In this article we will discuss about:- 1. Life History of Mendel 2. Rediscovery of Mendel’s Work 3. Technique of Artificial Hybridization 4. Monohybrid Cross 5. Summary 6. Trihybrid Cross 7. Physical Basis of Heredity 8. Ploidy.
Contents:
- Life History of Mendel
- Rediscovery of Mendel’s Work
- Mendel’s Technique of Artificial Hybridization
- Mendel’s Experiments—Monohybrid Cross
- Summary of Mendelian Principles
- Mendel’s Experiments – Trihybrid Cross
- Physical Basis of Heredity According to Mendel
- Ploidy
1. Life History of Mendel:
Gregor Mendel is regarded as the ‘father of genetics’. His experiments with garden peas (Pisum sativum) were elegant and the inferences together with his interpretations constitute the foundation of modern genetics.
The present-day knowledge and researches on heredity rest directly on a number of principles and laws advocated by Mendel. Since Mendel, many scientists have added to and extended these principles but it was he who set forth in a clear manner a logical explanation about the ways in which hereditary factors (genes) behave during a cross.
Johann Mendel was born in 1822 of German parentage in Heinzendorf, a village in Silesia. At the age of twenty- three, he joined the Augustanian Monastery of Brunn, Austria (now Brunn, Czechoslovakia) where he spent the remainder of his life (1884). After his entry into the monastery, he chose the name ‘Gregor’ and was thenceforth known as Gregor Johann Mendel.
Mendel conducted his hybridisation experiments in the garden behind the monastery. He had a scientific turn of mind and with painstaking and methodical researches he built up a tremendous amount of data which forms the very basis of all studies on heredity.
The classic paper of Mendel was written in 1865 and was published in the following year in an obscure journal, “Transactions of the Natural History Society” of Brunn. Here the investigation of Mendel remained buried till 1900 when the great contribution of Mendel was brought to the limelight by three Botanists of different nationalities.
2. Rediscovery of Mendel’s Work:
The men who brought to light the work of Mendel were de Vries of Holland, Tscher- mark of Austria and Correns of Germany. Their rediscovery of Mendel’s contribution was published separately only a few months apart and was followed by volumes of papers extending the application of Mendelian principles to both animals and plants.
3. Mendel’s Technique of Artificial Hybridization:
It is essential to know first the technique of Mendel for artificial hybridisation and its difference with normal self-fertilisation. The success of Mendel depends largely on the selection of garden pea as the experimental material.
Mendel chose the material because it is an annual plant with well-defined characteristics. Pea plants can be grown and crossed easily. The flowers of this plant have both male and female parts and self-fertilisation occurs ordinarily. Cross-pollination, i.e., transference of pollen from one plant to the stigma of another plant, does not occur in natural state to any great extent. It can be done by an experimenter.
Mendel chose seven pairs of contrasting traits in his experimental studies.
They are:
(i) Vines are either tall or dwarf;
(ii) Unripe pods are green or yellow;
(iii) Unripe pods are inflated or constricted between the seeds;
(iv) Flowers are either axial or terminal;
(v) Nutritive portions of the ripe seeds are either green or yellow;
(vi) Seed coats are either smooth or wrinkled and
(vii) Seed coats are white or gray.
Mendel did hybridisation experiments with many plants like beans, maize, snapdragon, etc. Of these, the experiments with garden peas are most fascinating. Peas are self-fertilising plants. Self-fertilisation occurs when two gametes which unite to produce a zygote that develops into a seed and subsequently into the adult plant of the next generation, are derived from the pollen and ovule of the same flower.
For artificial crossing of normally self-fertilising flowers of peas, Mendel carefully removed the stamens from one flower while the pollens are still immature and later he transferred to these depollenised flowers, ripe pollen from the flower of another plant of his choice.
The seeds produced by the cross-pollinated flowers were planted and the resultant offsprings (Hybrids) were carefully examined. Mendel then crossed the flowers amongst themselves to obtain another generation. From this second generation of hybrids Mendel formulated the fundamental principles of heredity.
In garden peas Mendel observed seven pairs of contrasting characters. These included smooth seeds and wrinkled seeds, yellow cotyledons and green cotyledons, inflated pods and constricted pods, yellow pods when unripe and green pods when ripe, flowers in the axils of the leaves and flowers at the end of stems, transparent seed coats and brown seed coats, tall plants and dwarf plants.
4. Mendel’s Experiments—Monohybrid Cross:
Experiment I:
Mendel noted that among the pea-plants some were tall attaining a height of six feet or more while others, though grown in the same soil, were dwarf reaching a height of eighteen inches only. He made a cross between a tall plant from a tall race and a dwarf plant from a dwarf race.
Results:
The resulting plants (progeny) of first filial (F1) generation were all tall.
Experiment II:
Mendel crossed two of these F1 plants together and raised as many plants as possible.
Results:
The resulting plants or progeny of second filial (F2) generation were both tall and dwarf. There were three times as many tall plants as dwarf ones.
Experiment III:
Mendel then self-pollinated the F2 plants.
Results:
(i) Dwarf plants when self-pollinated produced only dwarf progeny.
(ii) Of the tall plants in the F2, one-third bred true and two-thirds produced both tall and dwarf in the ratio of 3: 1 as the F1 plants did.
The most striking points from the above experiments were (a) only one of the two characters concerned in the first experiment showed up in F1. All plants were tall and not dwarf.
The dwarf character, though disappeared in F1 was not lost and made its appearance again in F2.
The logical conclusion arrived at by Mendel from these points was that F1 plants must have carried a ‘hidden factor’ for dwarfness. Thus parents: Tall X Dwarf.
F1 — Tall (Dwarf)
The second conclusion arrived at by Mendel was that if the F1 plants contain two ‘factors’ for height (one responsible for tallness which showed up and one for dwarfness which remained hidden) any plant of any generation must possess two ‘factors’ for height. A tall plant from a tall race contains two factors for tallness and similarly a dwarf plant has two factors for dwarfness.
Thus Mendelian experiments could be visualised in the following way:
Pure tall X Pure dwarf F1—All Tall
F1—All Tall X All Tall
F2—Three tall: One Dwarf
Of the three tall plants, one breeds true that is the plants on being allowed to self- pollinate produce tall plants. Other two on being allowed to self-pollinate or on making a cross between themselves produce tall and dwarf plants in the ratio of 3: 1. The dwarf plant of the F2 generation breeds true (Fig.2.1). Fig.2.2 relates the monohybrid cross between black and brown coloured guinea-pigs.
5. Summary of Mendelian Principles:
From the various crosses made by Mendel the following principles were formulated:
a. Principle of unit character:
The inheritance-of various traits by an organism is controlled by factors (genes) and these factors occur in pairs.
b. Principle of dominance:
One gene in an allelic pair may be instrumental in suppressing the expression of the other member of the pair.
c. Principle of segregation:
In the formation of haploid reproductive cells as gametes or spores, only one gene of each allelic pair is received by the gamete or spore.
d. Principle of independent assortment:
Hereditary characters are independent units which segregate upon crossing regardless of temporary dominance.
In the F2 generation, four types of seeds were obtained. They were round yellow seeds, round green seeds, wrinkled yellow seeds and wrinkled green seeds and the approximate ratio of the four types were 9:3:3:1 respectively. Of these four types of individuals two were like the grandparents (yellow round and wrinkled green) while two were entirely new combinations (round green and wrinkled yellow).
Thus, it appears that the seed colour character is independent of seed shape character and they are never tied. The formation of new combinations (round green and wrinkled yellow) shows that the yellow colour can dissociate itself from round shape and vice versa.
Similarly, green colour can dissociate itself from wrinkled shape and vice versa. This property of dissociation leads to the formation of new combination and it is obvious from the above mentioned behaviour of characters that segregation of seed colour is independent of seed shape.
From this experiment (Dihybrid cross), Mendel postulated his third law which states that hereditary characters are usually independent units which segregate upon crossing regardless of temporary dominance. This is known as the law of Independent Assortment.
6. Mendel’s Experiments – Trihybrid Cross:
Crosses between parents that differ in three contrasting characters, i.e., combinations of three monohybrid crosses operating together are regarded as the trihybrid crosses.
When a cross is made between a seed parent having a tall vine and yellow, round CDDGGWW) with a pollen parent with a dwarf vine and green, wrinkled seed (DDGGWW), the first generation (F1) of the cross is represented as follows:
Parents: DDGGWW X ddggww
Gametes: DGW X dgw
F1 Generation: DdGgWw
(Tall vine and yellow, round seed)
When a F1 plant is crossed with a full recessive type (DdGgWw X ddggww) eight varieties of gametes (DGW, DGw, DgW, Dgw, dGW, dGw, dgW, dgw) will be produced by the F1 parent while one kind of gamete (dgw) will be produced by the full recessive parent.
The results have been summarised in the Table—Genetics-1.
When the Ft plants are self-crossed (DdGgWw X DdGgWw) eight kinds of gametes are produced by both the parent plants.
The gametes are:
DGW, DGw, DgW, Dgw, dGW, dGw, dgW, dgw. If the F1 x F2 cross is represented by a checker-board 64 squares will be required. The results of such trihybrid cross are summarised in Table Genetics-2. The phenotypic ratio is 27:9:9:9:3:3:3:1.
7. Physical Basis of Heredity According to Mendel:
Mendel’s discovery failed to impress scientists at the time when it was published. For nearly thirty five years his findings remained unknown. The biologists of those ages, as a result of tremendous impact of Darwin’s publication of On the Origin of Species by means of Natural Selection, were much interested in problems relating to differences between the species and not on differences within the species.
Moreover, in those days quantitative methods were unheard of in most areas of Biology. Obviously, a paper on plant hybridisation crowded with numerical ratios and letter symbols were brushed aside as irrelevant.
But re-discovery of the Mendelism became an imperative necessity. Interest on problems of evolution was tremendous. Serious doubts have been cast upon the validity of Lamarckian idea (accepted by Darwin) that variations in an organism are the resultant outcome of impacts between organism and environment.
Attention was focussed on variations which are not traceable to environmental differences. Particular interest upon discontinuous variation or more precisely on the contrasting unit character studied by Mendel reached its zenith. And once this was undertaken, the rediscovery of Mendelian work became inevitable.
In essence, Mendel’s laws are descriptions of what happen when various types of crosses are made. The next question that follows is how or why it happens. The answer to this question is to be found in the behaviour of chromosomes.
Chromosomes are thread-like structures found in the nuclei of cells. These are the threads of life and form the basis for segregation and other phenomena of heredity. Unfortunately, Mendel knew nothing of the chromosomes and even their existence as the intimate knowledge of cell-structure had to await certain critical technical improvements which were accomplished in the beginning of twentieth century.
The chromosomes:
During cell division, chromatin masses present inside the nucleus become hinged together and form definite bodies or structures called chromosomes. Chromosomes are universally present in the nuclei of living organisms.
The number of chromosomes present in a given species is constant and they are present in like pairs excepting the sex-chromosomes. In number, the chromosomes vary in different species. There is a single pair of chromosomes in Ascaris univalens.
In the fruit-fly, Drosophila, there are four pairs while in man there are 23 pairs. In many organisms, the number of chromosomes are many and are counted with considerable difficulty. The number of chromosomes of a species is never an index to the complexity or degree of differentiation of the species concerned.
Species which are apparently related often differ widely with respect to their number of chromosomes. The chromosomes are capable of maintaining their individuality. This individuality remains unaffected in different physiological conditions such as growth, metabolism and reaction to stimulus.
The genes, located in the chromosomes, constitute the physical basis upon which heredity hinges. (For Architecture, behaviour of chromosomes during meiosis see any standard book of Cytology).
Salivary gland chromosomes:
Most amazing of all chromosomes are the salivary gland chromosomes found in the salivary glands and other larval tissues of two-winged flies (Order Diptera). The chromosomes were first noticed in 1881 and have become an important tool to cytologists and cytogeneticists in the last twenty years.
The salivary gland chromosomes are of great size. They are 70 to 110 times as long as the chromosomes in some oogonial cell. The largest chromosome may reach a length of half of a millimeter. The chromosomes generally lie well separated in the nucleus.
The chromosomes are provided with conspicuous transverse bands. These bands can be recognised even in living and unstained nucleus. 2000 such bands may be counted in a long chromosome. The bands form a pattern that is constant for a given chromosome.
The salivary gland chromosomes furnish valuable data regarding the structure and composition of chromosomes. It has formed a sort of biological spectrum indicating the organism’s genetical constitution (Fig. 2.6).
Lampbrush chromosomes:
During oogenesis in some animals like shark, amphibia, bird, etc., the diplotene chromosomes become large and are comparable to salivary gland chromosomes of Drosophila. In amphibia,’ the pachytene stage may prolong for two years and the chromosomes during this long period exhibit peculiar change.
The paired chromatids lie side by side and show series of swellings along their length. Sometimes the chromosomes form loops and give off filamentous bodies. The exact nature of this peculiar behaviour of chromosome is still inadequately known.
Chromosome number in some animals and plants are given in Table Genetics-3:
The gene hypothesis:
The ultimate but hypothetical hereditary unit is the gene. Castle has defined a gene as the smallest part of chromosome capable of varying by itself.’ The genes are domiciled on the chromosome in a linear order. They are the beads on the chromosome necklace.
Genes are ultra-microscopic. Still today no optical instrument has been able to bring them within the range of human sense organs. They are made up of complex molecules. The molecules of different genes vary in chemical composition.
Genes are present in duplicate pairs, one member of which comes from the father while the other from the mother. They are present in all the chromosomes of every cell from zygote onwards. Each chromosome houses many genes.
Each gene is located in the same particular chromosome and at a definite region of the chromosome. The specific arrangement of genes on chromosome is never thrown at random. Genes are like chemical catalysts and never die in action.
Contrary to atoms and molecules, the genes grow by assimilation, reproduce their own kind and mutate or change by loss, addition and rearrangement. The genes, however, possess a high degree of stability.
A gene cannot survive without partner. Genes are not considered today as ‘unit determiners’ for single trait, in the Mendelian way. They act in conjunction with other genes upon organism as a whole.
True nature of genes is still to be known completely. The genes when remain together in a hybrid do not blend, contaminate or interact with each other. On the other hand, the different genes segregate, separate pure and uncontaminated and transmit to the different gametes formed by the hybrid and go to different individuals in the off-springs of the hybrid.
Genes are symbolised in letters. Dominant genes are represented by capital letters (as ‘T’ in tallness) and recessive genes are represented by small letters (as ‘t’ in dwarfness). This is a convenient method to follow up the transmission and distribution of genes in cross experiments. Thus T and t are allelic genes or alleles.
An individual arises from the union of two gametes as in Mendel’s experiments. It receives a gene responsible for tallness and a gene responsible for dwarfness. The true breeding tall plant is represented thus by TT and similarly the true dwarf plant is represented by tt.
When the two plants are crossed one T bearing gamete, be it male or female, is fertilised by one t bearing gamete. Thus the resultant zygote gets both T and t and its genetic formula is written as Tt.
Theory of the genes:
T. H. Morgan in his “theory of the genes” has summarised the following:
(a) Characters of an individual are referable to paired genes in the germinal material. They are held together in a definite number of linkage group.
(b) During maturation of gametes each pair of genes separates according to Mendel’s first law and as a result each gamete receives only one set.
(c) Genes in different linkage group assort independently according to Mendel’s second law.
(d) An orderly exchange of genes occurs between homologous chromosomes by the process of crossing over.
(e) The frequency of crossing over furnishes evidences for the linear arrangement of genes in each linkage group and their relative position to one another.
8. Ploidy:
The chromosomes are basically present in diploid (2n) condition in the cells. The number become reduced to half or haploid (n) during gamete formation. The basic number of chromosomes may be changed in both plants and animals.
This alteration in the basic number of chromosome may be found in the following forms:
Polyploid:
When the basic number of chromosomes get multiplied, viz., Triploid (3n) Tetraploid (4n), the condition is called polyploidy. When polyploidy arises directly through the increase of lower number— it is called autopolyploidy. When the increase in chromosome number is due to crossing with other species it is called amphidiploidy.
Monoploid:
The haploid chromosomal number (n) which is the lowest chromosome number or the primitive number in a group of animals or plants is called the monoploid or haploid. This basic number is retained in many plants like corn and insects like bees.
Aneaploid:
Instead of showing alteration in the complete set of chromosomes, some-time individuals occur with a single chromosome as extra or wanting. Thus a trisomic individual possesses the normal chromosomal complements plus one extra. A monosomic has one chromosome missing. Such individuals with an incomplete set of chromosomes are called aneuploids.
Blakeslee and Belling (1924) were the pioneer scientists to record the existence of aneuploidy in a common plant, Datura stramonium. This plant has 12 pairs of chromosomes in the somatic cells. They discovered a ‘mutant type’ having 25 chromosomes rather than 24. At the meta- phase of first meiotic division, one of the 12 pairs was observed to have an extra chromosome.
In this plant one trisome was present along with 11 disomes. The trisomic plant differs from other plant especially in shape and spine characteristics of the capsule. The chromosome complement is illustrated as 2n+l. Aneuploidy in Drosophila with trisomic X chromosomc has been illustrated by Bridges.
When white-eyed female fruit flies are crossed with red-eyed males, it is expected that only red-eyed daughters and white-eyed sons will be produced. Bridges observed a few red-eyed sons and a few white-eyed daughters.
This is because of the failure of the X chromosomes to disjoin or separate in reduction division. This nondisjunction results in the production of some eggs having two X chromosomes and others with only one.