Initial results of Gregor Mendel’s monohybrid experiment on seed form or shape (round- x wrinkled-seeded parents) showed that the cross-fertilized garden peas (F1, his ‘hybrid’) exhibited only one character (he called it dominant) of either parent, that is, the round seededness. He likewise found that the other parental character (wrinkled seeds) is only hidden in the F1 but reappeared in the F2 and in the succeeding generations. He called this character recessive. This observation was also true to the six other sets of characters that he investigated.
He demonstrated that the F2 progeny consisted of two phenotypic types: the dominants and the recessives. The dominants are those which exhibit the dominant character only while the recessives are those which exhibit the recessive character only. They occured in the average proportion of 3:1. Thus, as to seed shape, 3 were round-seeded and 1 was wrinkled-seeded in every four. But he did not stop right there. He proceeded further, and made more discoveries.
He continued growing pea plants using seeds harvested in each generation and properly recorded his observations. Since garden pea is a naturally self-pollinated plant, the next progenies (example F2) are largely selfed progenies of the next preceding generation (i.e., F1).
He evaluated the F3 progeny and found that the recessive plants bred true, that is, the resulting F3 progeny exhibited the same recessive characters. For example, the F2 plants that were propagated from wrinkled seeds also produced wrinkled seeds. Similarly, all F3 plants that were raised from seeds of F2 plants with terminal flowers also produced terminal flowers.
But the dominant F2 plants consisted of two types: the pure dominants and the hybrid dominants. The pure dominants are those in which the genotype is homozygous for the dominant character, like the round-seeded pea with the genotype RR. The characters remained constant in the next generation.
The hybrid dominants are heterozygotes in which the genotype consists of a gene pair in which both contrasting alleles are present, like a round-seeded pea with the genotype Rr where capital R denotes the allele for the dominant character and small r is the allele for the recessive character. Hybrid dominants exhibit the dominant characters but the next generation is a mixed population of dominants and recessives. On the average out of three dominants, Mendel found that two were hybrid dominants and one was pure dominant, equivalent to 2:1 ratio.
As to the hybrid dominant plants (heterozygote), they yielded offspring consisting of a mixed population exhibiting the dominant character (e.g. round seeds) and recessive characters (wrinkled seeds) at 3:1 ratio. This showed that F2 hybrid dominants produce the same phenotypic ratio in the next succeeding generation as the hybrid or F1 plants.
To be clear, however, it is not possible to identify by visual inspection whether a dominant plant in any generation after the F1 is a hybrid dominant (for example, a pea plant with axial flowers, genotype Tt) or a pure dominant (for example, a pea plant with axial flowers, genotype TT). Mendel did otherwise by looking further into the characters of the offspring of each plant in question.
As to position of flowers on the stem, he first selected 100 F2 plants with axial flowers and obtained 10 sample seeds from each. He then planted the seeds and evaluated the resulting offspring. If all ten F3 sample plants have axial flowers, then the F2 plant from which the seeds were obtained was a pure dominant. Otherwise, if the ten sample plants consisted of a mixture of plants with axial flowers and others with terminal flowers, that F2 plant was a hybrid dominant or a heterozygote. This was how he arrived to his 67:33 or 2.03:1 hybrid-to-pure dominant ratio using 100 F2 test plants and 10 samples per test plant.
F2 Phenotype (Dominant Characters)
Number of F2 Plants Evaluated
Numbers of F2 Hybrid Dominants
|F2 Hybrid Dominant to Pure Dominant Ratio (heterozygote to homozygous dominant ratio)|
|1. Round or roundish seeds||565 plants with round seeds||372 hybrids (yielded 3:1 smooth to wrinkled seeds), 193 pure dominants (produced round seeds only)|
|2. Yellow cotyledon||519 plants with yellow cotyledons||353 hybrids (yielded 3:1 yellow to green seeds), 166 pure dominants (produced seeds with yellow cotyledons exclusively)|
|3. Grey-brown seed coat*||100 plants**||64 hybrids (plants had offspring with grey-brown seedcoats and some with white seed coats), 36 pure dominants (all F3 sample plants had grey-brown seed coats)|
|4. Inflated pods||100 plants**||71 hybrids (plants had offspring some of which yielded inflated pods and some with constricted pods), 29 pure dominants ( all F3 sample plants had inflated pods)|
|5. Green pods||100 plants**||60 hybrids (plants had offspring some of which yielded green pods and some yellow, 40 pure dominants (all F3 sample plants had green pods)***|
|6. Axial flowers||100 plants**||67 hybrids (plants had offspring some of which yielded axial flowers and some with terminal flowers, 33 pure dominants (all F3 sample plants had axial flowers)|
|7. Long stem||100 plants**||72 hybrids (plants produced offspring some of which had long stems and some with short stems, 28 pure dominants (all F3 sample plants had long stems)|
*Seeds with gray-brown seed coats formed from violet-red flowers; white seedcoats from white-flowered plants.
**100 F2 plants displaying the dominant character were selected. Ten (10) seeds from each plant were planted for evaluation of the offsprings.
***Mendel repeated this particular experiment because he thought that the ratio of 1.5:1 departed so much from the rest of the results. In place of 60 and 40, 65 and 35 were obtained, equivalent to 1.85:1 ratio. With the new ratio (1.85:1), the computed average ratio for the 7 experiments is 2.1:1 instead of 2.05:1.
Mendel explained that the large deviations in 5 experiments (Experiments 3-7) was due to the small number of 100 test plants. He emphasized that the two experiments which yielded the ratios 1.93:1 and 2.13:1 gave an average ratio that is almost equal to 2:1 and that these experiments were more reliable because of the large numbers of test plants used. But without the correction, the computed average ratio would have been 2.05:1 instead of 2.1:1.
He concluded that of the F2 plants which exhibited the dominant character, two-thirds thereof have the hybrid character, that is, similar to the F1 (heterozygous, for example having the genotype Tt) while one-third has the homozygous dominant character (for example having the genotype TT). This is equivalent to 2:1 ratio.
Therefore, each hybrid dominant (heterozygote, for example with genotype Tt) produces the next generation which consists of homozygous dominants, hybrid dominants, and homozygous recessives. The ratio of pure dominant to hybrid dominant to recessive, or homozygous dominant to heterozygote to homozygous recessive (for example, the genotypic ratio TT:Tt: tt), is 1:2:1. This ratio, as well as 3:1, has found common application in Mendelian genetics.
MENDEL G. 1865. Experiments in Plant Hybridisation. (Translated by the Royal Horticultural Society of London). Retrieved Nov. 2, 2013 from https://ia600409.us.archive.org/15/items/experimentsinpla00mend/experimentsinpla00mend.pdf.
(Ben G. Bareja, Nov. 11, 2013)