The century old dexter phenomenon in cattle breeding (A critical review of literature)
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The Dexter cattle were originally bred in South-western Ireland and brought to England in 1882. The Dexter is one of the smallest cattle breed developed in the British Isles. The Irish Dexter descended from the ancient Celtic Black. The Dexter is a dual purpose breed known as ’beefy little milkers’ used for both meat and milk production. Traditionally Dexters were a cottage cow in Ireland providing the household with milk and also the production of a calf for beef. Dexters are now mainly distributed in Australia, New Zealand, Britain, Canada, Ireland, South Africa and America. The Dexter was selected out of the general cattle breed population from 1820’s. As time went on, these Dexter breed were exported to England and further bred with similar trait english cattle where the gene pool expanded. Suspected results over time were that a stable standardbred look and size would be achieved. But instead these cattle owners not only achieved the preferred shortleg type but also resulted in breeding normal looking and sized cattle which are referred to as longleg type. Also added to the gene pool was bulldog calves (deformed foetuses). These last two undesirable traits were and still are proving difficult to eliminate. According to records of births, it showed 25% longleg type, 50% shortleg type and a further 25% bulldog calves. Science has proven that the Dexter has a trait that gives them their short legged appearance is actually due to a form of dominant lethal genetic mutation. So as long as the mutation was the main selection, the 3 Dexter types would continue to be bred and no true Dexter would be bred. From an owner prospective, it came down to continuing to achieve this desired appearance of cattle and therefore choosing to live with this issue in order to do so. Once owners became more aware and up to date about the mutation, shortleg were bred with longlegged normal type and so 50% shortleg and 50% normal longleg was achieved and this ruled out the bulldog calf breed. But more recently, owners are continuing to breed both directions. This has resulted in the longleg (normal) breed to be discarded and only the shortleg to be considered the ’real Dexter’. In North America since 1980, owners breed small longlegs type. Therefore they had achieved a similar size and look of Dexter to the original but at the same time discarded using the lethal mutation. The occurrence of lethal bulldog calves was high. In England, the breed society recorded every bulldog calf so that the parents could be culled. When the Dexter was first brought to England and their small size and productivity for both milk and beef, they became very popular and were used as show animals. The small Dexter and the large Kerry cattle of Ireland came from the same base stock. When the increased demand of pure Dexters did not meet the supply, dwarf – type Kerries were shipped to England as Dexters. Due to this concentration of unusual small Kerries in these English herds gave rise to the appearance of bulldog calves in the English strain. There are basically 2 kinds of Dexters : Shortleg: classic Dexter , dwarf, achondrodysplastic or chondrodysplastic dwarf, beef type, heterozygote, carrier, affected. Longleg: normal, Kerry type, proportionate, homozygous normal. (Davidson ,6) The bulldog trait is chondrodysplasia. An animal that has one bulldog allele will display chondrodysplasia and have a disproportionate confirmation and an abnormal short gait. If an animal has 2 chondrodysplastic alleles, it will be severely affected and be aborted as a defective bulldog calf. Affected Dexter’s should not be bred as they will never breed true.Regarding the mutation chondrodysplasia in Dexters, it causes a defective bone growth. This therefore gives rise to cattle with heavily bodied appearance on short legs. The single gene mutation produces a huge variety of Dexter types, such as proportionate and desirable where as others badly disproportionate and dwarf like. The expressed dwarfism is not consistent, and variation is not completely understood. Carriers of the gene, have more bulging muscle due to the muscle being compressed onto the shortened bone, giving a much more heavily muscled appearance. Also a pot bellied appearance is seen due to the smaller skeletal framework but yet have normal sized organs. The chondrocytes are affected by the mutation and instead of normal organisation of the chondrocytes this is disrupted, and palisading does not occur. Chondrocytes take an irregular pattern not allowing bone ossification to be completed. Dexter chondrodysplasia inheritance is a 4 character mutation in the gene for longitudinal growth. The mutation causes a stop action of the gene. Paired genes work together which gives rise to 3 results which are not the same and depends on gene combination. The Dexter bulldog is always born dead. The foetus is usually aborted at 1-2 months or between 6-8 months in which the foetus gives a bulldog like appearance. The foetus has stumped legs, an abdominal hernia and a bulldog like head. The possible genetical combinations that can occur are as follows: Non carrier bull x non carrier cow = 100% chance of having a non carrier calf. (normal long bone growth ) Non carrier bull x carrier cow = 50% chance of having a non carrier calf and 50% chance of having a carrier calf. Carrier bull x carrier cow = 25% chance of having a non carrier calf 50% chance of having a carrier calf 25% chance of having a dead bulldog calf (aborted) With this dominant gene, many carriers can be identified by the visual eye regarding their lowered heights. But it is not always that clear. The true identification of the presence of the mutation is done by newly developed genetic testing. With this breeding tool, owners can make more informed and better breeding choices and avoid bulldog calves by only having minimum of one parent carrier.