DNA Test Summary Dex-Info

"A DNA Test for Chondrodysplasia in Dexter Cattle"
by J.A.L. Cavanagh, I Tammen, P.A. Windsor. P.C. Thomson, F.W. Nicholas and H.W. Raadsma

The authors are associated with ReproGen, the Centre for Advanced Technologies in Animal Genetics and Reproduction, a research and commercial centre at the University of Sydney in Australia, see www.vetsci.usyd.edu.au/reprogen. The first author, Julie Cavanagh, was the doctoral research student who undertook the bulk of the work and whose identification of the location of the chondrodysplasia gene so quickly was highly praised by her supervisors and colleagues.

Reports of the incidence of "mutant, aborted foetuses" (p.1) in Dexters goes back to at least the 19th century. Such foetuses, or bulldog calves, display "disproportionate dwarfism, a short vertebral column, abnormally short legs, a relatively large head with retruded muzzle, cleft palate, protruding tongue and a large abdominal hernia" (p.1). [Click here for photos of a bulldog calf - Note: graphic photos] These foetuses carry two copies of a defective gene. Carriers with one copy are "short-legged" Dexters. Dexter Cattle Australia initiated and supported research to develop a DNA-based diagnostic test to identify carrier animals.

This section of the paper describes how the research team conducted the project, using a panel of 21 Australian Dexters which included known carriers of the gene, as well as tissue samples of bulldog calves from Australia, New Zealand, Germany and the UK. Surprisingly, two different mutations were found in the same gene, one of which is restricted to the Australia and New Zealand offspring of Meadowpark Charles, a grade bull, and introduced from "a genetic source outside the Dexter breed" (p.5). Identifying these genes enabled a DNA test to be developed to identify Dexters carrying them. "Dexters have been selected [by their breeders] for their shorter, often dwarf-like appearance usually indicative of a carrier animal. The DNA-test will enable...avoidance of carrier x carrier matings...A major benefit of the test will be the ability to select desirable breeding animals of a short stature that do not carry the chondrodysplasia defect" (p.5 - emphasis in original).

What is the impact of the presence of the chondrodysplasia gene on the height of Dexters? The DNA test for chondrodysplasia now allows for rigorous comparisons of the height of carriers and non-carriers. The research team took measurements from 34 male Australian Dexters and 94 females (none of whom were offspring of Meadowpark Charles or any of his progeny). Among the measurements they made were the height at hip and wither, the girth, the length from wither to tail, cannon bone circumference and cannon bone length. Among the results were the following (taken from Table 1, p.7):

Measurement	Male Non-Carriers	Male Carriers	Difference
Height at wither (cm)	120.72	100.92	19.8
Height at Hip (cm)	119.37	101.63	17.74
Girth (cm)	191.33	159.55	31.78
Length (cm)	137.28	108.07	29.21
Cannon bone length (cm)	16.79	13.31	3.48
Metacarpal Index (cannon bone length divided by circumference)	1.008	0.963	0.045

The researchers concluded that the presence of the chondrodysplasia gene "has an effect on all measurements" (p.7). They then did an analysis on which measurement was best in discriminating between carriers and non-carriers. They found that cannon bone length scored highest at 83%, followed by the metacarpal index at 77%. Height at hip scored next at 67%, then height at wither at 63%, length at 62% and girth at 56%. A more complicated analysis found a formula that was a combination of four measures that was best for predicting whether a Dexter would be a carrier or not, scoring at 95%. However, as the researchers noted towards the end of the paper, the existence of a DNA test makes such discriminating measures redundant.

Breeders are often interested in non-carrier Dexters that are short in stature. The researchers compared each of 73 females in terms of height distribution and presence of the chondrodysplasia gene (Figure 1, p.9). Twenty-nine of the females examined were carriers and 44 were non-carriers. Using the height at hip measure, they found that nine were shorter than the Dexter Cattle Australia breed ideal of 97 to 110 cms. One of these nine (the eighth) was a non-carrier. Thirty-one animals fell within the breed ideal - with only two exceptions, the 22 shortest of these were carriers and the 10 tallest were non-carriers. The two exceptions were the the third and sixth shortest which were non-carriers. The remaining 33 were taller than the breed ideal and only one of these (the 14th of them) was a carrier.

In other words, in general, the shorter cows were carriers and the taller were non-carriers with only four exceptions - three non-carriers were among the shortest 15 and one carrier was amongst the tallest. [It is of course the three short non-carriers that breeders would be most interested in.]

The researchers also found that the animals that fell within Australian breed ideal for females and males were comprised predominantly of carriers with a few shorter non-carriers (p.11).

Discussion and Conclusion

The mature hip height of Dexters is of most interest to breeders as this is specified in the breed ideals. "As shown in the results, the hip height of a carrier female is approximately 10 cm shorter than a non-carrier female. The effect is even greater for males, namely a difference of approximately 18 cm" (p.11). It is advantageous for breeders to select shorter non-carrier animals but only a small proportion of animals within the breed ideal are non-carriers (22% for females and 13% for males).

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