Mendel, Gregor Johann

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Mendel, Gregor Johann

Donald R Forsdyke, Queen's University, Kingston, Ontario, Canada

Published online: April 2018

DOI: 10.1002/9780470015902.a0002544.pub2

Full Article on Wiley Online Library

Abstract
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Abstract

Gregor Mendel (1822–1884) was a Moravian biologist from whose quantitative plant breeding studies were derived laws of inheritance that founded a new branch of biology, genetics. He crossed varieties of peas and examined the distribution of characters among offspring over successive generations. Hybrids resulting from crossing two varieties with contrasting characters did not breed true. They were not the ‘constant hybrids’ much sought by professional breeders that might lead to independent species. In the 1880s Mendel's work was appreciated by Wilhelm Focke and George Romanes, who initiated similar studies with animals. Studies with plants were initiated in the 1890s by European botanists who discovered Mendel's work in 1900. A relationship of hereditary units that determined characters – now known as genes – to chromosomes was noted by cytologist Michael Guyer in 1900. The major early advocate of Mendel's work, zoologist William Bateson, agreed with Mendel that new species could emerge discontinuously.

Key Concepts

For his experiments it was essential that Mendel chose characters in pea plants that would breed true.
He found that hybrids between lines which themselves bred true, did not breed true. They were not ‘constant hybrids’.
At that time many thought that constant hybrids would indicate a continuous species origination process.
From ‘brother‐sister’ matings of pea hybrids, Mendel derived quantitative laws.
His units that determined characters – now known as genes – were located to chromosomes by Michael Guyer.
Without knowledge of Mendel's laws, European botanists rediscovered them in plants.
William Bateson confirmed that Mendel's laws applied to various animal and plant species.
Mendel's laws were challenged by mathematical biologists (biometricians).
Mendel followed the statistics of his time and his results have withstood the test of time.
Mendel's view that new species can arise discontinuously without the involvement of natural selection has gained support.

Keywords: heredity; constant hybrids; Mendel's laws; genetics; chromosomes; speciation; Carl von Nägeli; George Romanes; Michael Guyer; William Bateson

	Figure 1. Mendel, Gregor Johann. Courtesy of the National Library of Medicine, USA.
	Figure 2. Pure lines breed true. Mendel sought pure lines (varieties) of peas. The units (elements) that represent contrasting pea characters are here symbolised by black or white circles. Members of pure lines bred true when crossed among themselves (1: black pollen with black ova and 2: white pollen with white ova). On crossing, any pairs from the first generation (F1 hybrids) should produce a second generation (F2 hybrids), all members of which would retain the parental character. The black line never produced a white. The white line never produced a black. Mendel was then prepared to study a cross between the lines (3).
	Figure 3. Theoretical outcomes of crosses between different lines (symbolised as black or white). Mendel sought pure lines that when crossed displayed nonblending inheritance among child plants. Only one of the alternative parental characters would be displayed in a hybrid offspring (1–3). However, professional breeders were more interested in blending inheritance (here symbolised as grey; 4). In this case new characters such as a better flower colour in plants, or better wool in sheep, might emerge. When all hybrids displayed the black character (1), then black was deemed ‘dominant’ and white ‘recessive’. Conversely, when all hybrids displayed the white character (2), then white was dominant and black recessive. If a mixture resulted (3), there was neither dominance nor recessiveness.
	Figure 4. Mendel's discovery of the 3:1 ratio when one character is dominant. In the F1 generation all hybrids appear identical (1). This equals the first case in Figure, with black symbolising the dominant character. The crossing of any pairs selected randomly from the F1 generation produced an F2 generation with a minority of recessives (white) in the ratio (black:white = 3:1). Here, the three black forms appear equal. With blending inheritance (2), which Mendel did not study, there would be visible variation among the three nonwhite forms (explained in Figure).
	Figure 5. Mendel's recognition that his chosen parental characters (here symbolised as black or white) existed in dual forms in each parent. These forms were separated when pollen and ova were formed (gametogenesis). In the F1 generation all hybrids are equal (black and white are both represented), but as black is dominant, they all appear black. The crossing of any pairs from the F1 generation will result, statistically, in an F2 generation with, for each one white, one pure black and two with black and white, both represented but appearing black. Applying the same interpretation with the blending model shown in Figure, these two would appear grey and, if crossed with each other, would not breed true.
	Figure 6. Statue of Mendel as erected in 1910 (a) and as it is today (b). Courtesy of the Mendelianum, Moravian Museum, Brno.

Article Title:	Mendel, Gregor Johann
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