i - SCAN®
Micropthalmia is a recessive gene disorder that causes a developmental malformation in the eyes of affected lambs. Typically such animals are entirely blind, although some retain the ability to detect movement.
Micropthalmia occurs when a lamb inherits from both parents, damaged copies of a gene needed for eye development. An animal which inherits a damaged gene from only 1 parent is termed a 'carrier'. Carrier animals have normal, functional eyes because they also carry a copy of the normal functional gene. In an animal inherits 2 copies of the damaged gene, it will be born with the eye disorder. When blind progeny are observed, there are likely to be multiple carriers in the flock and DNA testing may be required to remove the condition from the flock.
Recessive disorders like this are hard to remove from a ewe flock without a DNA test. Typically a small population of unobservable carrier ewes will remain in a flock, even when parents of Micropthalmia affected lambs are identified and removed.
i-SCAN® is a predictive DNA test that identifies carriers of the damaged gene that causes Micropthalmia. i-SCAN® allows a sheep breeder to screen their flock to identify animals with a high probability of carrying the damaged gene. With i-SCAN® results, a breeder can plan matings to avoid breeding blind lambs and progressively remove Micropthalmia carriers from a flock.
The i-SCAN® test examines the DNA profile of each animal in the region of the Micropthalmia gene amd compares it to information in a large database of profiles from blind lambs and known carriers. The data is analysed to determine the probability of an animal having:
- no copies of the damaged gene - i-SCAN® clear
- one or more copies of the damaged gene - Micropthalmia carrier
i-SCAN® can predict the status of an animal without reference to pedigree information, however inclusion of pedigree and pedigree links to known blind lambs in particular, can assist in interpretation of results.
i-SCAN results
The i-SCAN® DNA results report the probability that an animal is a carrier of the Micropthalmia gene. the probability estimates are given in a range from zero to one hundred. Zero (0%) represents a nil probability, and one hundred (100%), a near certainty that the animal is a carrier.
|
Probability |
Result |
Explanation |
|
0% |
i-SCAN® CLR |
The animal tested is clear of any DNA profiles associated with Micropthalmia |
|
0% - 95% |
At Risk |
There is a chance (indicated by the probability) that this animal carries the Micropthalmia gene disorder |
|
>95% |
Micropthalmia Carrier |
There is a high likelihood this animal carries the Micropthalmia gene disorder |
i-SCAN results and breeding strategy
The i-SCAN® test can help breeders verify which animals are i-SCAN® CLR and identify which animals are carriers. Breeding only from i-SCAN® CLR animals will rapidly reduce the prevalence of the gene carriers in the flock. The most direct breeding strategy is simply to test all animals, identify the carriers and avoid using these in breeding.
As with any gene test, a degree of balance is required; cullinf simply on the i-SCAN® test is not recommended. Some compromise should be made if carrier animals are identified which are of high genetic merit: these are critical to genetic progress within the flock. In this case, the high genetic merit carrier can be mated to i-SCAN® CLR stock and the offspring tested. This type of strategy avoids the loss of genetic progress from culling high value animals solely on the basis of their i-SCAN® status.
While this strategy is direct, the upfront cost of testing may be proibitive for some breeders. The alternative is testing potential breeding sires only and then mating just the i-SCAN® CLR rams. No blind lambs will be born with this strategy but carrier ewes will persist and pass on the gene within the flock for many generations. Tests would have to be maintained for a long period of time (5-10 years) to have any chance of eliminating Micropthalmia carriers from the flock.
Possibly the optimum strategy is to focus testing on potential sires and strategically test dam lines where carrier families are detected.
Ultimately, the choice of which strategy to undertake will vary depending on the goals of the breeder, how prevalent the Micropthalmia gene is within the flock, genetic merit of Micropthalmia carrier animals and the financial situation of the farm.