Mendel's laws of Inheritance

Some Important definitions

Gene : (Mendel called them factor) In modern sense an inherited factor that determines a biological character of an organism is called gene (functional unit of hereditary material).

Allelomorphs or alleles : Alleles indicates alternative forms of the same gene. e.g., Tall TT and dwarf tt are alternation forms of the same gene etc.

Gene locus : The specific place on a chromosome where a gene is located.

Wild and mutant alleles : An original allele, dominant in expression and wide spread in the population is called wild allele. An allele formed by a mutation in the wild allele, recessive in expression and less common in the population is termed as mutant allele.

Homozygous (Bateson and Saunders, 1902) : Both the genes of a character are identical is said to be homozygous or genetically pure for that character. It gives rise to offspring having the same character on self-breeding e.g., TT (Homozygous dominant) or tt (Homozygous recessive).

Heterozygous (Bateson and Saunders, 1902) : Both the genes of a character are unlike is said to be heterozygous or hybrid. Such organisms do not breed true on self fertilization e.g., Tt.

If we know the number of heterozygous pairs we can predict the following :

Number of types of gametes =ਂⁿ

Number of F₂ phenotype = 2ⁿ

(Where n is the number of heterozygous pairs).

Number of F₂ genotype = 3ⁿ

Genotype : The genotype is the genetic constitution of an organism. TT, Tt and tt are the genotypes of the organism with reference to these particular pairs of alleles.

Phenotype : External feature of organisms, colour and behaviour etc.

Pure line : Generations of homozygous individuals which produce offsprings of only one type i.e., they breed true for their phenotype and genotype.

Monohybrid, dihybrid and polyhybrid : When only one allelic pair is considered in cross breeding, it is called monohybrid cross. Similarly when two allelic pairs are used for crossing, it is called dihybrid cross and more than two allelic pairs in a cross are called polyhybrid cross.

Reciprocal cross : The reciprocal crosses involve two crosses concerning the same characteristics, but with reversed sexes.

Genome : Total set of genes (DNA instructions) in the haploid set of chromosomes and inherited as unit from parents to offspring is called genome.

Gene pool : All the genotypes of all organisms in a population form the gene pool.

F₁ Generation : F₁ or first filial (filus-son, filia-daughter Bateson, 1905) generation is the generation of hybrids produced from a cross between the genetically different individuals called parents.

F₂ Generation (Bateson, 1905) : F₂ or second filial generation is the generation of individuals which arises as a result of inbreeding or interbreeding amongst individuals of F₁ generation.

Punnet square : It is a checker-board used to show the result of a cross between two organisms, it was devised by geneticist, R.C. Punnet (1927). It depicts both genotypes and phenotypes of the progeny.

Back cross : It is cross which is performed between hybrid and one of its parents. In plant breeding, back cross is performed a few times in order to increase the traits of that parent.

Test cross : It is a cross to know whether an individual is homozygous or heterozygous for dominant character. The individual is crossed with recessive parent. The ratio will be 50% dominant and 50% recessive in case of hybrid or heterozygous individual. In case of double heterozygote (e.g., RrYy) crossed with recessive (rryy) the ratio will be 1:1:1:1. Test cross helps to find out genotype of parents.

Self cross/selfing : It is the process of fertilization with pollen or male gametes of the same individual.

Observed Vs expected results : Experimental results confirm to the ones expected through the theory of probability if the size of the sample is small but they tend to approach the latter if the sample size is large.

Hybrid : The organism produced after crossing of two genetically different individuals is called hybrid.

Heredity and variations in sexual and asexual reproduction

Sexual reproduction : Variations are common in animals and plants which reproduce by sexual means. The reason for this is that the sexual reproduction is biparental, involves meiosis and fertilization, and the offspring receives some traits from father and some from mother.

Asexual reproduction : Those organisms which reproduce by asexual means e.g., bacteria, amoeba, euglena, rose etc. The asexual reproduction is monoparental, involves mitosis and the organism produced by it, inherits all the traits of its single parent. With the result, it is almost a carbon copy of the parent and is known as ramet. A group of ramets is called a clone.

Mendelian period

Gregor Johann Mendel first "geneticist", also known as father of genetics was born on July 22, in 1822 in Silisian, a village in Heizendorf (Austria). In 1843, he joined Augustinian monastry at Brunn (then in Austria, now Brno Czechoslovakia). In 1856, Mendel got interested in breeding of Garden pea (Pisum sativum). He selected pure breeding varieties or pure lines of pea. Breeding experiments were performed between 1859 - 1864. The results were read out in two meetings of Natural History Society of Brunn in 1865 and published in 1866 in "Proceedings of Brunn Natural History Society" under the topic "Experiments in Plant Hybridisation". Mendel died in 1884 without getting any recognition during his lifetime.

Rediscovery of Mendel&aposs work : In 1900, Hugo de Vries of Holland, Carl Correns of Germany and Erich von Tshermak of Austria came to the same findings as were got by Mendel. Hugo de Vries found the paper of Mendel and got it reprinted in 𠆏lora’ in 1901. Correns converted two of the generalisations of Mendel into two laws of heredity. These are law of segregation and law of independent assortment.

Reasons for Mendel’s success

Method of working : He maintained the statistical records of all the experiments and analysed them. He selected genetically pure (pure breed line) and purity was tested by self-crossing the progeny for several generations.

Selection of material : Mendel selected garden pea as his experimental material because it has the following advantages.

It was an annual plant. Its short life-cycle made it possible to study several generations within a short period and has perfect bisexual flowers containing both male and female parts. The flowers are predominantly self-pollinating because of self-fertilization, plants are homozygous. It is, therefore, easy to get pure lines for several generations and also easy to cross because pollens from one plant can be introduced to the stigma of another plant by removing anthers (emasculation) and bagging. In addition to that there was one reason more for his success. He studied seven pairs of characters which were present on four different pairs of chromosomes.

Selection of traits : Mendel selected seven pairs of contrasting characters as listed in the table. Luckily all were related as dominant and recessive.

Mendel&aposs experiments

Monohybrid cross : Experiments with garden pea for single pair of contrasting characters.

Mendel crossed pure tall and dwarf plants. The plants belonged to F₁ generation all tall hybrid were self-pollinated. The plants of F₂ generation were both tall and dwarf, in approximate 3:1 ratio phenotypically and 1:2:1 genotypically.

Mendel’s explanation : Mendel explained above results by presuming that Tallness and dwarfness are determined by a pair of contrasting factors or determiners (now these are called genes). A plant is tall because it possesses determiners for tallness (represented by T) and a plant is dwarf because it has determiners for dwarfness (represented by t). These determiners occur in pairs and are received one from either parent. On the basis of this behaviour the tallness is described as dominant character and dwarfness as recessive (law of dominance). The determiners are never contaminated. When gametes are formed, these unit factors segregate so that each gamete gets only one of the two alternative factors. When F₁ hybrids (Tt) are self pollinated the two entities separate out and unite independently producing tall and dwarf plants (law of segregation). Monohybrid test cross ratio is 1:1.

Dihybrid cross (Crosses involving two pairs of contrasting traits).

Later on Mendel conducted experiments to study the segregation and transmission of two pairs of contrasting traits at a time. Mendel found that a cross between round yellow and wrinkled green seeds (P₁) produced only round and yellow seeds in F₁ generation, but in F₂ four types of combinations were observed. These are :

Round yellow 9 Parental combinations

Round green 3 Non-parental combinations

Wrinkled yellow 3 Non-parental combination

Wrinkled green 1 Parental combination

Thus the offsprings of F₂ generation were produced in the ratio of 9 : 3 : 3 : 1 phenotypically and 1 : 2 : 2 : 4 : 1 : 2 : 1 : 2 : 1 genotypically. This ratio is called dihybrid ratio.

Mendel’s explanation : Mendel explained the results by assuming that the round and yellow characters are dominant over wrinkled and green so that all the F­₁ offsprings are round yellow. In F₂-generation since all the four characters were assorted out independent of the others, he said that a pair of alternating or contrasting characters behave independently of the other pair i.e., seed colour is independent of seed coat.

Therefore, at the time of gamete formation genes for round or wrinkled character of seed coat assorted out independently of the yellow or green colour of the seed. As a result four types of gametes with two old and two new combinations i.e., YR, Yr yR, yr are formed from the F₁ hybrid. These four types of gametes on random mating produce four types of offsprings in the ratio of 9:3:3:1 in F₂ generation (Law of Independent Assortment). Dihybrid test cross ratio is 1 : 1: 1 : 1.

Trihybrid cross : The offsprings shows 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1 ratio is found in trihybrid cross. This suggests that a di, tri, or polyhybrid cross is actually a combination of respectively two, three or more monohybrid crosses operating together. Trihybrid test cross ratio is 1 : 1 : 1 : 1 : 1 : 1 : 1 : 1.

Mendel&aposs laws of inheritance

Mendel&aposs laws are still true because these take place in sexually reproducing organisms or parents are of pure breeding. He enunciated two major laws of inheritance i.e., law of segregation and law of independent assortment.

Law of segregation (Purity of gametes) : The law of segregation states that when a pair of contrasting factors or genes or allelomorphs are brought together in a heterozygote (hybrid) the two members of the allelic pair remain together without being contaminated and when gametes are formed from the hybrid, the two separate out from each other and only one enters each gamete as seen in monohybrid and dihybrid cross. That is why the law of segregation is also described as law of purity of gametes.

Law of independent assortment : If the inheritance of more than one pair of characters (two pairs or more) is studied simultaneously, the factors or genes for each pair of characters assort out independently of the other pairs. Mendel formulated this law from the results of a dihybrid cross.