Essay on the Mendel’s Law of Inheritance | Genetics

In this essay we will discuss about:- 1. Law’s of Hereditary 2. Reasons for Mendel’s Success 3. Reasons for Overlooking of Mendel’s Results 4. Extensions of Mendelian Concepts.

Essay on the Law’s of Hereditary:

Mendel laid the foundation of the science of genetics through the discovery of basic principles of heredity. He conducted his experiments with garden pea (pisum sativum) in a small monastery garden for over seven years (1856-1864) and discovered two important laws of heredity, viz., 1. law of segregation, and 2. law of independent assortment. These are briefly presented below.

I. Law of Segregation:

This law states that alleles segregate or separate from each other during gamete formation and pass on to different gametes in equal number. In other words, when alleles for two contrasting characters come together in a hybrid, they do not blend, contaminate or affect each other while together.

The different genes separate from each other in a pure form, pass on to different gametes formed by a hybrid and then go to different individuals in the offspring of the hybrid.

Thus main features of this law are as follows:

i. When a dominant and a recessive allele of a gene come together in a hybrid after crossing between two plants having contrasting characters, they do not mix or blend together.

ii. They remain together in pure form without affecting each other. For this reason, law of segregation is also known as law of purity of gametes.

iii. They separate into different gametes in equal number. Each gamete has only one type of allele (say either A or a).

iv. Separation of two alleles of a gene during gamete formation takes place usually due to the separation of homologous chromosomes during meiosis (anaphase 1), because alleles are located in the chromosomes.

v. With complete dominance, segregation leads to phenotypic ratio of 3 : 1 in F₂ for characters governed by single gene, and 9:3:3:1 ratio for characters controlled by two genes.

vi. If crossing over does not take place, segregation of genes takes place during anaphase I. If crossing over occurs, segregation of genes will take place during anaphase II.

Example:

When we make a cross between red (RR) and white (rr) flowered plants, we get red colour of flower in F₁. In the F₁ both the alleles R and r remain together without blending or mixing with each other, though only the effect of dominant allele is visible. In F₂, allele for red flower colour and white flower colour segregate during gamete formation and pass on to the gametes in equal number.

Thus two types of gametes, viz., R and r are formed. Each gamete has either R or r allele. When the F₁ is self-pollinated, individuals with three genotypes, viz., RR, Rr and rr are obtained in F₂. Here RR and Rr are all red and only rr individuals are white (Fig. 7.1). Thus a phenotypic ratio of 3 red: 1 white is obtained. The overall mechanism is represented below.

When selfed seeds of RR were grown in F₃, they all produced all the true breeding individuals for red flower colour. The Rr individuals showed segregation in F₃ similar to segregation in F₂ generation. Individuals with rr genotypes were found true breeding for white flower colour when their selfed seeds were raised in F₃ generation.

II. Law of Independent Assortment:

This is the second law of inheritance discovered by Mendel. This law states that when two pairs of gene enter in F₁ combination, both of them have their independent dominant effect. These genes segregate when gametes are formed, but the assortment occurs randomly and quite freely.

Thus main features of this law are given below:

i. This law explains simultaneous inheritance of two plant characters.

ii. In F₁ when two genes controlling two different characters, come together, each gene exhibits independent dominant behaviour without affecting or modifying the effect of other gene.

iii. These gene pairs segregate during gamete formation independently.

iv. The alleles of one gene can freely combine with the alleles of another gene. Thus each allele of one gene has an equal chance to combine with each allele of another gene.

v. Each of the two gene pairs when considered separately, exhibits typical 3 : 1 segregation ratio in F₂ generation. This is a typical monohybrid segregation ratio.

vi. Random or free assortment of alleles of two genes leads to formation of new gene combinations.

Example:

When plants of garden pea with yellow round seeds are crossed with plants having green wrinkled seeds, we get yellow round seeds in F₁. Thus yellow colour of seed exhibits dominance over green and round seeds shape over wrinkled independently.

The F₁ produces four types of gametes, viz., yellow round (YR), yellow wrinkled (Yr), green round (yR), and green wrinkled (yr). Selfing of F₁ gives rise to all above four types of individuals in 9 : 3 : 3 : 1 ratio (Fig. 7.2).

The behaviour of all these genotypes was studied in F₃ generation. Out of nine yellow round individuals only one (YYRR) was found true breeding in F₃ generation. The other eight individuals showed segregation of various types.

Similarly, out of 3 yellow wrinkled individuals only one (YYrr) bred true and others segregated in 3 : 1 ratio. Same thing happened with green round individuals. The green wrinkled individual was also true breeding (Table 7.2).

Reasons for Mendel’s Success:

Investigations to unravel the mechanism of inheritance were made by several workers. However, only Mendel could get success in explaining the laws of inheritance.

The important factors which are responsible for Mendel’s success are briefly described below:

I. Proper Maintenance of Records:

Mendel had a systematic record of all the observations which he recorded on various characters in different generations. This helped him in proper understanding of laws responsible for the transmission of characters from one generation to other.

II. Study of Individual Character:

Mendel laid emphasis on the study of individual character, which helped him in systematic analysis of various characters. He studied seven characters of garden pea. Other workers studied the individual as a whole which confused the whole issue and they could not come up with concrete results.

III. Choice of Material:

Mendel selected garden pea for his investigations. Garden pea is a hermaphrodite, self-fertilized and short duration crop. This helped Mendel in maintaining the purity of material and raising more than one generation in a year. Moreover, flowers of garden pea are also ideal for hand emasculation and pollination.

IV. Maintenance of Purity:

Mendel always used pure breeding parents or lines for hybridization, which helped him in studying the inheritance of individual character in a systematic way. He used to maintain purity by growing different lines in separate plots to avoid mechanical admixture or contamination. Use of heterozygous material poses several difficulties in drawing general conclusions.

V. Knowledge of Shortfalls of Earlier Workers:

Mendel systematically recorded the reasons for the failure of earlier workers in investigating the mechanism of inheritance. This helped Mendel to think in new direction and better planning of his work.

VI. Mathematical Background:

Mendel had very good background of physics and mathematics. He studied mathematics at the University of Vienna where he was sent for higher studies by the monastery. His mathematical knowledge proved boon for him, which helped Mendel in explaining the segregation of characters in F₂ and F₃ generations in terms of segregation ratios.

He was able to understand that in natural population, it is very difficult to get segregation perfectly in a particular ratio. A slight deviation from the exact ratio will generally be observed.

VII. Proper Choice of Characters:

In garden pea, Mendel could easily select contrasting (opposite) forms for various characters, which helped him in getting clear-cut groups in F₂ and F₃ generations. Moreover, all the seven characters which Mendel studied in garden pea, were qualitative in nature.

Such characters always display discontinuous variation, which permits classification of individuals into different clear-cut groups for these characters. This also helped Mendel in generalizing his results.

Reasons for Overlooking of Mendel’s Results:

Mendel presented results of his seven years hard and devoted work in the meetings of the Natural History Society of Brunn on 8th February and 8th March, 1865 in two papers. His papers on “Experiments in Plants” were published in detail in the annual proceedings of the society in 1866 in German language.

These proceedings were distributed to several libraries in Europe and America. However, Mendel’s work remained overlooked for 34 years. Mendel died in 1884 and his work came to light 16 years after his death when three different scientists viz., de Vries (Holland), Correns (Germany) and Tschermak (Austria) independently reached at the same conclusions in 1900 which were drawn by Mendel 34 years ago, i.e. in 1866.

The important reasons for the neglect of Mendel’s revolutionary findings related to mechanism of inheritance for such a long time are briefly presented below:

i. Mendel generalized his results based on his studies on garden pea. Later on he worked on hawkweed (Hieraceum) on the advice of C.V. Nageli. Mendel could not prove his results on this plant as the embryo is formed from the ovule without fertilization (diploid parthenogenesis). This created doubt in the mind of scientists about the results of Mendel.

ii. Since Mendel has a good background of mathematics, he explained his results with the help on mathematics. The scientists at that time did not appreciate this approach.

iii. Mendel could not support his findings through cytological investigations as cytological studies were not well developed at that time.

iv. After his failure to demonstrate the results on hawkweed, Mendel lost interest in research work and devoted most of his time with the work of monastery. Moreover, he did not give proper publicity to his work and kept quiet.

Extensions of Mendelian Concepts:

The basic principles of heredity were initially discovered by Mendel in 1866 and rediscovered by de Vries, Correns and Tschermak in 1900. Later on these principles were clarified and confirmed by several researchers and some new concepts were investigated. Some of the new concepts were at variance with the findings of Mendel.

These are called as Mendelian deviations or exceptions or anomalies.

Such investigations include:

i. Incomplete dominance,

ii. Co-dominance,

iii. Multiple alleles,

iv. Linkage,

v. Lethal genes,

vi. Gene interactions,

vii. Pleiotropic gene effect,

viii. Polygenes,

ix. Environmental effects, and

x. Cytoplasmic or maternal effects.

Mendel did not come across these findings. In fact these are extensions of Mendelian concepts. A brief description of these concepts in presented below.

I. Incomplete Dominance:

Mendel always observed complete dominance of one allele over the other for all the seven characters which he studied in garden pea. Later on cases of incomplete dominance were reported. For example, in four ‘o’ clock plant (Mirabilis jalapa) there are two types of flowers, viz., red and white.

A cross between red and white flowered plants produced plants with intermediate flower colour, i.e., pink colour in F₁ and a modified ratio of 1 red : 2 pink : 1 white was observed in F₂ (Fig. 7.3).

II. Co-Dominance:

In case of co-dominance both alleles express their phenotypes in heterozygote. The example is AB blood group in human. The people who have blood type AB are heterozygous exhibiting phenotypes for both the I^A and I^B alleles. In other words, heterozygotes for co-dominant alleles are phenotypically similar to both parental types.

The main difference between co-dominance and incomplete dominance lies in the way in which genes act. In case of co-dominance, both alleles are active, while in case of incomplete dominance only one allele (dominant) is active.

III. Multiple Alleles:

Mendel always observed two allelic forms of a gene. Now cases are known where a gene has more than two allelic forms, although only two can exist in a diploid cell at a time. Existence of more than two alleles for a gene is called multiple alleles. Examples of multiple alleles are ABO blood group alleles in human, coat colour in rabbit and self-incompatibility alleles in tobacco.

IV. Linkage:

Mendel always observed independent assortment of genes. Later on cases of linkage were reported by Bateson and Punnett in 1906 in pea, Hutchinson in maize and Morgan (1910) in Drosophila. In a di-hybrid test cross, they observed higher frequencies of parental types than recombinants instead of 1 : 1 : 1 : 1 ratio. This led to modification of the concept of independent assortment.

V. Lethal Genes:

Gene which causes the death of its earner when in homozygous condition is called lethal gene. Mendel’s findings were based on equal survival of all genotypes. In the presence of lethal genes, the normal segregation ratio of 3 : 1 is modified into 2 : 1 ratio. Lethal genes have been reported in both animals as well as plants. In mice, allele for yellow coat colour is dominant over grey.

When a cross is made between yellow and grey, a ratio of 1 : 1 for yellow and grey mice was observed. This indicated that yellow mice are always heterozygous, because yellow homozygotes are never born because of homozygous lethality. Such genes were not observed by Mendel. He always got 3 : 1 ratio in F₂ for single gene characters.

VI. Gene Interactions:

When the expression of an allele of one gene pair depends on the presence of a specific allele of another pair, it is known as gene interaction. Mendel observed 9:3:3:1 ratio in F₂ from a dihybrid cross. Later on many deviations of this phenotypic ratio were observed in dihybrid crosses. The modified ratios included 9:7, 9:3:4, 12:3: 1, 13:3: 15: 1 and 9:6:1 in different crop plants.

VII. Pleiotropic Gene Effects:

Mendel observed that one gene controls the expression of only one character. Later on cases were observed in which one gene was found to govern the expression of two or more characters. Example is white eye allele in Drosophila. This allele affects eye colour, shape of spermathica, fecundity and testicular membrane.

VIII. Polygenes:

Mendel always observed that each character is governed by a single gene. Later on Nilsson Ehle observed that some characters are controlled by several genes and each of such gene has additive effect in the expression of character. This concept led to the foundation of polygenic inheritance.

East (1916) demonstrated that polygenic characters were perfectly in agreement with Mendelian segregation and later on Fisher and Wright provided a mathematical basis for the genetic interpretation of polygenic characters.

IX. Environmental Effects:

Genes can interact not only with other genes but also with the environment to produce the final phenotype. Thus phenotype is the result of the interaction between genotype and environment. It leads directly to the concept of penetrance and expressivity. The importance of environment was first realised by Johannsen. He coined the terms genotype and phenotype.

X. Maternal Effects:

Mendel did not observe any difference between direct and reciprocal crosses. Later investigations revealed the presence of significant difference in the reciprocal crosses, which led to the concept of cytoplasmic inheritance.