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The below mentioned article will highlight you about the Mendel’s four postulates and laws of inheritance.
The Mendel’s four postulates and laws of inheritance are: (1) Principles of Paired Factors (2) Principle of Dominance(3) Law of Segregation or Law of Purity of Gametes (Mendel’s First Law of Inheritance) and (4) Law of Independent Assortment (Mendel’s Second Law of Inheritance).
Mendel laid the foundation of the science of genetics through the discovery of basic principles of hereditary. He conducted his experiments with garden pea (Pisum sativum) for over seven years (1856-1864) and advocated four postulates, including two important laws of inheritance.
Postulate-I. Principles of Paired Factors:
A character is represented in an organism (diploid) by at least two factors. The two factors lie on the two homologous chromosomes at the same locus. They may represent the same (homologous, e.g., TT in case of pure tall pea plants) or alternate expression (heterozygous, e.g., Tt in case of hybrid tall pea plants) of the same character. Factors representing the alternate or same form of a character are called alleles or allelomorphs.
Postulate II. Principle of Dominance:
“When two homozygous individuals with one or more sets of contrasting characters are crossed, the characters which appear in the hybrids of F1 generation are always the dominant characters and those do not appear in F1 offspring’s are always the recessive characters”.
During the course of investigations of the principles of inheritance, Mendel crossed plants of a variety of Pisum sativum six feet tall with plants of a variety one foot in height on an average, (i.e., parents or P generation). When the seeds from this cross were planted they produced plants not intermediate between the two parents, as might be expected, but all tall, like the six-foot parent (Fig. 5.1).
Mendel made crosses to study the inheritance of six other sets of characters (given below) and observed that in every case the hybrid resembled one of the parents with respect to the character. It follows then that one factor or gene in a pair masks or inhibits the expression of the other. Thus, in the cross described, the tall factor masks, or inhibits the expression of the dwarf factor in the F1 (first filial generation); therefore, the tall factor is called the dominant factor, and the dwarf factor is referred to as the recessive factor, or gene.
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Other six sets of characters that Mendel studied and classified as dominant and recessive were as follows:
(1) Round form of seeds dominant over wrinkled.
(2) Yellow colour of cotyledons dominant over green.
(3) Axillary position of flower dominant over terminal position.
(4) Green colour of unripe pod dominant over yellow.
(5) Inflated condition of ripe pod dominant over constricted.
(6) Purple colour of flower dominant over white.
Postulate III. Law of Segregation or Law of Purity of Gametes (Mendel’s First Law of Inheritance):
The two factors (alleles) of a trait which remain together in an individual do not get mixed up but keep their identity distinct, separate at the time of gametogenesis (i.e., gametes formation) or sporogenesis (i.e., spores formation), get randomly distributed to different gametes and then get paired again in different offspring’s as per the principle of probability. Since two alleles remain together in pure form without mixing, affecting or blending each other, the law of segregation is also known as “law of purity of gametes”.
Main features of this law are as follows:
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1. 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.
2. They separate into different gametes in equal number. Each gamete has only one type of allele (say either A or a).
3. Separation of two alleles of a gene during gamete formation takes place usually due to the separation of homologous chromosomes during meiosis (anaphase I), because alleles are located on the chromosomes.
4. With complete dominance, segregation leads to phenotypic ratio of 3: 1 in F2 generation for characters governed by a single gene, and 9: 3: 3: 1 ratio for characters controlled by two genes.
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5. 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:
The principle of the law of segregation can be explained by means of a monohybrid cross.
Analysis of Monohybrid Cross:
A cross in which only a single pair of alleles is considered is called a monohybrid cross. Figure 5.2 is a graphic analysis of the cross between tall and dwarf peas in terms of Mendel’s interpretation.
In this, T is the symbol which stands for the factor or gene controlling tallness, and t is the symbol used to denote the factor or gene controlling dwarfness. The factors or genes as also postulated by Mendel, always occur in pairs. Both tall and the dwarf plants which are crossed are homozygous (i.e., both the genes in a pair are identical). These plants are “pure” for tallness and dwarfness respectively, and if self pollinated will always breed true, producing only tall and dwarf plants respectively.
In the present monohybrid cross the tall parent, which is homozygous, is shown as TT, and the dwarf parent is shown as tt. During the course of sexual reproduction both kinds of plants produce gametes; these gametes contain but one factor of each pair (i.e., either T or t). The gametes produced by the tall plant contain T gene, while the gametes of dwarf plant possess t gene.
The fusion of a gamete from the tall plant with a gamete from the dwarf plant produces a tall plant in the F1 generation, because the gene for tallness (T) is dominant over that of dwarfness (t). The new plant in the F, generation is shown in the diagram as Tt. It is a heterozygous plant because it possess a pair of homologous chromosomes carrying one allele for tallness and one for dwarfness.
The heterozygous plants produce two kinds of gamete or sex cell, male gametes and female gametes. Half of the male gametes contain T gene and half possess t gene. Similarly half of the female gametes possess T gene and half contain t gene. During the process of fertilization which follows these two kinds of gametes (i.e male and female) unite at random and produce F2 (second filial) generation.
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As a result of these chance combinations, an approximate phenotypic ratio of 3 tall plants to 1 dwarf plant (i.e., 3: 1 ratio) is normally obtained. All plants with TT and Tt genes will be tall, and the plants possessing tt (both recessive) genes will be dwarf.
Further self breeding of these plants shows that the dwarf plants breed true (tt), i.e., produce only dwarf plants. Amongst tall plants, 1/3 breed true, that is, yield only tall plants. The remaining 2/3 of the F2 tall plants or 50% of the total F2 plants behave as hybrid plants and produce both tall and dwarf plants in the ratio 3: 1. Therefore, the F2 phenotypic ratio of 3: 1 is genotypically 1 pure tall: 2 hybrid tall: 1 dwarf (1: 2: 1 ratio is also called Mendel’s Monohybrid Genotypic Ratio).
Postulate IV. Law of Independent Assortment (Mendel’s Second Law of Inheritance):
After being satisfied with monohybrid crosses, Mendel took into consideration two pairs of contrasting characters and studied their inheritance (i.e., di-hybrid cross).
According to this law “the two factors (genes) of each contrasting character (trait) assort or separate independently of the factors of other characters at the time of gamete formation and get randomly rearranged in the offspring”.
Following are the main features of this law:
1. This law explains simultaneous inheritance of two plant characters.
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2. 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.
3. These gene pairs segregate during gamete formation independently.
4. The alleles of one gene can combine freely with the alleles of another gene. Thus, each allele of a gene has an equal chance to combine with each allele of another gene.
5. Each of the two gene pairs when considered separately, exhibits typical 3: 1 segregation ratio in F2 generation. This is a typical di-hybrid segregation ratio.
6. Random or free assortment of alleles of two genes leads to formation of new gene combinations.
Example:
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The principle or law of independent assortment can be studied by means of di-hybrid cross.
Analysis of Di-hybrid Cross:
In the di-hybrid cross Mendel crossed pure (i.e., homozygous) plants of round seed and yellow cotyledons variety of pea with those having wrinkled seed and green cotyledons. He had already studied these characters and had observed that roundness was dominant over wrinkleless, and yellow colour of the cotyledons was dominant over green colour. As shown in the figure 5.3 one homozygous parent is expressed as RRYY (Round seed and yellow cotyledons) and the other is expressed as rryy (wrinkled seed and green cotyledons).
The former, as expected, will produce gametes with YR genes, and the latter will produce gametes with ry genes. The two kinds of gametes fuse to produce F1 individual with genetic constitution RrYy. Phenotypically these individuals possess round seeds with yellow cotyledons because roundness is dominant over wrinkleless, and yellow colour is dominant over green. F1 individuals are thus heterozygous round and heterozygous yellow.
When Mendel self-fertilized the F1 individuals, in F2 generation he observed plants of four kinds in the following phenotypic frequencies:
Thus the four categories of plants appeared in approximate phenotypic ratio of 9: 3: 3: 1. (Called Mendel’s Di-hybrid phenotypic Ratio) (Fig. 5.3). The most noteworthy feature of this di-hybrid cross that struck Mendel was the appearance of two new categories of plants besides the parental-ones i.e., Round Green, and wrinkled yellow. These two new categories were in fact the re-combinations of the parental characters. This led Mendel to postulate the law of independent assortment.
It can also be proved by studying the individual character of seed colour and seed shape separately:
Seed colour:
Yellow (9 + 3 = 12): Green (3 + 1 = 4) or 3: 1
Seed Shape:
Round (9 + 3 = 12): Wrinkled (3 + 1 = 4) or 3: 1
The result of each character is similar to the monohybrid ratio.
Shortcomings of the Law of Independent Assortment:
The principle or law of independent assortment is applicable to only those factors or genes which occur on different chromosomes. Actually, a chromosome bears hundreds of genes. All the genes or factors present on a chromosome are inherited together except when ‘crossing over’ takes place.
The phenomenon of inheritance of a number of genes or factors together due to their occurrence on the same chromosome is called linkage. Mended himself found that white-flowered pea plants always produced white seeds, while red-flowered plants always yielded grey seeds.
