GENETIC CONDITIONS
certain conditions, the genotypes of their offspring can be predicted. The following Punnett Squares show the possible offspring genotypes for scrapie resistance, spider lamb syndrome, and dwarfism. Sires and dams each randomly contribute one of the two genes in their own genotype to their offspring. In turn, offspring inherit one gene at random from each of their parents. The combination of these two random genes forms the genotype of the offspring. The four quadrants of the Punnett Square, as shown below, show the range of possible offspring genotype(s), and their relative likelihood, for different matings between parents of known genotype(s).
Breeders face many challenging decisions as they attempt to produce better sheep for themselves and their customers. Animal breeding uses selection and mating decisions to increase desirable genes and decrease or eliminate undesirable ones. Fortunately, the inheritance of some of the most serious genetic defects is relatively predictable. In many cases, these conditions are controlled by only a single gene, and their expression is not influenced by the environment. For recessive conditions, the defect is visible when an individual carries two copies of the gene. Modern genetic testing allows the genotype of parents to be known and, for
SPIDER LAMB SYNDROME
SCRAPIE
DWARFISM
GENETIC ABBREVIATIONS NN – Non-Carrier, free of spider gene
GENETIC ABBREVIATIONS RR – Least susceptible, and all offspring will have at least one “R” gene QR – Least susceptible (but some offspring can inherit the “Q” gene) QQ – Most susceptible if exposed WHAT IS IT - Fatal brain disease affecting the nervous system in sheep and goats. WHY IS IT A CONCERN - Clinical signs are progressive and fatal once they develop. Infected animals may not become ill for years. There is no treatment. MANAGEMENT - Biopsy of the brain or lymphoid tissue. Genetic testing is available to determine an animals susceptibility. Select the least susceptible animals. “RR” breeding stock have been favored because all of their offspring will inherit a copy of the “R” (least susceptible) gene. In 2001, the USDA created National Scrapie Eradication Program in efforts to eradicate scrapie in the U.S. sheep and goat population. Through this program official USDA Scrapie tags must be in place for sheep sold, exhibited, or slaughtered. USDA Scrapie tags identify where the sheep originated by premise ID and the animal’s individual number.
GENETIC ABBREVIATIONS FF – Free of the dwarf mutation, non-carrier
FD – Carrier – has only one copy of the dwarf mutation. DD – Has two copies of the dwarf mutation and exhibits dwarfism. WHAT IS IT - Animals are very slow growing and may never reach normal size, which is contrary to the rapid early growth and efficient feed conversion reflected in the Suffolk breed standard. WHY IS IT A CONCERN - Animals are very slow growing and may never reach normal size. MANAGEMENT - Genetic testing is available to determine if the animal is a carrier of the dwarf mutation. Breeders should select against the dwarf (D) mutation. For USSA sanctioned events, sheep may not be entered/sold/exhibited if they carry even one copy of the D mutation (2022 Board Decision). Recent research indicates the existence of an additional mutation in the same gene as the original D mutation and which also causes dwarfism, but which appears to delete a portion of the gene containing the site of the D mutation. Since the missing region of the gene is not even detected by most current testing methods, the observed genotype is determined only from the intact gene. Therefore, an animal with one copy of this new mutation, let’s call it “X”, will be called FF if it’s other copy
NS – Carrier of spider gene SS – Exhibits Spider gene
WHAT IS IT - Inherited disorder associated with severe skeletal deformities. Affected animals have long splayed (bent) legs and exhibit twisted spines, flattened ribs and facial defects. WHY IS IT A CONCERN - Lambs have trouble standing and rarely survive beyond 6 months of age. There is no treatment. MANAGEMENT - Genetic testing is available to determine if the sheep is a carrier of the gene. Select resistant animals. This condition was first found in Suffolk sheep in the 1980s. Thanks to genetic selection by Suffolk breeders, this is no longer a commonly seen condition.
of the gene is F (it is actually FX) and DD if the other copy of the gene is D (it is actually DX). Interestingly, there will be no FD animals carrying the X mutation, since both copies of this gene can be detected. The problem is that an FX
animal, which tests FF by current methods, can produce a dwarf when bred to an FD or FX (such an animal will be DX and a dwarf). Fortunately, it appears that the “X” mutation is much rarer that the “D” mutation and a test for it should soon be available .
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