Father of Genetics
Gregor Johann Mendle
Gregor Johann Mendle was born in 1822. He was a priest by profession, but an amateur scientist by genius. |
| Gregor Johann Mendle |
Due to his unrewarding services to the genetics, he is called as "father of genetics". Mendel started his work on garden pea Pisum sativum in 1854, in his monastery garden. He performed a series of experiments and complied his data numerically.
Why mendle choosed pea for his experiments
Short growing season
Pea produces a large number of fertile seeds within a relatively short growing season. As the time gap between generations is short, many generations can be raised easily within a short time.Pea is hermaphrodite
Pea is a hermaphrodite plant as it has both male and female organs on the same flower. Owing to hermaphrodite, it normally self fertilizes because its stamens and carpels are completely enclosed by two petals, which fuse together to form a keel around them. |
| pea (flower) |
Contrasting pairs
Cross' fertilization of pea Pea has many sharply distinct traits. Each trait has two clear cut alternative forms or varieties. Mendel called these varieties 'antagonistic pairs' or contrasting pairs. He settled on seven such pairs for his study. |
| seven contrasting characters |
Mendle's experiment#1
He first isolated and perpetuated lines of pea that bred true.He self fertilizes with such pure breeding lines and produced offsprings like the parents. . After establishing 14 true breeding lines or varieties of seven characters, he did hybridization or cross fertilization. He worked with patience. First he did hybridization for just one character at time. He cross-fertilized plants differing in one contrasting pair only.The offspring of such a cross are called monohybrids. Then he made monohybrid cross between monohybrids by self fertilization.
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| Transmission of Menle's factors |
He worked with large number of plants to produce fairly large number of offspring. He counted the progeny and did statistical analysis, which gave him clear ratios. He compiled his results and then formulated law of segregation.
Then he studied the inheritance of two traits at a time and made dihybrids.Then he crossed dihybrids by self fertilization.He counted the progeny of dihybrids and did analysis of his results. He got clear-cut ratio even now and thus,formulated the law of independent assortment.
Seven Contrasting Pairs of Traits:
Mendel chosed seven contrasting pairs of traits of pea plant for his work. Each trait had two sharply distinct varieties. These seven contrasting pairs are given below: Seed shape may be round or wrinkled. Inflated or pinched pod Axial or terminal flower Tall or short stem . Seed colour may be yellow or green. Flower height may be tall or short. Flower colour may be white or purple. Pod colour may be green or yellow.Mendel made a cross fertilization between a true breeding 'tall' male plant and a true breeding 'short' female plant. He called this first parental generation (Pi).By crossing the above plants he obtained a generation and called it first filial generation (F1). Filial means progeny.
All the offspring of F1 were tall like one of the parents. Parents could not appear. Tall dominated short completely because no offspring was intermediate in size. There was no mixing or blending.
Dominent trait
"The trait which appeared in F1 generation is called dominent trait"Recessive trait
"The trait which was hidden in F1 generation is called recessive trait"Reciprocal Cross:
"The cross which is reciprocal to the normal cross in which we change the sex of a trait is called a reciprocal cross." In old days people believed in Aristotle's misconception that only the male parent contributed most to an offspring's inherited characters. The female parent just served to nourish. In order to test it, Mendel made a reciprocal cross by reserving sexes of Pi. The result was again the same. Again all Fi monohybrids were tall.Monohybrid Cross:
"A cross between two monohybrids organisms is called as a monohybrid cross." Mendel's Monohybrid Cross: Mendel self-fertilized Fi tall plants to raise F2 generation. A clear cut ratio of 3 : 1 was found. It happened for all the seven contrasting pairs of traits. From above cross, Mendel concluded that both parents have equal contribution to the genetic makeup of the offspring.Capability of expression in F1 is a trait's own characteristic. The reappearance of short in F2 indicated that this phenotype though not expressed in F1 did exist in some masked form. The dominant trait might have temporarily masked its expression, but it could not change or modify its determining nature.F1 tall was actually very different from Pi tall because self-fertilization of Pi tall produced 100% tall offspring but self fertilization of Fx tall gave 75% tall and 25% short offspring.
Fi tall was not true breeding rather it was monohybrid. Mendel got the same results and similar 3 : 1 ratio in offspring of monohybrid crosses for all, the seven contrasting pairs of traits. Mendel also raised F3 generation by self-fertilization of F2. F1 x F2 He found all of F2 tall produced only tall, while all of F2 tall produced both tall and short in 3 : 1 ratio. But F2 short produced only short.
Mendel's Interpretation:
Mendel interpreted his results intelligently. He imagined that contrasting form of a trait, tall or short was determined by heredity factors. Each individual had a pair of these factors. One factor of this pair had come in him from his male parent while the other from his female parent. Both of these factors together controlled expression of trait in him.Mendel assigned symbols to these factors. He designated dominated factor with capital latter and recessive with small latter, e.g. 'T' for tall and Y for short Mendel's factors For Mendel his factors were discrete physical units of inheritance. Johanson coined the term 'gene' and people started calling them genes. Today for us these factors are parts of DNA, the base sequences that carry the biological information to determine a trait. Mendel's factors are alleles of genes.
Test Cross
"Test cross is a mating in which an individual showing a dominant phenotype is crossed with an individual showing its recessive phenotype."Importance of a Test Cross
This is very important to note that recessive parent is always homozygous (tt). Mendel used this cross to find out genotype of an individual who had dominant phenotype. This test also determines whether the dominant parent is homozygous or heterozygous. Examples: Case I: .If an individual is homozygous tall (TT), it will form all gametes with a 'T' allele. Short is always homozygous recessive. It will form all gametes with a 't' allele.
Result:
1/2 Tt = Tall, 1/2 tt = short offspring. The phenotypically dominant heterozygous.
Misfortunately Mendel's work lay neglected for 34 years. In 1900, 16 years after his death, his work was rediscovered and acknowledged by the foundation of classical genetics. Mendel was a genius man. He worked differently from other workers of his period. He wisely chose garden pea plant called Pisum sativum for his experiments, because he knew its advantages which are given below.
Law of segregation
: "The two coexisting alleles of an individual for each trait segregate (separate) during gametes formation so that each gamete gets only one of the two alleles.Alleles again unite at random fertilization of gametes." To explain law of segregation, Mendel made a cross between true breeding tall (TT) and dwarf (tt) pea plants and got Fi generation.
Case I
All the plants in Fi were tall but heterozygous (Tt).
All tall offspring. The phenotypically dominant individual is homozygous. Tt
Case II:
If the tall individual is heterozygous (Tt), it will form half the gametes with a T and half with a't' allele. Short will form only't' type of gametes. Fertilization will result into 50% tall and 50% short offspring.
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| Test cross (case 1) |
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| Test cross (case 11) |
This indicated that tall character is dominant to dwarf. Experiment 2: self crossed plants of Fi generation and got 1 : 2 : 1 genotype and 3 : 1 phenotype in F2 generation, which meant the two alleles (T and t) stayed together in Fi generation but at the time of gamete formation in Fx, these alleles untie randomly as genotype All the plants were round yellow seeded due to dominance. As Fi contains two trail together it was called dihybrid.
Law of Independent Assortment:
Mendel allowed to self fertilize the Fi generation. He got F2 generation with two new recombinations in addition to the parent types. The new recombinations were phenotypically different from parent i.e. round green and wrinkled yellow. He got (9:3:3:1 ratio among F2. Appearance of recombination phenotypes in F2 indicated that some "shuffling" of allele had occurred. Mendel inferred the mechanism of thi shuffling as independent assortment of alleles inti gametes.Alleles for seed colour and seed shape were no fixed or tied in parental combinations i.e. 'R' with 'Y' ant V' with Y; rather they were free to assort independently 'R' could go with 'Y' or y in any gamete with equa probability. Similarly 'r' could go with 'Y' or y in any gamete with equal change.
Four kinds of gametes were producec in equal number with a perfect ratio 1 : 1 : 1 : 1 . Random fertilization of these gametes woulc produce 9 : 3 : 3 : 1 ratio. Monohybrid cross showing genotypic and phenotypic ratio among offspring.
This law states "when two pairs of contrasting traits (genetic character) are brought in the same cross,.-they remain together in offspring of Fi generation but .assort (separate) and reunite in new combination independently."
After thorough study of single trait inheritance, Mendel decided to make a cross between two traits at the same time. He selected two traits, one was "seed colour" and other was "seed shape". The seed colour had two phenotypes, yellow or green. Similarly the other trait, seed shape had two distinct phenotypes, round or wrinkled.
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| LAw of Independent assortment |
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