Genetic diseases

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Genetic diseases are being recognized with increasing frequency in veterinary medicine. This is due, in part, to heightened awareness and control of environmental diseases such as infections and nutritional deficiencies. It may also be the result of the influence that a popular sire can have on a breed. This chapter covers recognizing genetic diseases, basic genetics: segregation analysis, gene discovery strategies, DNA tests.

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7.1 Patients presented with young-onset spastic paraplegia. It is critical to consider all the differential diagnoses and investigate the extended family history before concluding that a disease is hereditary. Pure-bred English Pointer with a history of littermates similarly affected. Although the history makes a hereditary disease more likely, this patient had an infectious disease (). Stray cat from the streets of St. Louis with hereditary muscular dystrophy.
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7.2 Organic acids are intermediary steps in the metabolism of nutrients to produce energy and substrates for synthesis. Methylmalonic acid is a step in the metabolism of branched chain amino acids. A hereditary deficiency in the enzyme methylmalonyl CoA mutase or a dietary deficiency of cobalamin (a key cofactor), results in the accumulation of methylmalonic acid which can be detected in the urine ( ).
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7.4 Neonatal ataxia in Coton de Tulear dogs is an autosomal recessive trait. The parents are carriers with one normal allele (A) and one mutant allele (a). On average 25% of the offspring receive two normal alleles and are normal; 50% of the offspring receive one normal and one mutant allele and are carriers; and 25% of the offspring receive two mutant alleles and are affected. Since the affected puppies are readily identified, 66% of clinically normal puppies are carriers ( ).
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7.5 The tailless trait in Manx cats is an autosomal dominant trait. One parent is affected with one mutant allele (A) and one parent is homozygous for the normal allele (a). On average 50% of the offspring receive the mutant allele from the affected parent and are affected. The other 50% are homozygous for the normal allele and are clinically normal ( ).
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7.6 Hereditary cerebellar ataxia in English Pointers is an X-linked trait. The female parent is heterozygous for the recessive trait (x). The male parent is normal. On average 50% of the female offspring receive the mutant allele and are carriers. In addition, 50% of the male offspring receive the mutant allele, but since the Y chromosome does not contain a normal allele, these animals are affected ( ).
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7.7 Markers are DNA sequences that can vary between individual chromosomes. In this pair of chromosomes, markers A and B are polymorphic and the different alleles (1 and 2) have been labeled. Markers A1 and B1 are on the chromosome with the normal allele (D). Markers A2 and B2 are on the chromosome with the disease causing recessive allele (d). A2 and B2 tend to segregate with the disease and thus are markers for the mutant allele. When the chromosomes undergo meiosis, they separate and recombine, producing a different combination of markers. In this case, the marker A2 is closer to the disease causing allele (d) and thus more tightly linked. Recombination is more likely to occur between the disease causing allele (d) and the marker B2, which is further away. Such recombinations are helpful in narrowing the locus in linkage studies, but can lead to error when using linked markers to identify carriers of a trait ( ).
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7.8 In GWAS the probability that an SNP marker is more common in an affected animal than in a control animal is represented graphically in a ‘Manhattan plot’, where each individual chromosome is expressed in a different colour along the Y axis. The peak of significant association seen in chromosome two (red dots, second from the left) indicates that the disease causing mutation resides within this area of the chromosome.

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