Understanding Genetics

Posted: Fri Mar 09, 2007 9:20 am Post subject:

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Hi Ghozze,

Below is an extract from a useful article on avian genetics. The full article can be read at http://www.oldworldaviaries.com/text/styles/genetics.html

Please note that I had used the expressions "X" and "Y" in my earlier post to avoid confusion as these were the terms used in the extract from "Canary Tales" that I had quoted. The proper terms are “Z” and “W” which refer to the chromosomes of birds instead of “X” and “Y” which refer to those of mammals.

As you know, genetics is complex and even the experts do not fully understand it. What I tried to do in my earlier write-up was to provide very basic information that the beginning breeder might find useful.

You will note that the author of the article is against inbreeding. Without some degree of inbreeding or line breeding, it is impossible to fix the characteristics that we desire. Each breeder will need to decide for himself, the extent to which he will or can go with his inbreeding or line-breeding.

A brief review of avian genetics by Darrel K. Styles, DVM
Quote:


…….. Without getting overly complicated and boring, let's start with the basics. Birds have a mother and a father, just as we all do, and as a result, inherit characteristics of both. The instructions to produce these characteristics are encoded into the DNA of the bird. Each characteristic, such as feather color, is determined by one or more genes, ……….
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Each chromosome carries thousands or hundreds of thousands of genes, all uniquely encoded by DNA. The presence of genes makes up the "genotype" of an animal. The expression of genes makes up the "phenotype." For example, if a bird has one gene for brown eyes (a dominant trait) and one for red eyes (a recessive trait), then the genotype is brown/red, but the phenotype is brown because brown is dominant over red.

Now let's discuss sex chromosomes. We are all familiar with the concept of genetic sexing. And, many of us know that in humans XY is male and XX is female. These chromosomes carry most of the genes that determine our phenotypic sex. The XY combination is called "heterozygous" meaning different genes, and the XX combination is called "homozygous" meaning the same genes.

Birds are different in that their male and female sex-chromosome roles are reversed from mammals, meaning that the female is heterozygous and the male is homozygous. Also, in birds we use Z and W instead of X and Y. (These letters refer to the general shapes of the chromosomes.) So, a male bird is ZZ, and a female bird is ZW. This leads us to the topic of sex-linked traits.

A sex-linked trait is carried on one of the sex chromosomes, Z or W but usually Z. ………. [Note: As I mentioned in my earlier post, the female will carry the Z chromosome of her father and if we want to propagate the father's genes, the use of the female offspring will more likely enable us to achieve our aim than the use of the male offspring. This is especially so with the first generation progeny.]

……………. The secret of understanding genetics is keeping good records and comparing the results of crossings. In this way you can determine what the actual genotype of the parents is and enhance your chances of predicting the outcomes.

Often, the goal of studying avian genetics is to help in the propagation of valuable mutations. This is most often accomplished by inbreeding. But remember, if you inbreed (cross closely related individuals), you will not only concentrate the desirable genes, such as the lutino cockatiel gene, but also the bad genes, such as bald spots behind the crest. The bald spots appear to be "linked" to the lutino genes and are passed along with them. Also, inbreeding weakens the line. …….

Finally, there are no clear-cut rules on how to proceed with crossings. Experience and time are the best teachers along with your own avicultural intuition. My rule is to avoid inbreeding at all costs unless you are trying to concentrate a specific desirable trait, then proceed with caution.