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In this article we will discuss about:- 1. Mendel’s Experiment 2. Observations and Results of Mendel 3. Predictions 4. Principles 5. Di-Hybrid Cross and Mendel’s Law.
Mendel was the pioneer of classical geneticists. Gregor Mendel (1822-1884) was an Austrian monk and is popularly known as the ‘Father of genetics’. His experimental work became the basis of modern hereditary theory. Mendel was born on July 22, 1822 in Heinzendorf, Czech Republic. He entered the Augustinian monastery at Brünn, Czech Republic and became actively engaged in investigating variation, heredity, and evolution in plants.
Between 1856 and 1863 he cultivated and tested at least 28,000 pea plants, carefully analysing seven pairs of seed and plant characteristics. He delivered his first lecture on pea experiments in the year 1865 and published his paper ‘Experiments on plant hybridisation’ in the year 1866. His experiments resulted in the formulation of the laws of heredity or Mendel’s laws. He coined two terms in genetics, dominance, for a trait that shows up in an offspring; and recessiveness, for a trait that does not appear or remain marked.
Mendel published his work on heredity in 1866, but his work made no impression for the next 34 years. Only in 1900 was his work recognised more or less independently by three investigators, a Dutch botanist Hugo Marie de Vries, De Correns of Germany and Tschmarck of Austria. He died in Brünn on January 6, 1884.
The reasons why Mendel’s work was neglected during his time are as follows:
a. The biologists were preoccupied with Darwin’s origin of life which appeared in the year 1859.
b. The journal in which Mendel’s work was published was not recognised.
c. The scientists of earlier times were not familiar with the statistical analysis of experimental data.
Mendel’s Experiment:
Mendel was very careful about the selection of the plant for his hybridisation experiments.
He selected the garden pea as the experimental plant because of the following reasons:
a. The plant was easy to grow in pots or in open grounds.
b. It had a short life cycle.
c. The plants had self-pollinating flowers.
d. Cross-pollination was also possible.
e. The plant possesses distinct contrasting heritable characters.
Mendel chose seven pairs of characters for his study which is summarised in Table 1.
Experimental Procedure of Mendel:
1. Selection of the Parental Lines:
The first step in the experiment involved the selection of the parental lines of the chosen plant. Mendel ensured that the seeds of the parents were ‘pure’ for each character. This was done-by allowing the plants to self-fertilise for many generations until the plant produced exactly its own kind.
For example, to get a pure line of a tall plant, he self-fertilized a tall pea plant for several generations till the off-springs were tall and in turn produced only tall plants. Similarly, he selected the ‘pure’ dwarf plant by adopting the same technique.
2. Crossing of the Two Selected Parental Lines:
The two pure lines were crossed by removing the stamens of all tall flowers and dusting the stigma with the pollen derived from the dwarf plants. The removal of stamens is known as emasculation. The crossing of the two varieties i.e. tall and dwarf is known as hybridization. Later in the same year, the mature pod could be opened and the first generation pods were collected and recorded.
3. Self-Fertilisation of the First Generation Pods:
Mendel sowed the seeds of the first generation or F1 and recorded the characters of the stem, leaves and flowers of mature plants that grew from these seeds. These plants were allowed to self-fertilise and the characters of the off-springs of subsequent generations were recorded.
Observations and Results of Mendel:
To understand the observations and results of Mendel, it is important to become familiar with a few genetic terms.
a. Gene:
Gene is the unit of inheritance. The term ‘gene’ was coined by Johannsen in the year 1905.
b. Alleles or Allelomorphs:
‘Y’ or ‘y’ are alternate forms of a gene. They are located at the same locus on homologous chromosomes.
c. Locus:
Position of an allele within a chromosome.
d. Homozygous:
It is the diploid condition in which the alleles at a given locus are identical (YY) or (yy).
e. Heterozygous:
It is the diploid condition in which the alleles at a given locus are different (Yy).
f. Dominant Allele:
Of a pair of allele, the allele that expresses itself in homozygous or heterozygous condition is called a dominant allele. For example ‘Y’ is expressed in a homozygote (YY) and heterozygote (Yy). It is usually represented by a capital letter.
g. Recessive Allele:
The allele that cannot express itself in the presence of its contrasting factor is called a recessive allele. It is usually represented by a small letter and can express only in a homozygote state (yy).
h. Phenotype:
Any measurable characteristic or distinctive trait possessed by an organism i.e. tall or dwarf is called phenotype.
i. Genotype:
The genetic constitution of an organism is known as genotype.
j. Parental Generation:
The plants with distinct contrasting characters which were crossed are called parental generation or P generation.
k. Hybrid:
The progeny or offspring obtained from a cross between two individuals that differ from each other in at least one character is called a hybrid.
l. Hybridisation:
The process through which the hybrids are produced is called hybridization.
m. First Filial Generation:
The hybrids of the P generation are known as the first filial generation or F1.
n. Second Filial Generation:
When F1 progeny is allowed to self-fertilise they produce the second filial generation or F2.
o. The subsequent generations produced are called F3, F4, F5 or F6 etc.
p. Reciprocal Cross:
The cross in which the characters chosen are the same but the sexes are reversed.
Mendel observed that the F1 hybrids contained the character of only one parent. In F2 both the parental characters appeared. The character that appeared in was called the dominant character, while the hidden character that made its appearance in F2 was known as the recessive character.
He also noticed that the dominant character remained the same even when he conducted reciprocal cross. He tested each of the seven pairs of contrasting characters for the phenomenon of dominance and recessiveness. The results are given in Table 1.
Mendel observed that the recessive character appeared in the F2 off-springs in an average ratio of 3:1. This 3:1 ratio was observed in the hybridisation cross between parents which differed from each other with respect to one character. This ratio is known as the monohybrid ratio and the cross is known as the monohybrid cross.
In the F3 generation that was obtained by self-pollinating each of the F2, the following observations were recorded:
i. The recessive form of F2 gave rise to only the recessive variety showing that they were true breeding.
ii. About 1/3rds of the dominant forms of F2 were true breeding while the remaining 2/3rds produced both the forms i.e. dominant and recessive. Mendel’s original results for the seven pairs of characters have been tabulated in Table 2.
Mendel’s Predictions:
The science of cytology was in its primitive state during Mendel’s time. However, Mendel visualised the cause of inheritance as factors or elements, which were later named as genes by Johannsen in 1909. According to Mendel, each male and female parent contains a pair of factors and each parent passed only one factor of a pair to their off-spring.
He also predicted that each factor retained its individuality from generation to generation. The factors contributed united randomly to produce the characters of the hybrid. Thus, he indirectly predicted the reduction in chromosome number during gametogenesis and the physical hereditary mechanism.
Mendel’s Principles of Inheritance:
Mendel did not publish any law as erroneously described in the various text books. The re-discoverers of Mendel’s work were responsible for our understanding of Mendel’s principles. They felt that Mendel’s work could be represented by laws of heredity. These laws are the law of dominance, law of segregation and the law of independent assortment.
To understand Mendel’s work, post-mendelian geneticists have evolved a genetic terminology to understand the phenomenon.
Symbols Used to Represent Mendel’s Laws:
According to the classical method of symbolism, the dominant allele is represented by the capital letter while the recessive allele by the small letter. Thus tallness will be represented as ‘TT’ and recessive character by ‘tt’. In the modified method, according to the abnormal recessive allele the symbol is chosen.
For example the condition of albinism is characterised by the lack of melanin pigment in the skin, hair, eyes etc. This condition is a rare condition caused by the recessive allele in homozygous condition. The symbol in this case is ‘a’ for recessive allele and ‘A’ for the normal allele.
Another method is followed in wild plants, bacteria and viruses. When out of two phenotypes, one is more common in the population than its alternative form, it is referred to as the wild phenotype. The rare form is called the mutant phenotype. The symbol + is used to indicate the normal allele for wild type and the base letter is borrowed from the name of the mutant type.
Law of Dominance:
According to the law of dominance,
i. Different characters are controlled by units called factors
ii. Factors occur in pairs
iii. Of a pair, one factor dominates the other.
Law of Segregation:
According to the law of segregation or law or purity of gametes, in a heterozygote the dominant and recessive allele remain together without mixing with each other. The alleles separate or segregate from each other during gametogenesis, so that each gamete receives only one allele, either dominant or recessive.
During his experiments, Mendel encountered some traits that did not follow the laws he had encountered. These traits did not appear independently, but always together with at least one other trait. Mendel could not explain what happened and chose not to mention it in his work. Today, it is understood that these traits are because of alleles located on the same chromosome.
Di-Hybrid Cross and Mendel’s Law of Independent Assortment:
Di-hybrid cross is a cross between two parents which differed from each other with respect to two characters. The law of independent assortment is based on the di-hybrid cross.
According to the law of independent assortment or recombination, when gametes are formed the members of the different pairs of alleles segregate independently of each other and recombine in all possible ways.
Let us understand with an example:
In the case of a di-hybrid cross, two characteristics i.e., seed colour and seed texture are considered. The cross was made between plants with yellow round seeds and plants with green wrinkled seeds. The F1 progeny obtained are heterozygotes for the two gene pairs. The di-hybrid is also often referred to as the double heterozygote. All the F1 hybrids obtained had yellow round seeds.
When the F1 seeds were self-fertilised, four types of gametes – YR, Yr, yR and yr were produced during gametogenesis. In a di-hybrid cross, the gametes of the two parents are determined by Mendel’s law of segregation. These gametes unite randomly during fertilisation and produce sixteen types of individuals as shown in the Fig. 4.
The ratio of the progeny obtained is 9:3:3:1.
Back and Test Cross:
Back cross is a cross between the F1 hybrid and one of its parents. Test cross is a cross between a hybrid and a homozygous recessive parent. A test cross is conducted to know whether an individual is homozygous or heterozygous for a dominant character. Every test cross is a back cross but every back cross is not a test cross.
The phenotypes of the offspring produced by the test cross reveals the number of different gametes formed by the parental genotype. A monohybrid test cross gives a phenotypic ratio of 1:1 (Fig. 5). A di-hybrid test cross gives a phenotypic ratio of 1:1:1:1 indicating that two pairs of factors are segregating and assorting independently.
