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The below mentioned article provides an overview on Mendel and his Laws of Heredity:- 1. Introduction to Mendel and his Laws of Heredity 2. Mendel’s Experiments 3. Terminology used in Mendelian Laws 4. Mendel’s Laws of Inheritance or Laws of Heredity 5. Biological Importance of Mendelism.
Contents:
- Introduction to Mendel and his Laws of Heredity
- Mendel’s Experiments
- Terminology used in Mendelian Laws
- Mendel’s Laws of Inheritance or Laws of Heredity
- Biological Importance of Mendelism
1. Introduction to Mendel and his Laws of Heredity:
The credit goes to Gregor Johann Mendel for making first effort in the field of heredity and formulating the basic laws of inheritance. He is now appropriately famous as ‘the father of science of heredity’ (Fig; 13.1).
He was, of course, not the first man to discover any general law of inheritance as Kohlreuter and several other predecessors had already obtained the results similar to those of Mendel but had failed to explain the problem Mendel had two very important qualities with him which enabled him to achieve a good scientific record.
These were:
1. His ability to ask the right questions from nature, and
2. His ability to interpret nature’s answers patiently and correctly.
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Mendel was born in the family of an ordinary farmer in a Sicilian village of Heinzendorf in 1822. His father created deep interest in him in gardening. Mendel received his early education at his home and then he attended a preparatory schooling. After completing preparatory course, he completed two- year course in philosophy.
During the latter part of preparatory course he had to meet financial burdens of the family. He entered Augustinian religious establishment of St. Thomas at Brunn in Maravia, Austria (now Czechoslovakia) on the advice and help of his teacher. Professor Franz. At the age of 25 he became priest (Monk) in 1847.
Later Mendel was appointed as substitute teacher of Physics and natural history in a secondary school of Brunn. There he served for some fourteen years. In his spare time Mendel grew several varieties of pea in his monastery garden.
He conducted his famous garden pea experiments in his garden during the years 1856 to 1864. He also studied science and mathematics at the University of Vienna. There he realised the value of statistics. He became abbot of the monastery in 1868 and died in 1884.
2. Mendel’s Experiments:
Mendel’s predecessors working on several varieties of plants differing from one another in many complex characters produced fertile hybrids by artificial pollination, but they failed to explain the mechanism of heredity.
The following were the two reasons behind their failure:
1. They considered simultaneously many characters in which parents differed. This made them confused. Due to confusion they could neither trace the individual characters through successive generations nor they could maintain the complete numerical records of results.
2. Secondly, they believed that the hereditary characters of two parents (mother and father) become thoroughly mixed in the offspring.
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Mendel did realise the above two causes of his predecessors’ failure. In order to overcome those difficulties he carefully planned experiments exactly on the same pattern as his predecessors had already followed. He began his breeding experiments with different varieties of garden peas (Pisum) in 1856.
He performed those experiments not with the intention ship between testing direct action of foreign pollen or to see if two kinds of pollen acted identically, but to discover the relation of hybridization and the history of evolution of organic forms.
He spent first two years in selecting varieties with distinctive and contrasting characters and in making sure that the parental stocks under study were pure, i.e., their characters were permanent and true breeding.
In other six years he set up experiments for breeding the varieties with contrasting characters. As indicated in his field diary, Mendel studied several sets of such characters. Out of some twenty or thirty characters, he finally selected the following seven sets of contrasting characters.
Mendel studied all the above seven pairs of differing characters individually; first in original stocks and then in the hybrids which presented combination of such characters in different generations. He classified the progenies according to their characters and also maintained complete counts of individuals belonging to particular classes.
It is only on account of quantitative method of investigation Mendel could achieve success in his work. One of the most important experiments which Mendel performed is briefly described below.
In an artificial cross two pure breeding plants, one with tall stem (6ft. in height) and the other with small or dwarf stem (about 1 ½ ft. in height), were cross pollinated. Mendel accomplished this cross pollination by opening immature flower bud before its anthers had matured. Firstly, he removed the stamens from flower bud and then pollinated its stigma with pollens from the desired stock.
The cross-pollinated flowers were covered in order to prevent foreign pollens of undesired stocks from reaching the stigma. The seeds obtained from that cross on germination developed plants which represented the first filial generation (F1).
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They were not intermediate between the tall and dwarf as might be expected, but were all tall plants. Similar results were obtained regardless of whether pollen grains were taken from dwarf plants (stamenate) and tall was used as the female (pistillate) parent or vice versa.
Mendel made crosses to study the inheritance of six other pairs of characters and noticed that of the two contrasting characters of two different parents only one appeared in the F1, hybrids and the other remained latent or unexpressed.
The particular parental character which appeared unchanged in the hybrid plants of F1, generation was, therefore, called “dominant” and the unexpressed character as “recessive”. Thus by this definition, the tall character is dominant and dwarf is recessive in the above cross.
Mendel allowed the hybrid plants of first filial generation to self-pollinate (selfing). The seeds obtained from F1, plants were planted which developed into plants of second filial generation (F2). In F2 generation both tall and dwarf characters appeared in the ratio of approximately 3 tall to one dwarf (3: 1).
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Mendel took 1064 F2 hybrid seeds and grew them into plants. Among F3 plants, 787 were tall and 277 short (dwarf), i.e., in the ratio of 2.84 tall to I dwarf or on allowing for experimental error due to chance, it comes to 3: 1. Plants of F2 generation were allowed selfing and the F3 generation was raised.
Mendel then noticed that F2 dwarf plants produced seeds which developed into dwarf plants on germination or in other words F2, plants with recessive characters bred true on selfing and of the F2 tall (3/4 of the total F2 population), 1/3 produced only tails whereas the remaining 2/3 produced both tall and dwarf plants in 3:1 ratio in F2 generation.
The experiment can be summarized in the graphic form as follows:
Mendel was very much impressed when he found that all the seven pairs of differentiating characters which he combined by crossing gave similar results.
He found in each of his crosses that:
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1. One of the two contrasting characters was dominant and the other was recessive;
2. The recessive characters did not disappear permanently but only their appearance was checked by dominant ones in F, generation. The parental characters combined in the hybrids reappeared in second generation in the ratio of 3 dominant to one recessive;
3. the F2 recessives always bred true to their characters but of the F2 dominant types one-third were pure whereas 2/3 were hybrid types which when allowed selfing segregated into dominants and recessives in the ratio 3 : 1 in F3 generation.
Mendel continued his valuable investigation for eight years and examined in great detail about 10,000 plants. The data and inferences drawn from experiments were set forth in a paper entitled “Experiments in Plant hybridization.” Mendel read his paper before the members of the Brunn Natural History Society on February 8, 1865 and got that published in the proceedings of that society in 1866.
Unfortunately, his work failed to attract the attention of the people and remained unrecognised in the life of Mendel. Importance of his work was realised after his death when three biologists, De Vries in Holland, Correns in Germany and Von Tschermak in Austria worked independently on the same problem and arrived at the similar conclusion in 1900.
3. Terminology used in Mendelian Laws:
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The knowledge of the following technical terms is essential for understanding Mendel’s Laws of inheritance:
1. Hybrid:
The product of a cross between two stocks belonging to the same genus or species or between two different genera is called hybrid.
2. Hybridization:
It is a phenomenon which speaks about the mixing of two unlike genetic constitutions.
3. Alleles (allelomorphs):
This term was coined by Bateson and Saunders (1902) for characters which are contrasting or differing from one another.
4. Monohybrid cross:
It is a cross between two parents differing in only one pair of alternating characters.
5. Dihybrid cross:
It is a cross between two parents differing in two pairs of contrasting characters.
6. Polyhybrid cross:
This is a cross between two stocks differing in more than two sets of alternating characters.
7. Reciprocal crosses:
A set of two reciprocal crosses means that the same two parents are used in two experiments in such a way that in one cross one individual is used as female parent and the other as male parent and in the second cross of the same genotype sexes of the parents are reversed, as for example, cross between a tall male and a dwarf female is reciprocal of a cross between a tall female and dwarf male.
8. Dominant character:
Mendel introduced the term for a character which manifests itself in all the members of first filial generation from a cross between two pure breeding parents differing in respect of this character, i.e., in a cross involving two different alleles one which suppresses the other character is called dominant.
9. Recessive:
The suppressed character which does not appear in first filial generation is called recessive.
10. Back cross:
It is a cross between a pure parental strain and its hybrid progeny from previous cross.
11. Homozygous (homozygote):
This term was coined by Bateson and Saunders (1902). When the diploid individuals carry sets of similar traits or characters (example, TT or tt) they are said to be homozygotes. They develop when both the gametes (male and female), each carrying the same factor, fuse. Such plants breed true to their characters (Gk. Homo, alike).
12. Heterozygous (heterozygote). (gk. hetero, different):
This term was proposed by Bateson and Saunders (1902) for a zygote or a diploid individual developed from it (zygote) which carries both factors of a pair of alleles. It develops when two gametes with different genetic constitutions fuse. Such individuals do not breed true.
13. Phenotype:
The term was coined by Johannson (1909) for the visible characters or external appearance of an organism with respect to particular character or a group of characters.
14. Genotype. The term was proposed by Johannson (1909) for hereditary or genetic constitution of an individual.
4. Mendel’s Laws of Inheritance or Laws of Heredity:
Mendel crossed different varieties of garden pea (Pisum sativum) with the intension to discover relationship between hybridization and history of evolution of organic forms. He classified and recorded the number of individuals in progenies of each cross and compared the proportion on mathematical pattern. This led Mendel to discover a precise pattern and so to formulate basic laws of heredity.
These laws are:
1. Mendel’s first laws or law of segregation or law of purity of gametes.
2. Mendel’s second law or law of independent assortment.
1. Mendel’s First law or Law of Segregation or Law of Purity of Gametes:
Mendel formulated his first law from the conclusions drawn from his monohybrid experiments (i.e., from crosses in which only one set of differentiating characters was taken into consideration in the parental stocks).
This law states that if a pair of contrasting characters or allelomorphs originally found in two parental stocks (male and female) are brought together in the hybrid, they remain unchanged without contaminating each other.
In the first filial generation (F1) only one of the two characters appears in the hybrids (dominance) and the other character (allele) remains latent or unexpressed. These two coexisting dominant and recessive characters of F1, hybrid segregate or separate from each other during formation of gametes.
This is law of segregation. Since in F2 generation recessive types reappeared, it was apparent to Mendel that heredity constitution of an individual was not necessarily revealed by its external appearance.
The results of monohybrid experiments led Mendel to conclude that the germ cells or gametes from each parent, though at that time little was known of the cytology, carried “something” which was responsible for the appearance of particular character in adult plant. To that “something” he called germinal unit or factor.
These are now known as genes. He maintained that those factors remained unchanged even when the characters did not appear in the progeny, as for example, the unit for recessive character in F1, generation.
The theory also suggested that the factors separated when the germ cells were formed in the next generation. Professor Charlotte Auerbach in his book “Science of Genetics” writes, “This theory were so far ahead of what was then known about the cell structure and fertilization that modem geneticists stand amazed at the brilliance of mind which starting from very simple observations could push so far, so confidently and so daringly into completely unexplored territories.”
When the segregation takes place in F1, hybrids, two types of gametes are formed and a gamete contains only one unit character of each allelomorphic pair. Therefore, half of the gametes will have dominant factor and the remaining half will carry recessive factor. This rule applies to both male and female gametes.
Thus the gametes formed from F1, hybrids are always pure for a particular character and one may carry either dominant or recessive factor but not both. This is why law of segregation is known as “law of purity of gametes”. If self-pollination is allowed, two kinds of eggs will form all possible combinations with two kinds of male gametes.
Now there appear two probabilities:
(i) The union may occur between male and female gametes carrying similar factor or alleles. In this case the zygote will receive either both dominants or both recessives.
(ii) The union may take place between female and male gametes, one carrying dominant unit and the other with recessive unit.
Zygotes will be of different behaviour depending upon the ways in which the gametes unite. The zygotes which possess set of identical alleles will always develop into pure breeding individuals. These are called homozygotes. They may be either pure recessives or pure dominants.
The second types of zygotes which contain both the factors of a set of differentiating alleles are termed heterozygotes and individuals developing from them are called heterozygous. They always develop into individuals which will never breed true to their characters. Thus, in the segregation process hybrid types produce parental types as well as hybrids.
In other words, the law of segregation may be defined as the non-mixing of alleles in the hybrid. What it means is that if good allele and bad allele come together in the hybrid neither they will fuse nor will they influence each other, but will continue to be together in the hybrid generation after generation.
A good allele is as good when it leaves the hybrid as it was when it entered the hybrid and the bad allele is just as bad as it was when it entered the hybrid.
Mendel found that in the second filial generation both dominant and recessive characters appeared in 3 to I ratio. Further analysis of F2 individuals has shown that they are in the ratio of one homozygous dominant: two heterozygous dominants: one homozygous recessive (1:2:1). What was the significance of these simple numerical ratios? These ratios enabled Mendel to formulate his first law of heredity (law of segregation).
Mendel explained his results making use of letter symbols. He introduced symbol “A” for dominant and ‘a’ for recessive factors. Capital letter is used for dominant character and small letter for recessive one. This use of letter symbol is most significant as it implies for the existence of “something” or factor or character (gene).
Now returning to a cross between tall and dwarf peas, if we assume that the tallness is governed by factor ‘T’ and dwarfness by factor ‘t’ then the genetic composition of pure tall and pure dwarf parents will be TT and tt respectively. Both these parents are homozygous and if selfing is allowed in them, the tall will always produce only tall plants and similarly the dwarf plant will produce dwarf plants.
During development of reproductive cells or gametes the segregation or separation of factors or genes occurs. Here each gamete produced by homozygous tall parent will carry only one ‘T’ gene and similarly each gamete from homozygous dwarf parent will carry only one ’t’ gene.
Therefore, fusion of gametes from tall parent with gametes from dwarf parent will produce zygotes of only one kind, all with genetic composition ‘Tt’. Because the dominant factor T is present in all the F, offsprings, they all are tall (dominance).
The F1, generation now possesses a pair of genes, one for tallness and the other for dwarfness, so it should be called heterozygous (Tt). According to law of segregation, two genes (T and t) of heterozygous tall will segregate during the development of gametes and as a result, two kinds of gametes will be produced, half of the gametes will have ‘T’ gene and the remaining half will carry ‘t’ gene.
Here egg cells carrying T factor may be fertilized by male gametes carrying T or by those carrying t, and in the same way eggs carrying t factor may fuse with male gametes carrying either T or t factor, in other words, the fertilization may be at random. As a result of chance combination, approximate ratio 3 tall plants to 1 dwarf plant would be expected.
It is evident from the above graphic representation that among F2 tall plants there are two categories, some carry T T and some Tt. Though they are similar in their external appearance yet their genetic constitutions are different.
The entire genetic constitution of an individual, both expressed and latent, makes up its genotype, and the appearance or assemblage of characters which are expressed in the individual, as for example, tallness or dwarfness, constitutes its phenotype.
Back Cross:
When heterozygous individuals of F1 generation are crossed with one of the parental types (P) individuals it is known as back cross.
The back cross may be of the following two types:
(a) Out cross:
When the F1, hybrid individuals are crossed with dominant homozygous parent it is called out cross. In this type of back cross all the offsprings will show dominant trait but genotypes will be in the ratio 1 homozygous dominant. 1 heterozygous dominant, as is shown in the following table.
(b) Test cross:
When F1 hybrid individuals are crossed with homozygous recessive parent it is called test cross. When heterozygous tails of F1, are crossed with the recessive dwarf variety, half of progeny will be tall and half will be dwarf in F2 generation as illustrated in the following table. This is a back cross. Because it is used in detecting whether a given plant is homozygous or heterozygous, it is also known as test cross.
Other Example of Law of Segregation:
The law of segregation holds good not only in plants but also in animals. Here one original experiment conducted by a well known geneticist T.H. Morgan on Drosophila is cited to illustrate the law of segregation in animals (Fig. 13.2).
Morgan crossed a homozygous long-winged (wild type) Drosophila with a homozygous vestigial- winged fly and found that all the F1, hybrids were long-winged. When F1, hybrids were allowed mating among themselves, they produced long-winged hybrids and vestigial winged hybrids in three to one (3:1) ratio in F2 generation. The mechanism of segregation is illustrated in Fig. 13.2.
2. Mendel’s second law or law of independent assortment (dihybrid and tri-hybrid experiments):
Mendel conducted breeding experiments to study not only the monohybrid crosses but also the dihybrid and tri-hybrid ones in which the parental stocks differed in two and three characters respectively.
He planned his experiments on dihybrid and tri-hybrid crosses with the intention to discover the relationship amongst the different pairs of contrasting characters. Mendel also wanted to know whether the different pairs of alleles tended to remain together in parental arrangement in the progeny or they were independent.
Dihybrid Cross:
In a dihybird experiment, a pure breeding variety of pea with round (smooth) seeds and yellow cotyledons was crossed with a pure breeding variety with wrinkled seeds and green cotyledons. F1 seeds were found to be all round and yellow (round dominated over wrinkle and yellow cotyledon dominated over green one). It was immaterial which of the parents provided the pollen and which acted as pistillate parent.
It also made no difference whether both dominant traits had originally been in one parent or each of the parents had contributed one of the dominant alleles. When F1 hybrids were allowed self-pollination, in the F2 generation four different combinations were noted. Of the total 556 F2 seeds, 315 produced plants with round- yellow seed; 108 round-green; 101 wrinkled- yellow and 32 wrinkled green.
These different combinations, i.e., round yellow, round green, wrinkled yellow and wrinkled green, were found to fit very nearly into a ratio 9:3:3:1 which Mendel recognised as multiple of 3:1, i.e., (3+ 1)2 . Mendel saw that the ratio of 9:3:3:1 would be obtained if the two sets of traits inherit independently of each other, since two independent 3:1 ratio would be combined to simplify mathematical probability.
The results of dihybrid cross can be shown graphically as follows:
From the results of dihybrid experiment Mendel concluded the following points:
1. The members of two sets of alleles segregated in F2 generation.
2. the alleles of one set behaved independently with respect to those of the other set at the time of combination, i.e., they are independently assorted, as for example, round character appeared in combination with green and wrinkled appeared with yellow in the above cross.
The above two points are read in Mendel’s second principle called Law of Independent Assortment.
The independent assortment and random combination can be more easily illustrated if parent (P) are represented by letter symbols for 2 sets of alleles or genes (factors) under consideration. Here ‘R’ denotes for roundness and ‘r’ for wrinkleness and Y and y are used for the factors controlling yellow and green character of cotyledons respectively.
Now the mode of inheritance in the dihybrid cross can be explained in the following way:
After fertilization, the possible combinations will be 4 x 4 = 16.
This is shown in the following checkerboard (Fig, 13.3):
This method of representation is called punnett square and is named after an English geneticist, R.C. Punnett who first used it to illustrate possible combinations resulting from the crosses. The checker board method is time consuming so in its place the following easy and quick device is now used. This is known as forked line process.
It is explained below:
Round, yellow x Wrinkled, green
Here dihybrid cross is resolved into two monohybrid crosses. Thus round and wrinkle characters form one set and yellow and green form the other set. These two sets of contrasting alleles operating simultaneously will segregate in two monohybrid ratios 3:1.
Thus 3 round: 1 wrinkled and 3 yellow: 1 green will be produced in F2. Since these characters are combined at random, the 3: 1 ratio of round and wrinkled may be combined with 3: 1 ratio of yellow and green.
Thus:
In this way, we get the same dihybrid ratio 9:3 3:1.
The same ratio can be obtained by checker board method:
The genotypic expression may be illustrated as follows:
Summary:
Dihybrid Test Cross:
In the F2 progenies, only one of the four phenotypes, i.e., wrinkled and green will be homozygous for both the characters and the other three phenotypes can be homozygous or heterozygous for one or both the characters. In order to find out whether the individual is homozygous or heterozygous for both characters, test crosses are made.
If the individual is heterozygous for both the characters (as if F1,) and it is crossed with a double recessive, a phenotypic ratio 1:1:1:1 will be obtained.
This is clear from the following example:
Tri-hybrid experiment:
Mendel also crossed two parents which were different from each other in three sets of contrasting characters. In one trihybrid experiment, pure breeding plants with tall stem, round and yellow seeds were crossed with pure plants with dwarf stem, wrinkled and green seeds.
In this example 3 monohybrid sets operate simultaneously. Supposing that tall dominates over dwarf, round over wrinkled and yellow over green, the F1, progeny would be cent percent tall, round and yellow.
Here expected F2 results are shown mathematically by (3 + 1)3 which may be expanded to 27:9:9:9:3:3:3:1.
The result may be explained graphically by resolving the trihybrid cross into 3 monohybrid ratios and multiplying them in the following manner:
Phenotypic ratios:
It is evident from the above phenotypic expressions that some phenotypes have old combination of characters as are in parent but most of the combinations are new which are generally spoken as recombination’s. These combinations are good proofs to suggest that the characters are assorted independently.
Trihybrid test cross:
This type of test cross involves crossing of a trihybrid (e.g., TtRrYy) with triple recessive. In this, eight different types will be obtained in 1: 1: 1: 1: 1: 1: 1: 1 ratio in next generation.
Causes of Mendel’s Success:
The common garden pea plants were selected by Mendel as suitable material for his experiments for the following reasons:
1. The plants were annual with well defined characters.
2. The pea plants could be grown easily and crossed readily.
3. Flowers were self-pollinated and the hybrids were fertile.
Mendel succeeded in his experiment due to following points which he always kept in mind:
1. He was careful to work with pure varieties of peas which bred true to their characters.
2. He considered only one set of traits or characters at a time in the beginning.
3. Mendel recorded and classified offsprings of each individual in each generation and compared the results on mathematical pattern.
4. He maintained records of each cross at least up to third generation.
5. Mendel was the first man who decided the question of heredity and had this perception that characters of organisms were governed by a large number of separate units or factors. He also considered that single factor controlled a single character.
5. Biological Importance of Mendelism:
Mendel’s experiments have practical applications in plant and animal breeding. It is possible to produce by hybridization desired types which do not occur in nature. The desired traits carried in different varieties can be combined and maintained in a single individual variety.
Many new disease resistant and high-yielding crop plants and ornamental varieties are being developed by cross hybridization, for which Mendel’s law of segregation and law of independent assortment have provided basis.
