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Original
Article About Dexters
"The
Significance of an Achondroplasia-Like Condition Met With in Cattle"
Part
II
by F.A.E. Crew
in Proceedings of the Royal Society of London,
volume 95, 1923, pages 228-255.
Page 230 (Cont'd)
II -The Genetic Constitution of the Modern Dexter
A statistical study of the results of the mating of Dexter with Dexter shows
that four classes of calves are produced - Black Dexter-type, Red
Dexter-type, Black Kerry-type and Red Kerry-type. By Dexter-type is meant a
relatively massive head, a stout body and short limbs; by Kerry-type is
meant a relatively slender head, a slim body and long limbs. Of the four
classes, the Black Dexter-type is by far the most common, the Red Kerry-type
the least common, while the other two occur in more or less equal numbers.
The appearance of four phenotypes in such proportions suggests at once that
the Dexter is, itself, to be regarded as a Mendelian di-hybrid, and that its
parental stocks differed one from the other at least in respect of two pairs
of contrasted characters, coat colour and bodily conformation. The
contrasted characters concerned were black and red coat colours, and
Dexter-type and Kerry-type bodily conformations. The old type Kerry, black,
slender and long-limbed could have been one of, the parental stocks. If this
were the case, then the other must have been a red, stout, short-limbed
stock.
Wilson brings forward considerable evidence to show that such an animal was
imported into Kerry during the formative period of the Dexter, and that it
was crossed with the native Kerry stock. He quotes Wakesfield as saying
that: “In the South I met with some persons who had imported Devon cattle;
Lords Bantry, Shannon and Doneraile, Mr. Hyde and others possess
Page 231
considerable numbers of them.” Wilson also points out that in the
seventeenth and eighteenth centuries many English emigrants, sailing from
Bristol Channel ports, settled in Kerry and West Cork, bringing with them
their own red cattle.
Wilson sums up the evidence thus: “The probability, therefore, that Dexter
cattle are descended from Black Kerries and Red cattle of Devon type is very
high; and if further proof were wanted, it can be found by setting a Red
Dexter cow side by side with a Red Devon.” The modern Red Dexter is a
mottled red, showing delicate blackish traceries on the red ground; this
same mottling is seen in the modern Red Devon.
It is indeed probable that the Dexter had its origin in the mating of Black
Kerry and Red Devon. It would seem to be a simple matter to put this
suggestion to the test of direct experiment by mating a Kerry and a Devon.
But the modern Kerry is not the Kerry of the time when the Dexter was
produced, nor is the modern Devon genetically the same as the Devon taken to
Ireland by the English emigrants.
Supposing, as stated, that the original parents, Kerry and Devon
respectively, introduced the dominant factors B and S to denote black coat
colour and short-legged bodily conformation, it is easy to see that the
result of backcrossing to either type of parent would give 50 per cent Black
Dexter type, while inter-breeding between the heterozygous Black Dexters
would give Black Dexter offspring in the proportion 9/16. These Dexters
would be of four types according as they were homozygous for both dominant
factors, heterozygous for both, or homozygous for one and heterozygous for
the other. As soon as the picture of the ideal Dexter took shape in the
breeders’ mind, the production of a pure strain homozygous for both factors
followed automatically by the elimination of the Red Kerry and Red Dexter
off-types.
On to Part III
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