Pioneers of Mendelian Inheritance in Animals (PMIA)
1905
Davenport, C.B.
The origin of black sheep in the flock.
Science
22: 674-5
This paper is the first to report Mendelian inheritance of black sheep.
Davenport starts this paper with the statement “The phrase 'Every flock has its black sheep' connotes the sporadic nature of their appearance. They crop out in flocks of breeding ewes and rams that are wholly white”. In a letter to Nature in 1880, Charles Darwin had also discussed the same “occasional appearance” of black or partly-black sheep in otherwise white flocks. Now, just a few years after the rediscovery of Mendelism, Davenport follows the above two opening sentences with a Mendelian hypothesis: “When a quality suddenly arises from parents that have its opposite the probability is that the two opposed qualities follow Mendel's law in inheritance and that the new, filial character is recessive, the parental opposite dominant.” He then tests this hypothesis with wool-colour and pedigree data of 877 sheep in the flock of Dr Alexander Graham Bell (1904). The results of four types of matings (described in terms of the Mendelian hypothesis) were:
|
Mating |
White offspring |
Black offspring |
|
recessive homozygote (black) x recessive homozygote (black) |
0 |
18 |
|
recessive homozygote (black) x heterozygote (white) |
26 |
25 |
|
heterozygote (white with mixed ancestry) x heterozygote (white with mixed ancestry) |
40 |
7 |
|
recessive homozygote x dominant homozygote (white with no known mixed ancestry) |
15 |
0 |
Davenport was concerned that the proportion of black in the third type of mating was too low to be consistent with the Mendelian expectation of 25%. However, had he been aware of Pearson’s chi-squared test of independence (published only a few years earlier, in 1900), he would have been consoled to know that when the observed ratio of 40:7 is tested against the Mendelian expectation of 35.25:11.75, the resultant P value is 0.11, indicating consistency with the Mendelian hypothesis of 25% black.
Davenport’s justifiable final sentence was “The conclusion of the whole matter is that black wool color in sheep behaves like a Mendelian recessive characteristic.”
For further information on this trait, see OMIA 000201-9940: Coat colour, agouti in Ovis aries
References
Bell, A.G. (1904). Sheep Catalogue of Beinn Bhreagh, Victoria Co., Nova Scotia; showing the Origin of the Multinippled Sheep of Beinn Breagh and giving all the Descendants down to 1903, pp. 22. Washington. View this document
Darwin, C.R. (1880) Black sheep. Nature 23: 193. View this paper
Pearson, K. (1900). On the criterion that a given system of deviations from the probable in the case of a correlated system of variables is such that it can be reasonably supposed to have arisen from random sampling. Philosophical Magazine, 50: 157–175. View this paper