Inbreeding: Why & Why not?

Inbreeding is the breeding together of closely related birds. Pairing of siblings, parents to offspring, grand-parents to grand children, half-brothers to half sisters are all examples of inbreeding. Beginners in breeding, often ask why this practice should be avoided when expert breeders regularly use inbreeding to establish new mutations and desirable traits in their lines. This post will discuss why inbreeding should generally be avoided and why it becomes useful for expert breeders.

You don't need to be an expert in genetics to understand this article. I will only make use of the concept of dominant and recessive traits to explain the pros and cons of inbreeding. A recessive trait (feature) is one that will not appear in the children unless it is carried by both the parents. Both parents must pass on the same recessive gene to the child for it to become visible. To learn more, please read Dominant and Recessive Genes in my Genetics Tutorial.

Most undesirable features are genetically recessive in nature. If they were dominant, they would quickly spread to the whole population and threaten the well being of the species. Our birds may carry many such genes without showing any visible signs. For example, they may carry a gene for a short tail, a gene that causes infertility and another that causes crooked toes. However, our birds look perfectly fit and healthy because they only have a single copy of this gene. If they had two copies of the same gene they would show the effect of these genes by having a short tail or crooked toes or infertility. When we breed such a bird there is a 50% chance for each undesirable gene to be passed on to the offspring. Since there may be several undesirable genes in a single bird, it is very probable that a given offspring will inherit some of these undesirable genes.

Let us now consider what happens when we breed two siblings from this bird that was carrying the undesirable genes. Each of the two siblings has a 50% chance of inheriting each of the undesirable genes from the parent. There is a 50% chance that the male has inherited the gene for a short tail and a 50% chance that he has inherited a gene for infertility and a 50% chance that he has inherited a gene for crooked toes. The same is true for the female. Therefore there is a high probability that one or more of these undesirable genes is present in both siblings. When we breed them together, the chances of the same recessive gene being passed on from both parents to the same chick are much higher as compared to a pairing where the birds were unrelated. We may get one or more chicks that show one or more of the undesirable features described above. Even if we are lucky and don't get a bird that shows a defect, the same undesirable genes are now present in more of our birds than before. If we continue to inbreed, sooner or later we will start seeing birds that show these defects. The defects I have mentioned here are just examples. There may be hundreds of such genetic defects that are found in a single species. To summarize, inbreeding greatly increases the chances of combining undesirable genes leading to undesirable qualities in our birds.

Let us now look at the other side of the picture. If inbreeding is so bad why do expert breeders use it? What good can possibly come from it? To explain this I will use the exhibition budgerigars as an example. Exhibition budgerigars have been bred to their current size and form through almost a hundred years of selective breeding. Just like the undesirable features mentioned above, a lot of desirable features are also genetically recessive. They will not appear in the offspring unless both the parents are carrying the gene for this feature. Consider a breeder who has a line of very good quality exhibition budgerigars. He wants to further improve the quality of his birds and identifies that larger throat spots is what his birds need. He goes to a champion breeder, and buys the best male bird that he can afford with super throat spots. He must now use this bird to improve the quality of his own line. To begin with, he pairs the new male to his best hen. The birds breed and produce some chicks. But the chicks do not show spots nearly as good as the father! What happened? Has his money and effort gone to waste? To understand this, recall that many desirable features are also genetically recessive. The chicks are carrying the gene for the super spots but since the gene is recessive the chicks are not showing this feature. He must therefore plan to combine two copies of this gene in a single bird. In order to do this he needs a pair where both parents are carrying this gene. One option is to pair two siblings from the first clutch but this option carries the risks of inbreeding described earlier. To reduce the risks of inbreeding, he pairs the same super male to another one of his females. He now has two clutches from different mothers but with the same father. All chicks carry the gene for large throat spots from the father. He now pairs a male from one clutch to a female from another clutch. The pair being half-brother and sister. Some of the resulting chicks now display the great throat spots of the father. Inbreeding has helped this breeder in improving the quality of his birds. Inbreeding is also essential when establishing new mutations from a single mutant bird.

Even an expert, practicing inbreeding to improve his birds, may see the effects of the undesirable genes coming together and producing defective birds. Such situations may be dealt with by keeping inbreeding to a minimum, culling the defective birds and introduction of out-crosses. Inbreeding should be avoided as a rule unless the breeder has a very good reason for it and clearly understands his goals and the path leading to that goal. If done irresponsibly it can lead to defective birds, high mortality and infertility but with careful planning it can be used to breed some wonderful birds.

The genetics presented in this article have been simplified to facilitate the understating of beginners. Most features are polygenic in nature. This means that several genes are controlling a single feature with each being inherited independently. Furthermore, genes may also have other modes of inheritance and be incomplete-dominant or sex-linked recessive. The general theory and lessons however remain valid in most cases.


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