Dominant versus Recessive: When two pure breeding varieties are crossed, the parental character that expresses itself unchanged in the F1 generation hybrids is dominant: the one that does not appear in Fl but appears in F2 is called recessive. In the above cross three- fourths of the F2 progeny show the dominant character and one-fourth the recessive character.
Alleles: Factors which control contrasting expressions of a character are said to be alleles or allelomorphs of each other. In the above cross the character in consideration is height, and factors T and t which control tallness and dwarfness are alleles of each other.
Homozygous and Heterozygous: These terms were coined by Bateson and Saunders in 1902. Mendel had concluded that each character is controlled by a pair of factors. When both factors are identical such as TT and tt, the individual is said to be homozygous for that character.
When the factors are different (for example Tt), the term heterozygous is used. Mendel used capital letter of the alphabet to denote dominant factors, and small letters for recessive alleles.
Mendel’s factors were later replaced by the term 4gene’ by a Danish botanist Johannsen in 1909. The word genotype refers to the genetic constitution of an individual, whereas phenotype refers to the external appearance or manifestation of a character.
Mendel selfed members of the F2 progeny and found that out of the dominant types, one-third bred true for the dominant character, whereas two-thirds segregated into dominants and recessives in the ratio of 3 : 1. All the recessive plants of F2 generation when selfed bred true for the recessive character. Mendel found similar results in monohybrid crosses with all
the seven pairs of contrasting characters in Pisum sativum. After eight years of detailed investigations on thousands of pea plants, Mendel published his results in a paper entitled “Experiments in Plant Hybridisation” in the Proceedings of the Brunn Natural History Society in 1866. However, his work received no attention for 34 years until three scientists, DeVries in Holland, Correns in Germany and Tschermak in Austria working independently published their findings in 1900 and confirmed Mendel’s results.
The Test Cross: Not satisfied with his work, Mendel himself subjected his results to a test. In the cross between tall and dwarf pea plants, the F1 hybrids were all phenotypically tall but their genotypes were not only TT but also Tt. Consider a heterozygous hybrid plant Tt. When it forms gametes, the factors T and t segregate in the gametes in a 1: 1 ratio. This means
that 50% of the gametes of an F1 heterozygous hybrid carry the factor T and 50% the factor t. Mendel crossed such a hybrid plant (Tt) with a plant of the true breeding, dwarf variety (tt). All the gametes of the homozygous dwarf plant carried the recessive factor t. Every gamete of the recessive parent has 50% chance of combining with a gamete carrying T and 50% chance to
combine with a t gamete from the heterozygous parent. This should result in 50% of progeny showing the tall phenotype and genetic constitution Tt, whereas 50% of the progeny should be phenotypically dwarf with genotype tt as explained diagrammatically below:
P: Tall (TT) x Dwarf (tt)
Gametes: T (50%) t (50%) (all t)
Test Cross: Tall : Dwarf : : 1:1
Progeny : (Tt) : (tt)
Indeed Mendel’s results of this cross agreed with the theoretical expectations thus providing additional experimental proof of the correctness of his interpretations, ouch a cross where an individual is crossed to a double recessive parent to test and verify the individual’s genotype is called a testcross or backcross.
In order to determine genotypes of the F2 progeny, Mendel allowed the F2 plants to self- fertilise and produce a third filial or F3 generation. He found that the homozygous F2 tall plants could produce only tall plants on self-fertilisation. This indicated their genotype to be TT. Similarly the F2 dwarf homozygotes yielded only dwarf plants on selfing; their genotype
was tt. The F2 heterozygotes on self fertilising behaved identical to the F1 hybrids and gave rise to tall and dwarf phenotypes in the ratio 3:1. This proved that their genotype was identical to that of F1 hybrids i.e. Tt.
It is noteworthy that the genotypes of the parents are written as TT and tt instead of single T and t. This is in accordance with Mendel’s hypothesis that each parent has two factors for a character. There is also a cytological explanation. The somatic chromosomes of all plants and animals exist in homologous pairs, one member of each pair coming from the paternal
parent, other from maternal parent. A gene is a section of the chromosomal DNA which has information necessary for determination of a specific genetic trait. Suppose a hypothetical gene A occupies a particular site or locus on a given chromosome. The homologous chromosome contains at the identical locus an alternative gene a which controls the same trait as gene A,
but in such a way as to produce a different phenotype for the same trait. The alternative genes at the same locus A and a are also called alleles. It is an astonishing fact that though Mendel knew nothing about genes, he could predict the existence of factors, which later turned out to be genes. During the reduction division of meiosis (Metaphase I), chromosomes of a pair separate and go to the opposite poles. Consequently genes or alleles segregate from each other and pass into different gametes.