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title: "7.5 Breeding aspects of monogenic traits with negative effects (2024)"
canonical: "https://wiki.groenkennisnet.nl/space/TAB/296681642/7.5%20Breeding%20aspects%20of%20monogenic%20traits%20with%20negative%20effects%20(2024)"
format: markdown
---
In animal breeding in all species and in all breeds genetic defects caused by mutations require continuous attention. Due to mutations during the meiosis each individual carries mutations with negative effects. When such a mutated allele is dominant, it will have a visible or measurable effect in the carrier and the carrier is not viable or will not be used for breeding. The mutated allele will not be spread in the population: it will be purged. But when a mutated allele is recessive, it will not have an effect for the carrier and it will not be recognized. It will be spread in the population when the carrier is used extensively for breeding. The effect of the mutated allele will be recognized many generations later when, often by accident, two offspring of the carrier will be mated to each other. Then, with a chance of 25 % a homozygous carrier of the mutated allele is born. It might not be viable, malformed or suffer from a serious health problem early or later in life.

In animals a lot of malformations and malfunctions are known. Not all of them have a genetic origin. To determine the genetic background it is highly recommended to record all malformations and malfunctions and to analyze regularly the population for their frequencies. An observed increase might be caused by a certain parent, or ancestor in past generations, which gave more than one offspring with a defect. That is the first sign that the deviation is a genetic defect. Per species many genetic defects are known:



![image-20240720-100014.png](media://03f12b9c-61a6-4387-a428-60e306222a68)


The homozygous animals suffering from monogenic genetic defects pop up in populations were in the past a sire got a large number of offspring, much larger than the other sires used at the same time. This fact is illustrated in the figure below:

![image-20240122-100646.png](media://3d6b861a-ca5d-4671-9b37-9a7ace0b8ff7)


From this fact, it can be learned that in animal breeding it is not wise to create a large number of offspring from a selected parent. In well controlled populations this can be easily realized, but in less controlled populations this is very difficult. A directive is that each parent in a generation should produce less than 5 % of the animals in the subsequent population.

Thus, as all individuals carry alleles for genetic defects, it is impossible to discard all these alleles in a population. And once an allele is spread in a population it is possible to reduce its frequency to a low level, but without genetic markers it can never be discarded.

Even with a very low allele frequency (e.g. 0.05) for the genetic defect, you still have (Hardy-Weinberg expectation: 2pq = 2*0.95*0.05 = 0.10) a lot (10% in this example) of carriers in the population.