Friday, 16 September 2011

Genetic Diseases in Animals

Genetic Diseases in Animals

Report # 1

Genetic diseases result from chromosome abnormalities or mutant genes showing a specific pattern of inheritance. In addition, genetic factors are involved in susceptibility to some nongenetic DISEASES. As progress has been made in eliminating infectious diseases, genetic disease has come to represent an increasingly larger proportion of all disease. Genetic diseases among animals, particularly domestic species, have sometimes provided useful information about human counterpart disorders.

Report # 2

The majority of inherited diseases recognized in cattle are autosomal recessive. Due to the practice of "line-breeding", some recessive defects may become relatively common. In addition, undue emphasis upon estimated breeding values (EBVs) to maximize production may lead to high levels of inbreeding and an inevitable increase in prevalence of recessive defects. In contrast, inherited defects that are autosomal dominant are most likely limited to a single herd, or rarely, to a few associated herds.

Haemophilia A in Herefords is an example of an X-linked defect, where the clinical disease is most common in males. Potentially there will also be Y-linked defects and also those resulting from mutations in mitochondrial DNA.

Autosomal recessive defects are expressed in male and female progeny of clinically-normal, heterozygous parents. One quarter of the progeny of heterozygotes will be affected, half will be heterozygotes like the parents, and the remaining quarter will be homozygous wild-type (normal) - that is, they did not inherit the disease-causing mutation.

Registered animals of most cattle breeds have been derived from a limited gene pool. Within modern breeds, herd books are generally closed. Consequently, most recessive defects are usually "breed-specific" and are caused by founder effects relating to a single mutation. There are exceptions where multiple mutations cause the same clinical disease within and across breeds. An affected individual may be heterozygous for two distinct mutations.

The term compound heterozygotes is used to describe these animals. An example would be a Brahman calf affected with generalised glycogenosis if it inherited the exon7 mutation from one parent and the exon13 mutation from the other parent. Similarly, different mutations in the α-mannosidase gene cause α-mannosidosis in Angus and Galloways. Consequently, an affected Angus/Galloway calf could be heterozygous for both of the mutations. In like manner, a Poll Hereford cross Poll Shorthorn calf may be affected with maple syrup urine disease if it inherits the "breed-specific" mutations from the parents.

DNA tests are available for diagnosis of the following genetic diseases, or for detection of heterozygotes. With CVM in Holsteins and Dwarfism in Dexters, samples should be submitted direct to an external reference laboratory - addresses provided. With DNA-based assays, hair root samples are preferred as they avoid potential problems created by haemopoietic chimerism, which can occur when using tests based on analysis of DNA from cellular elements of blood. Bovine co-twins commonly share haemopoietic stem cells, resulting in DNA in cellular elements of blood containing nucleotide variants from both twins that may be different to the DNA variants found in gametes from an individual co-twin.


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