Hereditary defects
by Gisela Aach
(Any copies, or excerpts, only with permission from the author)
Every deviation from the normal phenotypical make-up of an organism that disturbs the carrier in its physical or psychical health and that is of genetic origin, is seen as a hereditary defect.
Basically, two types are differed:
1. Hereditary defects, caused by a single genetic locus: monogenic defects.
2. Hereditary defects, caused by several and additively working genetic loci: polygenic defects.
There is a genetic threshold for many polygenic controlled defects that, depending on the number of added defective genes, either makes the affected animal seem phenotypically healthy or makes it diseased to a certain degree. In most cases, genes causing defects react recessive (in opposition to dominant) towards healthy genes.
Genetic mistakes are always the result of a mutation. In which case the exchange or loss of one or more bases in the DNA-chain causes structural changes of enzymes or proteins, so that a specific enzyme can no longer do its job, or structures have been disturbed and now the tissues or organs are composed differently.
If a defective enzyme is present in a simple form, the animal is therefore heterozygous for the defective gene, the job is done by the normal gene determined through the enzyme, the mistake does not become manifest. Only if the defective gene is homozygous, a normal enzyme is missing completely, will the enzyme defect become phenotypically apparent, the affected animal carries the hereditary defect.
Not always will a mutation lead to the total disfunctioning of a determined enzyme, in many cases its spectrum of activity will only be limited, so that the enzyme can function normally under optimal environmental conditions. But slight changes in these optimal conditions can make it lose its ability to function and the organism becomes more susceptible to changes in the environment. Environment can act as a trigger in these cases.
The possibilty to get any of these environmentally triggered hereditary defects, among which fall many genetic diseases, is passed on from the parents to the offspring, but the disease itself will only come to life if certain environmental influences actually trigger it.
The most important rules for successfully dealing with genetic defects:
1. Recognize any phenotypical deviation and keep a record about it.
2. Furnish proof for the heredity by analyzing the family tree and by breeding experiments.
3. As far as possible, keep the breed free of these hereditary defects by excluding the carriers from breeding.
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