in his research, discovered that the phenotype of the F1 heterozygote (Tt) is phenotypically similar to the homozygous dominant (TT) parent. Then, he proposed that in a pair of dissimilar factors or alleles, one dominates the other (as in the F1) and hence is called the dominant factor or allele while the other factor is recessive (the one being masked). Concerning this, T (for tall) is dominant over t (for dwarf), which is recessive. The one that dominates is being expressed while the second is masked and can only be expressed in a duplicated form.
The recessive parental trait is shown with no blending in the second generation, one can state that, when both tall and dwarf plants produce sex gametes, during meiosis, the alleles of the parental pair segregate from one another and only one allele is transmitted to a gamete. The allele segregation is randomly done. Therefore, there is an equal probability or 50 percent chance of each gamete containing either dominant or recessive allele. One cannot separate organisms with the genotypes of homozygous TT and heterozygous Tt by physical examination. This is because, within the genotypic pair of Tt, only one character ‘T’ tall is expressed, whereas t is the mutant allele, and it is not expressed. Therefore, the trait represented by T or ‘tall’ dominates the other allele t or ‘dwarf’ character. This dominance of one trait over the other trait resulted in all the F1 being tall (though the genotype is Tt) and in the F2, 3/4th of the plants is tall (though genotypically 1/2 are Tt, and only 1/4th is TT). This results in a phenotypic ratio of 3/4th tall: (1/4 TT + 1/2 Tt) and 1/4th tt, i.e., a 3:1 ratio, but a genotypic ratio of 1:2:1 as earlier shown in fig. 21.1 b.
Mendel observed on monohybrid crosses and proposed two general rules to unify his understanding of inheritance in monohybrid crosses. These rules presently are called the Principles or Laws of Inheritance and are stated below.
MENDELIAN LAWS OF INHERITANCE
Law of segregation: it states that the two members of a gene pair segregate independently from each other into the gamete. Half of the gametes will carry one member of the pair while the other half will carry the other member of the pair. This idea was directly proven after the discovery of meiosis. In meiosis, the paternal and maternal chromosomes separate, and the alleles for the traits segregate into two different gametes, if not, the gamete of the zygote will contain a duplicated chromosome number being a whole chromosome from the mother and father. For instance, a human cell would contain 92 chromosomes.
Law of Independent Assortment or Inheritance Law: it states that gene pairs on different chromosomes are inherited independently at meiosis. In another way, it states that alleles of different genes assort or separate independently of one another during gamete formation. This law stands when there is no linkage. The genes involved are on different chromosomes, when on the same chromosome, they assort together. For sex-determining genes that show sex linkage, genes are inherited together.
Law of Dominance: This law explains the relationship between dominant and recessive alleles. If an individual is heterozygous for a trait, the dominant allele will determine the phenotype expressed by the organism, while the recessive allele will be masked and not expressed in the phenotype. While the dominant allele is symbolized with an uppercase letter, the recessive allele is symbolized with a lowercase letter.
This is seen in the study of inheritance of flower color in pea plants by Mendel. He observed that the dominant allele for purple flowers (P) masked the expression of the recessive allele for white flowers (p) in the phenotype. He concluded that only when an individual has two recessive alleles (pp) would white flowers be produced. Mendel’s work laid the groundwork for the modern field of genetics.
EXPERIMENTAL STRATEGIES EMPLOYED BY MENDEL
- For each trait he studied, he chose a set of plant and ignored other variations existing between the chosen plants.
- To reaffirm that the plants were true breeding for the trait of interest, he allowed them to self-fertilize.
- Crosses and reciprocal crosses were carried out on plants selected for traits of interest.
- Hybrid offspring collected after the crosses were allowed to self-fertilize across generations and critical observation and documentation carried out on them.
His involvement of mathematical logic and statistical analysis approaches in documenting his findings distinguished Mendel’s research from former researchers who recorded their findings quantitatively.