Gregor Johann Mendel (1822-1884) known as the “Father of Genetics”, was an Austrian monk. In 1856, he published his work titled ‘Experiments on Plant Hybrids’ in a journal and formulated the laws of inheritance. Mendel carefully carried out certain sets of hybridization experiments using different qualitative traits of pea plants. His study was based on statistical observation from various monohybrid and dihybrid crosses. He discovered that traits are inherited in the form of discrete “factors”. A diploid organism carries two versions of the same factor.
The experimental subject chosen by Mendel was Pisum sativum, the pea plant. It offered various advantages −
Easy cultivation and short life cycle
The plant produces many seeds
Self-fertilization is pre-dominant in pea
Easy hybridization
Well-defined distinct characters
The pea plant produces bisexual flowers
Mendel chose to work with seven qualitative characteristics. All the characteristics had two variants known as alleles.
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Between the years 1856-1863, Mendel conducted rigorous experiments with peas during which he crossed different varieties of pea plants with each other. He statistically analyzed the data he’d collected to understand the patterns of heredity.
It is a cross between two individuals that bear contrasting traits for a particular character.
Cross of a pure-breeding plant that produces yellow seeds and those that produce green seeds produced the first generation (Filial 1, F1 generation) which expressed only one of the two characters- i.e., yellow seeds.
Self-fertilization of the F1 progeny yielded F2 generation. The F2 generation produced both kinds of plants- those with yellow seeds and those with green seeds, in the phenotypic ratio 3 (yellow): 1 (green)
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The pattern of inheritance repeated for the other traits are stem height, flower position, etc. Only one trait from either of the parents would be expressed in the F1 progenies, and the other traits would resurface in a small proportion of the F2 progeny.
Mendel proposed that the F1 generation inherited discrete “factors”, one from each parent, but expressed only the dominant factor of the two. He termed the expressed character as the dominant trait (represented by a capital letter), while the other trait which didn’t show up in F1 was termed recessive (represented by a small letter).
Mendel took his quest a step further to determine the pattern of inheritance of two characters simultaneously. A dihybrid cross involves the breeding of individuals with two contrasting characters.
Mendel crossed a true-breeding pea plant with yellow and round seeds (RRYY) and a pea plant that produces wrinkled and green seeds (rryy)
Since yellow and round are dominant characters, The F1 progeny produced round and yellow seeds, with the genotype RrYy
Selfing of F1 produced interesting results- The F2 progeny showed new combinations apart from the two parental characters - round and yellow, wrinkled and yellow, wrinkled and green, and round and green.
This meant that the alleles for one character (seed shape) separated into gametes INDEPENDENTLY from the alleles of the other character (seed color) in the F1 generation.
Upon self-fertilization of the F1 gametes, 16 different genotypes were produced, corresponding to a phenotypic ratio of 9:3:3:1
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Through his experiments with pea plants, Mendel made some important conclusionssome specific patterns of inheritance, which he formulated into the following three laws known as Mendel’s Principles (or laws) of Inheritance.
From monohybrid crosses, Mendel proved that characteristics in offspring are not a blend of those of their parents. Instead, only one trait from either parent will be expressed phenotypically in the offspring.
This led Mendel to hypothesize that a sexually reproducing diploid organism carries two copies/variants (alleles) of a particular character. These copies may be the same (homozygous) or maybe contrasting (heterozygous).
Based on the observations, he put forward the following law of dominance −
In a heterozygous organism, the expression of one allele encoding a particular trait is always repressed by the presence of the other allele encoding the contrasting, dominant trait.
Based on monohybrid crosses wherein the true-breeding parents gave rise to pea plants that expressed only the dominant trait, and the F2 progeny that produced both parental traits, Mendel put forth the following law −
The two alleles determining a particular characteristic in a diploid organism are segregated when gametes are formed, with one allele going into each gamete.
From dihybrid crosses, Mendel observed that new combinations of traits different from the parental phenotypes were also produced. This led him to formulate the law of independent assortment −
The alleles of one character at a locus segregate into gametes independently of the alleles of the other character at the other locus.
Mendel’s factors, which are passed down from each parent to the offspring are now known as genes, which are the basic unit of heredity.
His idea that a diploid organism carries two copies of a factor has now translated into alleles, i.e.., variant forms of a particular gene.
Chromosomes were not known during Mendel’s time, yet his laws predicted their presence.
Mendelian genetics has made significant contributions in studying the inheritance of genetic diseases, studying family histories via pedigree analyses, etc.
The laws of inheritance are not applicable in the cases of linked genes, non-allelic gene interactions, sex-linked traits, and under the cases of codominance or partial dominance.
Mendel performed a series of breeding experiments with pea plants, to understand the inheritance of characters in offspring. Based on his experiments, he formulated three laws −
The law of dominance
The law of segregation, and
The law of independent assortment.
These laws explain the dominance of characters, the segregation of alleles into individual gametes, and the assortment of alleles encoding one character into gametes independently of the other characters. The significance of these laws is profound, however, Mendelian genetics does not apply to all situations.
Q1. How many varieties was Mendel able to create from his experiments?
Ans. By performing various combinations of crosses among the varieties of pea plants, he was able to produce 22 different varieties.
Q2. What are the different zygosities observed in diploid organisms?
Ans. A diploid organism may be homozygous (same alleles) or heterozygous (contrasting alleles, i.e., Aa). A homozygote may carry homozygous recessive (i.e., aa) or homozygous dominant (i.e., AA) alleles for a trait.
Q3. What are Codominant alleles?
Ans. Alleles that does not show dominant or recessive relation, and are expressed to the same degree are codominant allele. Example allele of M-N blood group in humans.
Q4. How does the inheritance of a gene vary with respect to the dominance of alleles?
Ans. The dominance of alleles does not have any influence on the inheritance of genes whatsoever. It only determines the phenotype of an individual.
Q5. What is the concept of incomplete dominance?
Ans. Incomplete dominance occurs when the heterozygote exhibits an intermediate phenotype that does not resemble either of the parents fully.
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