The intellectual troika that helped understand heredity

Continuous variation can be inferred from Mendelian heredity but heredity cannot be inferred from continuous variation

July 16, 2022 07:20 pm | Updated July 19, 2022 07:07 pm IST

Inherited: Mendel propounded that information on traits gets passed on from one generation to another. 

Inherited: Mendel propounded that information on traits gets passed on from one generation to another.  | Photo Credit: Getty Images

We are celebrating the 200th birth anniversary of Gregor Johann Mendel and Sir Francis Galton this year. Mendel was born on July 20, 1822; Galton on February 16. Both men sought to understand heredity, transmission of characteristics from parents to children. Galton was Charles Darwin’s half-cousin; they shared a common grandparent, Erasmus Darwin.

Galton was a polymath; an explorer, a geographer, a meteorologist, a psychologist and a statistician. And so was Charles Darwin; a naturalist, a geologist, a tireless walker, a brave adventurer and the originator of the theory of evolution by natural selection. Darwin published The Origin of Species in 1859, which as Galton has said “gave me freedom of thought”.

Influence of Darwin

In his book, Darwin had used fancy pigeons to illustrate how natural selection might work. This sparked Galton’s interest in whether humans can be improved through selective breeding. Galton’s bright mind reasoned that if the human stock were to be improved through selection, he would first have to show that desirable human characteristics were inherited.

Galton set out to identify men of accomplishment and determine whether any of their male relatives were also similarly accomplished. He generated evidence in support and concluded that talent and character were probably inherited. He published his findings in 1865 in an article titled ‘Hereditary Talent and Character’ in Macmillan’s Magazine, a popular Victorian periodical. 

Galton then collected and analysed family-trees of well-known statesmen, judges, poets, painters, musicians, military commanders, etc. His basic tenet was that if talent and character were inherited, the closest male relatives of a distinguished man (say, a son or father) would be more likely to be talented than those farther removed (e.g., an uncle or a nephew). He generated affirmative evidence using mental and intellectual qualities as well as physical ones such as height and eye colour.

He published his findings in the book, Hereditary Genius in 1869. The word ‘genius’ was used to denote high level of talent or ability. Galton propounded the ‘Law of Ancestral Heredity.’ He suggested that hereditary qualities were embedded in the reproductive organs and the germ plasm, which were passed on from one generation to the next.

It is noteworthy that when Galton was actively working on inheritance of characters in humans, Mendel was also studying the same problem in gardens of St. Thomas Monastery in Brünn (now Brno) using pea plants. Mendel presented his results to the Society for the Study of the Natural Sciences in Brünn in the same year (1865) that Galton published his findings in Macmillan’s Magazine.

Mendel propounded that information on “traits” gets passed on from one generation to another as particulate ‘elements’ and traits in the present generation can be traced back to past generations. 

Mendel died in 1884 and his laws lay buried until they were rediscovered in 1900. Galton and his supporters — notably Karl Pearson and W.F.R. Weldon — criticised Mendel because his ‘laws’ ignored ancestors; the genetic compositions of only the parents but not of other ancestors, matter in the determination of the characteristics of a person.

Galtonian ‘law’ stated that other ancestors also matter. Pearson claimed that observed correlations of characteristics between various types of relatives were higher than those expected under Mendel’s laws.

Mendelian inheritance

Indeed, it is difficult to conceive that continuous characteristic such as height can be controlled by transmissible particulate elements. It was the genius of Ronald A. Fisher — the father of modern statistical science — that created a synthesis showing, in 1918, that inheritance of continuous variation can be explained by Mendelian particulate inheritance.

Fisher’s role in resurrecting Mendelism is legendary. But Fisher also drew attention to some aspects of Mendel’s data. In 1911, Fisher, in a talk in Cambridge, noted, “It is interesting that Mendel’s original results all fall within the limits of probable error; if his experiments were repeated, the odds against getting such good results is about 16 to one.”

Fisher later carried out more extensive statistical analysis of Mendel’s published data and wrote in 1936 that “the data have evidently been sophisticated systematically”.

My own belief is that Mendel was able to deduce his laws from data of an initial set of experiments. In the later experiments, such observations that were wide of the mark from expectations may have been discarded. The goodness-of-fit of observations to expectations is therefore exceptional. Fisher also said that “… after examining various possibilities, I have no doubt that Mendel was deceived by a gardening assistant, who knew too well what his principal expected from each trial made”. 

We will never know how the data may have been doctored (‘sophisticated’ in Fisher’s terminology) because after Mendel’s death, the boxes of data-notebooks and other apparently worthless papers were destroyed by his successor abbot.  

(Partha P. Majumder is National Science Chair and Distinguished Professor and Founder, National Institute of Biomedical Genomics, West Bengal.)

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