Dex-Info Article Summary: "Genetic and Breeding Research in Dexter Cattle Since 1970"

"Genetic and Breeding Research in Dexter Cattle Since 1970"
by Andrew Sheppy

This Paper is from The World of Dexter Cattle: Proceedings of the First World Congress on Dexter Cattle, 1999, edited by A. Sheppy, published by the Dexter Cattle Society (UK), pages 69-91.

Andrew Sheppy is a lecturer, researcher and author on rare farm livestock breeds, especially poultry but also cattle and sheep. He is the founder of the Cobthorn Trust and of the British Rare Poultry Society. He has a longstanding involvement with the UK Rare Breeds Survival Trust, and has served as its Vice-President. He has a keen interest in Dexters, operating the Cobthorn stud. For the Cobthorn Trust website, see www.datazenith.com/cobthorn.

This Summary generally closely follows Sheppy's phraseology. Any comments in square brackets [ ] below are those of the webmaster, John Paterson.

In the first of four main topics discussed in the Paper, Sheppy looks at the genetics of size. Dexters are the smallest European breed, though some Zebu cattle are smaller. Sheppy points out that size is governed by many pairs of genes. "The different breeds are of course characterised by many different combinations, but it should be the case that Dexters carry fewer genes for increased size than the generality of cattle breeds, and that crosses between Dexters and other breeds should produce intermediate offspring" (page 70). Trials have been developed to test this view. Long-leg Dexter bulls were crossed with Holstein Friesian, Highland and Jersey cows, and long-leg Dexter cows were crossed to a Highland bull. The results were that all 29 offspring were of intermediate size between the the sire and dam. "It can therefore be assumed that the Dexter breed is genotypically small when compared to other breeds" (page 71). Sheppy argued that the large variation in the size of registered Dexters is at least partly attributable to "the large amount of upgrading and introgression now present" (page 72).

The first diagnostic description of the Dexter bulldog calf problem (a non-viable extreme dwarfism) was by Stock (1902) and Seligmann (1904). Sheppy believes that the condition - Classic Dexter Dwarfism - should be called chondrodystrophia rather than the term achondroplasia which had been used almost universally up until then. Many writers believed the gene causing Classic Dexter Dwarfism to be a dominant or incomplete dominant single gene which has a lethal effect in the homozygote [the bulldog calf], which is different from other forms of dwarfism which can be recessive [needing the same two alleles to be present to cause a dwarf]. Other dominant genes which are lethal when homozygous are known in the genetics literature.

There was significant interest in dwarfism in cattle in the 1950s and 1960s due to major problems in Hereford cattle, found to be recessive dwarfisms. "Although it had generally been possible to determine by visual assessment which Dexters were dwarf (often known as short-legs) and which were not (known as long-legs), the continuing confusion with other bovine dwarfisms, and an increasing demand for definite identification of animals capable of producing the bulldog calves underlined the need for research to be directed specifically at this problem. The situation was made worse by deliberate attempts to breed down the longer-legged non dwarf Dexters to a shorter legged conformation, resembling the dwarf type favoured by many breeders" (page 74).

In 1983, research was undertaken on behalf of the UK Dexter Cattle Society to study the availability of various bloodlines by artificial insemination. Part of this involved an analysis of bulls' breeding records entailing 4,273 births (90% being live births, 4% bulldog calves and 6% other dead calves) recorded in the 1970 to 1982 Herdbooks. A breeding index was calculated for every bull since 1970 that had more than 20 calves recorded. This index was based on an expectation of 90% live births. A score of +10 meant a perfect record [90% plus 10% = 100%], though any positive index indicated a better than average score. If a bull sired only dead calves, his breeding index would be -90. Indices of minus 10 or lower were found to be rare. Figures were provided for four bulls:

Sheppy pointed out that these figures could only be as accurate as breeders' submissions to the Herdbook. Bulls with negative indices are likely to be carriers of Classic Dexter Dwarfism, though this identification can be made only "after the event" [after bulls have sired a good number of calves].

Attempts have been made to find a reliable indicator of Dexters carrying Classic Dexter Dwarfism. Sheppy refers to Landauer's (1929) count of levels of eosinophils cells in blood samples, with high counts associated with dwarfism. In the early 1980s, under the auspices of the DCS (UK), blood samples were taken from 60 Dexters from five different herds to examine the relationship between eosinophil cell levels and dwarfism, but Landauer's results were not able to be replicated. Sheppy argues that "no system of external measurement [has shown] any real prospect of carrier detection" and the failure of Lindauer's blood test appears to have brought plausible lines of investigation to an unproductive end. The only other possibility is DNA analysis. Sheppy approached the Roslin Institute [University of Edinburgh] to get them to start a project on DNA analysis. A Report on this research - to that date unsuccessful - research was presented to the Congress. [It was not until the next Congress in 2003 that it was reported that the bulldog gene had just been identified by an Australian research team - click here for a summary of the 2003 Paper.]

Sheppy discusses crossbreeding trials at his Cobthorn Trust which supported the hypothesis that the dwarf gene was a single dominant one. He then explains an attempt to determine the pattern of inheritance of Classic Dexter Dwarfism. Calvings at the Cobthorn Dexter herd plus four other herds were monitored between 1970 and 1997. Three types of matings took place: both parents non-dwarf; dwarf/non-dwarf matings; and both parents dwarf. Results were:

Mating	Non-dwarf Calves	Dwarf Calves	Bulldog Calves	Ratio	Total Births
Non-dwarf x Non-dwarf	709 100%	Nil 0%	Nil 0%	1 : 0 : 0	709
Dwarf x Non-dwarf	274 50.55%	268 49.45%	Nil 0%	1.01 : 0.99 : 0	542
Dwarf x Dwarf	125 24.41%	285 55.66%	102 19.92%	0.98 : 2.22 : 0.8	512
All Matings	1108	553	102		1763

"It can be seen that the expected 1 : 1 ratio of offspring from a heterozygote/homozygote mating is the result produced by" Dwarf/Non-dwarf matings. Dwarf/Dwarf matings produced close to the expected 1 : 2 : 1 ratio for "matings between heterozygotes of a single gene" (page 78). It needs to be taken into account that it is likely that bulldog calves were under-reported due to difficulties in discovering them if abortion occurs around the fifth month of pregnancy while the cows are out on grass. "Taking all the results from the monitored herds, there is nothing to suggest any interpretation other than the standard view of the problem; namely that there is a dwarfing gene in the Dexter breed which in the heterozygote gives a short-legged animal, but when homozygous gives a bulldog calf" (page 79). Other explanations, such as that by Crew (1923) [available elsewhere on this website] and Curran (1986 and 1990), are not supported by this research.

Polling can be regarded as a simple dominant to horns in cattle, and can arise by mutation. Sheppy argues that mutations in cattle are quite rare, with rates ranging from 1 in 10,000 to 1 in 1,000,000. Mutation alone can produce only very slow changes of gene frequency. "As the gene frequency of the autosomal dominant poll gene in Dexters has been effectively nil, this quantifies the problem posed by the appearance of a number of polled animals in the breed" (page 81). The appearance of more than one polled Dexter in different herds in the space of less than four years in a breed of under 1,000 cattle is "statistically extremely unlikely" [to be due to mutations] (page 81). According to Sheppy, it would be "more reasonable" to look for the recent origin of polling in Dexters in introgression from other breeds.

The two most significant Dexter bulls responsible for polled calves are Whitegates Storm (DCS UK 2247), born 5 June 1983, and Migh Poldark (DCS UK 2447), born 24 April 1987. Whitegates Storm came off a commercial beef farm that included Aberdeen Angus, a polled breed. Sheppy contends that Dexters would have at times been yarded next to the beef bulls and that an illicit mating could very well have occurred "Quite possibly completely without the knowledge of the owners" (page 81). Migh Poldark's dam, Godstone Esmeralda (DCS UK 10531) "was claimed to be a novel mutation to polled, but was herself from a herd containing polled cattle upgraded from crossbreds, and with major inconsistencies in earmarks" (page 81). Furthermore, Migh Poldark was born on a farm which also ran polled Murray Grey cattle, resulting in a number of calves being recorded in the DCS UK Herdbook with Murray Grey as one of the parents, and a crossbred origin for Migh Poldark is also possible. Doubts about the parentage of these two bulls could be resolved by bloodtyping or DNA analysis, but these tests have not been done.

Sheppy believes that the low probability of mutation and the possibility of cross-breeding with a polled breed lead to the conclusion that "neither of these bulls ... can be regarded as purebred Dexters" (page 81).

Dexters are traditionally a black breed with red permitted, and the inheritance of black and red is a standard situation in cattle. Black is a simple dominant to the recessive red. Standard texts regard the Dexter as a black breed with a red recessive. A couple regard black as the only true Dexter colour with red appearing as a result of crossbreeding. In the period since 1970, duns have been observed in Dexters, along with brindles, pale reds (yellows) and combined shades of more than one colour.

Dun became an officially permitted colour in the DCS UK Herdbook in 1973. Some calves born prior to that were dun but were recorded as red. Sheppy suggests that dun Dexters have an inheritance pattern in relation to black that is the same as red. This is in direct contrast to other dun inheritance patterns in cattle where dun is a dilutant of black. Sheppy found at Cobthorn that dun calves have been the product of black to black mating or black to dun mating. "From this, and from the fact that Dexter duns generally appear of a browner shade than the crossbred and Highland duns, it would seem that a distinct gene has been demonstrated in the Dexter" (page 84). There are two main possibilities for the dun Dexter gene: a recessive brown of the 'B' series or a recessive dilution gene. The former has been suspected in cattle but never proven. The latter could be a gene from Channel Island breeds (Guernsey and Jersey), but this has yet to be determined.

The main source of dun Dexters in the UK is the Woodmagic herd, particularly the bull Woodmagic Mole (DCS UK 1920). Dun calves recorded in the 1996 Herdbook showed that they all had Woodmagic Mole in their pedigree, or known Channel Island blood, or both. Sheppy felt this indicated that "a recessive dilution ... is certainly a possible candidate for the 'dun' gene. Furthermore, the production of standard dun coloured calves from two black parents, one of which is upgraded from Channel Island, and the other Woodmagic bred, indicates that the same gene is present from both sources. Dun calves of this breeding have occurred in 16 distinct matings in the monitored herds" (page 84). [That Sheppy has drawn false a conclusion here, confusing phenotype with genotype, became apparent when the Dexter dun gene was identified and proven to be unique in later research reported elsewhere on this website.] Sheppy believes that another piece of evidence of Channel Island influence is that dun Dexters exhibit yellow fat in the carcase, a characteristic of Channel Island breeds. Black Dexters have a whitish body fat.

Another problem with colouring in Dexters is the tendency for the three main coat colours to change in significant ways as animals mature. As a result, some calves are registered initially as one colour but turn into another later. Sheppy argues that this was normal in the wild ox (aurochs), and reappeared when cross-breeding of domestic breeds was undertaken to try to recreate the aurochs. "It would seem reasonable to suspect that this phenomenon has again been recreated in Dexters by the multiple and diverse nature of the breeds introduced to the modern Dexter by introgression"(page 85). This can be seen particularly in the huge variation in modern reds. Jersey and Ayrshire have dusky faces and darker shading and star-like blotches in the coats of red Dexters like Templeton Conquest (DCS UK 2049) and Summerdale Bracken (DCS UK 7972) are "highly reminiscent" of Guernsey markings. Older Dexter breeders refer to the "even dark mahogany red" that used to be found in Dexters similar to the colour of the Red Devon "which is borne out by the photographs in early Herdbooks" (page 85).

There are three types of white markings to be found in Dexters of all colours: white on the underline, white patches elsewhere on the body, and white hairs in the coat.

White on the underline is attributed to a dominant gene for inguinal white which seems to be influenced by other white-producing genes. Extensive white on the underline was apparent in some of the earliest registered Dexters, such as Irisine and Rosemary (both listed in volume one of the Kerry & Dexter Herd Book, 1890). A certain amount of white on the underline and udder can be seen in most modern Dexters.

White markings on the head and body are the result of recessive white-spotting genes and are found in many cattle breeds, especially the Friesian, Shorthorn, Ayrshire, Shetland, Jersey and Guernsey. There are no indications of its occurrence in Dexters in historic records, and purebred Dexters should not be capable of producing recessive white spotted calves in first crosses with any other breed. "Most of the recent appearances of recessive white spotting [in Dexters] seem to be attributable to one bull, Parndon Charley Pudding 1928, and are frequently uncovered by the use of Dexters in crosses with other breeds that do have this gene" (page 86). There seems to be some link between inguinal white spotting and recessive white spotting, so that restricting white in the Dexter breed standards is a sensible measure.

The presence of individual white hairs is sometimes likely to be a "very low expression" of the recessive white spotting gene (page 86), though if they are widely distributed in the coat they may be an expression of another recessive gene found in the Jersey breed. Dexters showing a sprinkling of white hair generally have Jersey in their pedigree.

"The situation of the Dexter being either wholly black or wholly dark red with a very small amount of white permitted, as in the requirements of the standard, was almost exclusively seen in the early years of this century" (page 86). Shepy view's today's many colour variations in Dexters as the result of "considerable introgression from other breeds, particularly Channel Island" (page 87). There is very little evidence for the presence of these variations prior to the middle of the twentieth century, and the chances that they may be the result of spontaneous mutations are "statistically so extremely unlikely as to be an untenable hypothesis" (page 87).

Crew, F.A.E. (1923), The significance of an achondroplasia-like condition met with in cattle, Paper presented at the Proceedings of the Royal Society of London (B).

Landauer, W.E. (1929), A possible means of selection for non-deformity breeding in Dexter cattle, Veterinary Journal, 85(1), 11-15.

Seligmann, C.G. (1904), Cretinism in calves, Journal of Pathology and Bacteriology, 9, 311-322.

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