These purple-flowered plants are not just pretty to look at. Plants like these led to a huge leap forward in biology. The plants are common garden pea plants, and they were studied in the mids by an Austrian monk named Gregor Mendel. With his careful experiments, Mendel uncovered the secrets of heredity, or how parents pass characteristics to their offspring. You may not care much about heredity in pea plants, but you probably care about your own heredity.
People have long known that the characteristics of living things are similar in parents and their offspring. He did well in school and became a monk. He also went to the University of Vienna, where he studied science and math. His professors encouraged him to learn science through experimentation and to use math to make sense of his results.
Mendel is best known for his experiments with the pea plant Pisum sativum see Figure below. Gregor Mendel. The Austrian monk Gregor Mendel experimented with pea plants. He did all of his research in the garden of the monastery where he lived. This is the theory that offspring have a blend, or mix, of the characteristics of their parents.
For example, a tall plant and a short plant had offspring that were either tall or short but not medium in height. He found that all of the first-generation F1 hybrids looked like 1 of the parent plants. For example, all the progeny of a purple and white flower cross were purple not pink, as blending would have predicted. However, when he allowed the hybrid plants to self-pollinate, the hidden traits would reappear in the second-generation F2 hybrid plants.
Mendel described each of the trait variants as dominant or recessive Dominant traits, like purple flower colour, appeared in the F1 hybrids, whereas recessive traits, like white flower colour, did not. Mendel did thousands of cross-breeding experiments. His key finding was that there were 3 times as many dominant as recessive traits in F2 pea plants ratio. Mendel also experimented to see what would happen if plants with 2 or more pure-bred traits were cross-bred.
He found that each trait was inherited independently of the other and produced its own ratio. This is the principle of independent assortment. In addition, Mendel confirmed that, other than flower color, the pea plants were physically identical.
This was an important check to make sure that the two varieties of pea plants only differed with respect to one trait, flower color.
Once these validations were complete, Mendel applied the pollen from a plant with violet flowers to the stigma of a plant with white flowers. After gathering and sowing the seeds that resulted from this cross, Mendel found that percent of the F 1 hybrid generation had violet flowers. Conventional wisdom at that time would have predicted the hybrid flowers to be pale violet or for hybrid plants to have equal numbers of white and violet flowers.
In other words, the contrasting parental traits were expected to blend in the offspring. Importantly, Mendel did not stop his experimentation there. He allowed the F 1 plants to self-fertilize and found that plants in the F 2 generation had violet flowers and had white flowers. This was a ratio of 3. When Mendel transferred pollen from a plant with violet flowers to the stigma of a plant with white flowers and vice versa, he obtained approximately the same ratio irrespective of which parent—male or female—contributed which trait.
This is called a reciprocal cross —a paired cross in which the respective traits of the male and female in one cross become the respective traits of the female and male in the other cross. For the other six characteristics that Mendel examined, the F 1 and F 2 generations behaved in the same way that they behaved for flower color.
One of the two traits would disappear completely from the F 1 generation, only to reappear in the F 2 generation at a ratio of roughly Figure 8. Upon compiling his results for many thousands of plants, Mendel concluded that the characteristics could be divided into expressed and latent traits.
He called these dominant and recessive traits , respectively. Dominant traits are those that are inherited unchanged in a hybridization. From Eichling [ 25 ] and from a later seed-order we know that Mendel was in contact with Benary in Erfurt, Germany [ 26 ]. A subset of 15 of these, where the status of several Mendelian characters can be determined, is described in Additional file 1 : Table S1.
Among the 15 varieties for which we could obtain the most information, there are different parental combinations that could be used in crossing. Of these, 21 would segregate for a single Mendelian character.
Crosses for which 2, 3, 4 or 5 characters would segregate in the F2 can be found, and for three crosses, if it is assumed that pod colour is green where this is not specified, 6 characters would segregate. With his background in meteorology and physics and his appreciation for numbers, Mendel may have noticed that green or wrinkled seeds were typically ca. Where both segregate a ca. As Mendel had studied combinatorial theory under Ettinghausen in Vienna [ 33 ] this may have piqued his interest.
The 2 years of trialling that Mendel undertook with 34 varieties [ 11 , 3 ] would have provided information about purity of seed lots, similarity of varieties, and the stability both phenotypic and genotypic of the character states of these varieties. This would undoubtedly have assisted in his identifying the 22 varieties that he chose to cross for his experiments and the 7 characters he chose to follow in their offspring. The genes le and v are genetically linked, but le and p lie on different linkage groups see [ 23 , 38 ].
He also mentioned that this segregant bred true in the following generation presumably and that it became a popular variety cultivated in the monastery [ 25 , 5 ]. Furthermore, if this involved le and v rather than p then the linkage between le and v recombination fraction ca. In the four factor cross discussed by Eichling [ 25 ] the linkage was in repulsion, so the occurrence of double homozygous recessive types in the F2 would be rare see Additional file 2 , p5.
If this was in fact what happened, Mendel would have had difficulty in noticing that they behaved differently from the others he studied, i. If such an F2 population was used to generate an F3 in which both le and v were scored, then many but not all of the heterozygotes identified for each character would be the same F2 individual, consistent with them having similar segregation ratios as seen in Fig. Frequency distribution of segregation ratios.
This frequency corresponds to the likelihood, L m , y-axis with which that number occurs as determined by the binomial distribution; these values were obtained in Excel using the function BINOM. The values that Mendel obtained are ringed and the gene involved is indicated, see Ellis et al. For the differentiation between AA and Aa genotypes in the F2, it is necessary to examine the F3 progeny of each individual plant; on average 1 in 4 of F3 progeny of Aa genotypes will be aa homozygous recessive but none occur in the selfed progeny of AA genotypes.
If there are very few F3 seed it is possible that, by chance alone, no recessive types will appear in the F3, so for small F3 families some Aa genotypes may therefore be misclassified as AA. Analysis of the segregation of seed characters R vs r and I vs i is relatively easily done, because the F3 seed occurs in the pods on the F2 plants.
Therefore we can estimate that among these plants the frequency of misclassifying Aa genotypes as AA is low ca. In this situation it is reasonable to consider that the genotypic segregation ratio should approximate to the expected 1: 2 ratio AA : Aa. The seed characters discussed above can be scored using the F2 seeds that are in the pods of the F1 plant.
In order to score other characters plant height, pod colour etc. Thus, because of misclassification, Fisher expected the AA : Aa ratio should be 1: 1. Experimental design. The gene symbols are as described in Ellis et al. It may just have been luck; or it may be that the worthy German [sic] abbot, in his ignorance of probable error, unconsciously placed doubtful plants on the side which favoured his hypothesis' qtd.
Fisher later changed his mind and attributed these results to the work of an assistant. It should be noted that Mendel could not possibly obtain a result that was exactly either of these ratios because the ratio he obtained must consist of two integers that sum to Given this constraint, we can derive the expected frequency of each possible ratio and see how the distribution of segregation ratios Mendel obtained compares to their expected frequencies.
This relationship is plotted in Fig. We should note that the segregation ratios for le and v or p are very similar as would be expected if they derived from the same heterozygotes in the same seed lot.
In fact these two ratios are more alike than are the repeat trials with pod colour gp , consistent with their being the same actual plants rather than different samples from the same F2 see above. Mendel provided F2 data for these experiments, from which, assuming random mating of the gametes, the allelic ratios in these particular seed batches can be estimated, and this allows a comparison between the F2 and estimated F3 ratios Additional file 2.
From these data we can conclude that the F2 and F3 ratios are within the expected range of one another, but that for le and v or p the deviation is greatest, and again these two characters behave in a very similar way, consistent with the underlying genes being linked. As in the bifactorial cross, Mendel would have had an abundance of F3 seed on his mature F2 plants for the determination of seed characters, and Edwards [ 9 , 10 ] appropriately gives an expectation of a 1: 2 ratio for these characters.
Mendel did not state how many F3 plants were cultivated in this experiment; it could be that he used 10 F3 plants, as in the experiments discussed in the previous section, but he could have grown fewer or more.
The convenience of using leaf axil colour would have been obvious, so Mendel could have examined many more than 10 F3 segregants without needing to cultivate all the plants to maturity. The genes involved in the trifactorial experiment were R vs r , I vs i and A vs a , and Mendel reported the segregation of A vs a within each of the nine seed character classes Table 1 , Additional file 2 : Table S2.
Four classes have more Aa plants than a ratio predicts; the abundance of the aa class in the F2 of this batch of seed suggests the frequency of the a allele is 0. There are two points to consider about these ratios. The first is whether or not to assume that Mendel used exactly 10 F3 individuals selfed from each F2 and the second is whether to assume that the A and a alleles had an exactly equal ratio, or whether to estimate their ratio from the frequency of the aa class in the F2.
The simplest interpretation is that this reflects the experiment which Mendel performed. Either way the evidence is against the interpretation that the results were adjusted to fit to the 1: 2 ratio because, in this particular seed lot, they are closer to the estimate of the actual ratio rather than the theoretical 1: 2 ratio. One can applaud the lucky gambler; but when he is lucky again to-morrow, and the next day, and the following day, one is entitled to become a little suspicious.
In the 2nd experiment 32 yellow and only 1 green seed, but also 20 yellow and 19 green ones. For each of the first two experiments Mendel gave the number of seeds with dominant and recessive phenotypes for i the experiment as a whole, ii for the first ten plants as an example of variation and iii for two extremes of segregation, one where the number of dominant and recessive seeds was nearly equal and another where the number of recessive seeds was very low.
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