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In this article we will discuss about:- 1. Introduction to Gregor Johann Mendel 2. Mendel’s Experimental Material & Chosen Characters 3. Monohybrid Cross 4. Di-Hybrid Cross 5. Tri-hybrid and Poly-Hybrid Cross 6. Chromosomal Basis.
Contents:
- Introduction to Gregor Johann Mendel
- Mendel’s Experimental Material & Chosen Characters
- Mendel’s Assumption on Monohybrid Cross
- Mendel’s Assumption on Di-Hybrid Cross
- Mendel’s Assumption on Tri-Hybrid and Poly-Hybrid Cross
- Chromosomal Basis of Mendel’s Laws
1. Introduction to Gregor Johann Mendel:
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Gregor Johann Mendel (Fig. 6.1), known as father of Genetics was born in a farmer family near Brunn in Austria in 1822. He graduated in Philosophy in 1840 and became a priest in St. Augustinian Monastery in 1847. Later he went to University of Vienna for studying natural science.
After return, he was engaged in school teaching. He started his experiment with garden pea, and in 1865, presented a paper entitled “Experiments in plant hybridization” before the Natural History Society of Brunn. He died in 1884.
The implication of his work, which forms the basis of genetics, was realized in 1900 when Derives in Holland, Correns in Germany and Tschermak in Austria, working independently, obtained similar findings.
2. Mendel’s Experimental Material & Chosen Characters:
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Mendel took garden pea (Pisum sativum) as his experimental material due to certain suitable reasons:
1. Sexually reproducing;
2. Flowers bisexual;
3. Convenience in handling;
4. Existence of detectable variations;
5. Self-fertilizing;
6. Short life cycle (annual);
7. Large number of offspring’s;
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8. Controlled mating;
9. True-breeding lines available;
10. Fertile hybrids are produced.
Mendel selected seven pairs of characters for his experiments (Fig. 6.2):
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1. Seed shape – smooth and wrinkled;
2. Seed colour – yellow and green;
3. Flower colour – violet-red and white;
4. Pod shape – inflated and constricted;
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5. Pod colour – green and yellow;
6. Flower position – axial and terminal;
7. Stem height – tall and dwarf.
3. Mendel’s Assumption on Monohybrid Cross:
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A cross between two parents differing in one trait/character or in which only one trait is considered is called monohybrid cross. Mendel raised separately two varieties of garden peas, tall and dwarf. When the flowers of the tall variety were allowed to be fertilized with their own pollen, the offspring’s were all tall; the dwarf variety on self-fertilization produced only dwarfs.
He crossed these two varieties of garden peas. From the cross between the tall and dwarf parental (P) generation plants, the offspring’s in the first generation (F1-First filial generation, Latin word filial meaning progeny) were all tall.
There was no dwarf plant in the F1 generation. When these F1 tall plants were fertilized by their own pollen (selfed), the offspring’s of second generation (F2) were both tall and dwarf. About three-fourths of the plants were tall and one- fourth were dwarfs.
This showed him that the character of dwarfness which disappeared in F1, reappeared in F2. Mendel planted the F2 seeds to raise F3 progeny. About one-third of the tall F2 plants produced only tall progeny, whereas two- third produced both tall and dwarf plants. The dwarf F2 plants produced all dwarfs. Mendel carried out monohybrid experiments with other chosen characters and got the similar results.
Mendel assumed that:
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1. Soil and moisture conditions might have an effect on growth of the plants, but heredity was the main limiting factor under the conditions of his experiments.
2. Since the results from reciprocal crosses were identical (♀Tall x ♂ Dwarf = ♀ Dwarf x ♂Tall), both male and female parents make equal contribution to the development of characters in the progeny.
3. Each character (phenotype) of an organism is controlled by a specific factor (presently known as gene); each factor has two alternative forms called alleles or allelomorphs.
4. Of the two alleles for a trait, one is dominant and the other is recessive. The parental character which is expressed in F1 is the dominant character controlled by dominant allele and the character of the other parent, which is not expressed, is referred to as recessive, controlled by recessive allele.
5. Each somatic cell of the organism has two doses of each factor (genotype), either similar alleles (homozygous, pure) or dissimilar alleles (heterozygous, hybrid). The organism gets these factors from its parents, one from each.
6. Two different alleles for a trait do not mix or modify during their stay together. Each of these factors transmitted to the progeny as a discrete, unchanged unit through gametes. Gametes contain only one dose of each factor.
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7. The two alleles of a character separate from each other and transmitted to two different gametes. A random union between the male and female gametes occurs.
Explanation of Monohybrid Cross:
On the basis of above assumptions, Mendel explained the result of monohybrid cross. The tall and dwarf plants of P generation were both pure breeding and genotypically homozygous-TT and tt respectively. The gametes produced by the tall parent carry only T allele and dwarf parent carry only t allele.
Therefore, after fertilization, the zygote must have the genotype Tt and F1 plant will be phenotypically tall because of dominance of T allele. As the t allele is recessive, expression of dwarf character will not occur.
When the F, tall (Tt) plants were selfed, separation of the alleles T and t occurred during the formation of gametes. Half of the gametes will carry T allele and half t allele in both male and female organs. Two types of male gametes are free to unite with two types of female gametes. Therefore, both tall and dwarf phenotypes will appear-in F2.
As the male gamete and female gamete, both with t allele, unite to produce the genotype tt, the reappearance of dwarf plant will occur in F2 generation. Thus the F2 plants produced will be of three types of genotypes-TT,. Tt and tt in the ratio 1:2:1. Both TT and Tt plants will be tall and tt plants will be dwarf in the ratio 3:1 (Fig. 6.3). On selfing of F2 plants – TT tall plants will breed true, Tt tall plants will segregate in the ratio 3:1 and tt plants will also breed true.
Mendel’s Conclusion: Law of Segregation:
Mendel formulated his first law, the law of segregation, from the conclusion drawn out of his monohybrid experiments.
The law of Segregation States:
The alleles for each character existing in pairs in an organism do never blend, they segregate from each other and pass into different gametes in their original form. Thus each gamete contains only one allele for each character. A F1 mono- hybrid will thus produce two different types of gametes in equal frequencies. The law of segregation is thus also called as law of purity of gametes.
4. Mendel’s Assumption on Di-Hybrid Cross:
A cross between two parents differing in two traits or in which only two traits are considered called di-hybrid cross. Mendel raised separately two pure varieties of garden peas, one with yellow cotyledon, round seed and another with green cotyledon, wrinkled seed. From the cross between these two parental (P) generation plants, the offspring’s in the F1 generation were all with yellow cotyledon and round seed.
When these F1 plants were self-fertilized, the offspring’s of F2 generation were of four types in the ratio 9:3:3:1 –
(a) Yellow cotyledon, round seed
(b) Yellow cotyledon, wrinkled seed
(c) Green cotyledon, round seed and
(d) Green cotyledon, wrinkled seed.
The offspring’s showed that two pairs of contrasting characters combined in every possible way.
Mendel carried out di-hybrid experiments with all the chosen characters in different combinations and got the similar results.
Explanation of Di-hybrid Cross: Mendel explained the di-hybrid cross as follows:
1. As the parental plants were pure, so their genotypes will be homozygous – YYRR and yyrr producing YR and yr gametes respectively.
2. The F1 di-hybrid will be heterozygous for both the traits (YyRr).
3. As all the F1 plants were with yellow cotyledon and round seed, so allele Y for yellow cotyledon is dominant over allele y for green cotyledon and allele R for round seed is dominant over allele r for wrinkled seed.
4. The appearance of all the four possible phenotypic combinations in F1 in the ratio 9:3: 3 :1 is possible if the two pairs of characters are believed to behave independent of each other. Each pair of contrasting characters bear no permanent association with particular other character.
5. If the F1 plant (YyRr) produces only parental gametes (YR, yr), then in F2 only two types of phenotypes (parental) are expected. But the appearance of four types of phenotypes in F2 (two parental and two new types) confirms the production of four types of gametes (YR, Yr, yR, yr) in equal frequency.
The appearance of two new types of phenotypic combinations – yellow cotyledon, wrinkled seed and green cotyledon, round seed in addition to parental phenotypic combinations requires the production of Yr and yR gametes in addition to YR, yr gametes by F2 plants.
6. Thus the allele Y may be associated with the allele R as well as r in equal frequency, giving rise to YR and Yr gametes respectively. Similarly, the allele y may be associated with the allele R as well as r in equal frequency giving rise to yR and yr gametes respectively. Thus four types of gametes viz.’, YR, Yr, yR and yr will be produced in the ratio 1 : 1 : 1 : 1.
7. These four types of gametes (both male and female) will unite in sixteen possible combinations to produce nine types of genotypes in the ratio 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1 and four types of phenotypes in the ratio 9:3: 3 : 1 (Fig. 6.4).
8. The similar ratios will result even if the characters are present in different parental combinations: yellow cotyledon, wrinkled seed X green cotyledon, round seed. This further proves that the inheritance of individual character is independent of the other characteristics.
Mendel was fortunate in selecting his experimental material. It is self-fertilizing species but fertile hybrids can be produced and all the seven characters chosen by him showed independent assortment without any linkage.
Mendel’s Conclusion:
Law of Independent Assortment:
Mendel formulated his second law from the conclusions drawn out of his di-hybrid experiments.
The law of Independent Assortment states:
When the two parents differ from each other in two or more pairs of contrasting characters or factors, then the assortment of alleles of one character is independent of assortment of alleles of other characters. Each member of an allelic pair may combine randomly with either of another pair during the formation of gametes.
5. Mendel’s Assumption on Tri-hybrid and Poly-Hybrid Cross:
In tri-hybrid cross, three pairs of characters are involved, such as round and wrinkled seed; yellow and green cotyledon; gray-brown and white seed coat. The F1 hybrid presents with three dominant and three recessive genes and thus will be heterozygous.
The gametes will be eight different types both on female and male sides and the progeny would show 64 (8 x 8) combinations, in the phenotypic ratio of 27: 9: 9: 3: 9: 3: 3: 1 (Fig. 6.5).
A cross between two organisms differing in more than three pairs of contrasting characters is called poly-hybrid cross. In case of genes increasing beyond three, the numbers of possible phenotypes and genotypes show exponential increase. In such cases, the rules of probability are to be applied.
Probability implies the likelihood of the occurrence of event. The probability of simultaneous occurrence of two or more independent events is summation of the probability of their occurrence as independent events. The types of gametes of F1 and kinds of genotypes, phenotypes in F2 and their ratios may be predicted in poly-hybrid cross according to the Table 6.1.
Back Cross & Test Cross:
Crossing of F1 organism with either of the parents is called back cross (Fig. 6.6). When an organism is crossed with other organism having recessive phenotypic trait (recessive homozygous genotype) is called test cross. This is called test cross because it helps to test the genotype of an organism. In monohybrid cross, tall pea plant of F2 may be homozygous (TT) or heterozygous (Tt). Test cross results confirm it (Fig. 6.7).
In monohybrid test cross, the ratio is 1:1. In di-hybrid test cross, the expected ratio is (1 : 1) (1 : 1) = 1 : 1 : 1 : 1 (Fig. 6.8).
Terms Related to Mendelian Genetics:
Alleles: Each trait of an organism is controlled by a specific factor (presently known as gene); each factor has two alternative forms called alleles. T and t are the alleles for height of pea plant.
Genotype & Phenotype:
Allelic constitution for a particular trait or character is called genotype; expressed character (outward physical manifestation) is called phenotype; e.g., TT is the genotype for the phenotype tall of pea plant.
Dominant Allele & Recessive Allele:
Of the two alleles of a trait in a hybrid, which expresses its phenotype is called dominant allele and whose phenotypic expression is suppressed called recessive allele. In tall hybrid (Tt) pea plant T allele is dominant and t allele, is recessive.
Homozygous & Heterozygous:
When the two alleles for a trait are of one type is called homozygous genotype; when the two alleles are of different kinds is called heterozygous genotype; e.g., TT or tt are homozygous genotypes and Tt is a heterozygous genotype for height of pea plant.
Pure & Hybrid:
When an organism breeds true (on selfing the phenotype remains unchanged) is called pure; but when the organism on selfing produces new phenotype in addition to parental phenotype is called hybrid; e.g., in pea plant tall with homozygous TT genotype is pure while tall with heterozygous Tt genotype is hybrid.
Monohybrid & Di-hybrid Cross:
A cross between two parents differing in one trait/ character or in which only one trait is considered, is called monohybrid cross, e.g., Tall (TT) pea plant when crossed with dwarf (tt) plant. A cross between two parents differing in two traits/ characters or in which only two traits are considered is called di-hybrid cross; e.g., yellow round (YYRR) pea plant when crossed with green wrinkled (yyrr) plant.
Selfing & Crossing:
Fertilization within a plant or the cross between the same genotypes is called selfing, e.g. Tall (Tt) x Tall (Tt). When the fertilization occurs between the plants differing in one or more trait(s) or the cross between the different genotypes is called crossing, e.g.. Tall (TT) X Dwarf (tt).
Punnett Square:
It is the probability diagram illustrating the possible offspring of a mating.
Step 1:
Definition of alleles and determination of dominance.
Step 2:
Determination of alleles present in all different types of gametes.
Step 3:
Construction of the square.
Step 4:
Recombination of alleles into each small square.
Step 5:
Determination of genotype and phenotype ratios in the next generation.
Step 6:
Labelling of generations; P, F1, F2, etc.
NOTATIONS
Parental Generations (P1 and P2)
First Filial Generation F1= P1x P2
Second Filial Generation F2 = F1 x F1
Back Cross one, BC1 = F1 x P1
Back Cross two, BC2 = F1 x P2
6. Chromosomal Basis of Mendel’s Laws:
Sutton and Boveri (1902-1904) formulated Chromosome Theory of Mendelian inheritance in which they showed clearly that, the chromosomes exhibit a behaviour during meiosis and fertilization which is exactly parallel to the behaviour of Mendelian factors in segregation and recombination (Table 6.2 and Fig. 6.9).
In view of the existence of a complete parallelism between the behaviour of Mendelian factors and the behaviour of chromosomes in cell division, it is confirmed that Mendelian factors are located on chromosomes and chromosomes are the bearer of hereditary factors (Figs. 6.10, 6.11).
