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"Congenital Defects [in Dexters]"

by Lawrence Alderson

from Alderson's The Chance to Survive: Rare Breeds in a Changing World, published by Cameron & Tayleur, London, 1978. This extract, pages 124-127, is from Chapter 5 "Commercial Consideration".

Lawrence Alderson has long been active in the UK Rare Breeds Survival Trust. He is an international consultant on livestock breeding and production and on genetic conservation. Born on a Pennine farm, he went on to gain degrees in agriculture and education at Cambridge. He has assisted the creation of rare breeds conservation groups in many countries and is a Trustee and Founder President of  Rare Breeds International. He is author of  "The Observer's Book of Farm Animals"(1976) and as well as a number of other books and articles. "The Chance to Survive: Rare Breeds in a Changing World" (1978) is considered the standard work on genetic conservation. Alderson owns the Dynevor herd of White Park cattle which can be traced back to the ninth century.

Page 124:

[In a section sub-headed "Congenital Defects", Alderson discusses how the commercial value of some farm livestock breeds has suffered from the occurrence of detrimental genes. He initially outlines problems with scrapie, a disease of the nervous system.]

It is now relatively easy to identify a fault in a

Page 125:

minority breed, but the small size of the population and the limited number of bloodlines make drastic and immediate elimination of the defect a threat to the breed's existence, and more moderate methods must be employed to eradicate it.

This is the case with the dwarf defect in Dexter cattle, which is encouraged by the favour recently bestowed on small animals in the show-ring. The dwarf factor is frequently given as an example of a partially dominant achondroplastic gene, which causes shortening of the bones and is lethal when both genes are "dwarf" (i.e. the genetic make-up is homozygous), but in the heterozygous state produces a dwarf animal. Young conceived with the lethal condition are known as "bulldog" calves, which are grossly deformed. It has been claimed that 25 per cent of Dexter calves are "bulldogs", and are thus aborted, about 50 per cent are heterozygous dwarfs (i.e. Dexter), and about 25 per cent are longer-legged animals which are free from the defect, but unacceptable as typical Dexters. This simplified account is no longer acceptable, and my observations have shown that animals within the breed cannot be divided into these distinct categories, but rather that they are found in a full gradation of type from extreme examples, exhibiting pronounced dwarf abnormalities, to animals which,

Page 126:

although still small, are true miniatures of pleasing proportions without any unnatural features. The extreme dwarf type is only at one remove from the "bulldog"; it may be identified by its short body and high tail-head, heavy head and forequarters, turned out elbows and twisted fetlock joints, and a tendency to arthritis and dislocated stifle, as well as difficulties in conceiving and calving. Breeders who avoid cows with these characteristics are more likely to eliminate the "bulldog" problem and a few of the leading breeders have now adopted a policy which will achieve this objective. However, progress will be slow because the dwarf characteristic is widespread in all bloodlines of the breed.

Size variations: Two Dexter bulls with their owner (Pages 126 and 127, Alderson, 1978).

The majority of genetic defects [in livestock] are less severe in their effect than the "bulldog" gene, but may be equally difficult to eradicate in a minority breed.

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