Page 36 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

28 Chapter 2 Mendel’s Principles of Heredity

Figure 2.19 The science of genetics begins with the rediscovery of Mendel. Working independently near the beginning of
the twentieth century, Correns, de Vries, and von Tschermak each came to the same conclusions as those Mendel summarized in his laws.
(a, c): © SPL/Science Source; (b): © INTERFOTO/Alamy; (d): © ullstein bild/Getty Images

(a) Gregor Mendel (b) Carl Correns (c) Hugo de Vries (d) Erich von Tschermak

chance variation observed in a sample of individuals within Mendel’s work might have had an important influence
a population is compatible with a genetic hypothesis. on early debates about evolution if it had been more widely
appreciated. Charles Darwin (1809–1882), who was unfamil-
Mendel’s Genius Was Unappreciated iar with Mendel’s work, was plagued in his later years by
Before 1900 criticism that his explanations for the persistence of variation
in organisms were insufficient. Darwin considered such vari-
Mendel’s insights into the workings of heredity were a ation a cornerstone of his theory of evolution, maintaining
breakthrough of monumental proportions. By counting and that natural selection would favor particular variants in a
analyzing data from hundreds of pea plant crosses, he in- given population in a given environment. If the selected com-
ferred the existence of genes—independent units that de- binations of variant traits were passed on to subsequent gen-
termine the observable patterns of inheritance for particular erations, this transmission of variation would propel evolution.
traits. His work explained the reappearance of hidden traits, He could not, however, say how that transmission might oc-
disproved the idea of blended inheritance, and showed that cur. Had Darwin been aware of Mendel’s ideas, he might not
mother and father make an equal genetic contribution to the have been backed into such an uncomfortable corner.
next generation. The model of heredity that he formulated For 34 years, Mendel’s laws lay dormant—untested,
was so specific that he could test predictions based on it by unconfirmed, and unapplied. Then in 1900, 16 years after
observation and experiment. Mendel’s death, Carl Correns, Hugo de Vries, and Erich
With the exception of Abbot Napp, none of Mendel’s von Tschermak independently rediscovered and acknowl-
contemporaries appreciated the importance of his research. edged his work (Fig. 2.19). The scientific community had
Mendel did not teach at a prestigious university and was not finally caught up with Mendel. Within a decade, investiga-
well known outside Brünn. Even in Brünn, members of the tors had coined many of the modern terms we have been
Natural Science Society were disappointed when he pre- using: phenotype, genotype, homozygote, heterozygote,
sented Experiments on Plant Hybrids to them. They wanted gene, and genetics, the label given to the twentieth-century
to view and discuss intriguing mutants and lovely flowers, science of heredity. Mendel’s paper provided the new dis-
so they did not appreciate his numerical analyses. Mendel, cipline’s foundation. His principles and analytic techniques
it seems, was far ahead of his time. Sadly, despite written endure today, guiding geneticists and evolutionary biolo-
requests from Mendel that others try to replicate his studies, gists in their studies of genetic variation.
no one repeated his experiments. Several citations of his
paper between 1866 and 1900 referred to his expertise as a
plant breeder but made no mention of his laws. Moreover, at The Influence of Molecules on Phenotype
the time Mendel presented his work, no one had yet seen the Determines Whether Alleles are
structures within cells, the chromosomes, that actually carry Dominant or Recessive
the genes. That would happen only in the next few decades
(as described in Chapter 4). If scientists had been able to see We now know that genes specify the proteins (and RNAs)
these structures, they might have more readily accepted that cells produce and that dictate cellular structure and
Mendel’s ideas, because the chromosomes are actual physi- function. Recently, two genes were identified that are likely
cal structures that behave exactly as Mendel predicted. to correspond to Mendel’s genes for seed shape and seed