Genetics is a mystifying subject, which can lead to considerable vexation of the aviculturist when trying to produce a desirable mutation. I am not a geneticist, but I will try to relate my somewhat simplistic understanding of the subject and elucidate this perplexing area.
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, which are simply discrete segments of a specific DNA molecule. Multiple genes are carried on units called chromosomes. We humans have 46 chromosomes in our cells, 23 of which come from mother and 23 from father resulting in 23 pairs. (Humans are said to have a "chromosome number" of 23. Chromosomes are counted in pairs.) These chromosomes are pair matched. For example, if chromosome 22 carries a gene for eye color, then two chromosome 22s, one from mom and one from dad, will each carry a gene for eye color.
Different species often have different chromosome numbers. For example, some plants, such as day lilies, have over 1000 chromosome pairs compared to our pitiful 23. It is the same for avian species. Many species have different chromosome numbers. And, although some species may have the same chromosome number, the genes and their arrangement on the chromosomes make the unique differences.
This diversity in chromosome number plays a major role in making crosses of different species possible or impossible, and if possible, determining the fertility of the offspring. For example, two similar animals, a donkey and a horse, can produce a viable offspring, the mule, which is infertile. This is because the donkey's and horse's chromosome numbers differ by one but their genetics are similar--close enough to produce offspring but different enough for the offspring to be infertile.
With birds most species are confined to their genus because chromosome numbers are more likely to vary among genera, and even if the numbers are the same, the gene patterns on the chromosomes can differ significantly. For example, Amazons may cross with other Amazons but not with African greys. However, exceptions exist. I have seen macaws cross with larger conures.
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.
Although genetics is much more complicated than dominant versus recessive, let's stick with classical Mendelian genetics, where this holds true. If a gene is dominant, then it is expressed. So, if a dominant and a recessive gene are present, then we will see the effect of the dominant gene only. To see the recessive gene, it must be present in pure form, that is, on both chromosomes of the pair that contains the gene. In the example of eye color, a red-eyed bird must have the genotype red/red.
Let's use the eye-color example because it is simple. We can analyze the outcome of a cross with a table called a Punnet square, if we know the parents' genotypes for a given trait. For example, look at the following table (not a Punnet square) where Br = dominant brown eyes and rd = recessive red eyes. (In genetics notation, dominant genes start with capital letters; recessives are lower case.)
We can see that only rd/rd produces red eyes because the trait is recessive. Now let's set up a Punnet square to analyze the crosses of the first two of the above four birds. The square shows us all possible outcomes for the brown/red eye genes for these two birds. (The genotype for bird 1 is given in bold in the first row; for bird 2 in the first column.)
From the square, we can see that 100% of the offspring will have brown eyes (Br/Br or Br/rd) because the dominant Br gene is present in all crosses. But 50% would be "split" to red (Br/rd) because they carry the recessive gene for red, which is not expressed. (Split just means that a dominant and a recessive gene for a trait are both present with the dominant gene being expressed.)
Now, let's set up a square for crossing the two Br/rd birds.
As you can see, this cross would yield one Br/Br, two Br/rd's, and one rd/rd. So, 75% would have brown eyes, 25% would have red eyes, and two thirds of the brown-eyed birds would be split to red. This is a very general example of how the outcome works, but it explains why two brown-eyed birds can have a red-eyed offspring, otherwise known as the "milkman syndrome" in human circles. This should give you a simplified understanding of how dominant and recessive genes work.
Any change from the normal or commonly expressed genes is called a mutation. For example, lutinos are mutations of the original green or gray bird. Mutations can occur for a variety of reasons and are part of the evolutionary picture of survival. When we as aviculturists find a mutation that is desirable to us, such as a yellow bird versus a green one, we propagate the mutation. In the wild, such a mutation would be easily spotted by a predator and might not have the chance to reproduce.
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. For example, lutino cockatiels are sex linked. Let's examine some arrangements.
|Gray male (not split)||ZZ||Gray|
|Gray male (split)||Z(l)Z||Gray|
Here, the split gray male has the mutated gene (l) for lutino on one of his Z chromosomes. However, the lutino gene is recessive. Therefore, it is not expressed because the other Z chromosome carries the normal, dominant, gray gene, resulting in a gray bird.
Here both Z chromosomes carry the recessive lutino gene. Therefore, the bird's coloration is lutino.
If the female carries the lutino gene, she must express it even though it is recessive, because the W chromosome doesn't carry a gene for color to mask it. Therefore, you cannot have a sex-linked, split hen. You can have an autosomal (non-sex gene) split hen.
To get 100% lutino babies in cockatiels, you must cross lutino with lutino. Crossing a lutino hen [Z(1)W] with a split male [Z(l)Z] yields: 25% lutino males [Z(l)Z(l)], 25% split gray males [Z(1)Z], 25% lutino females [Z(1)W], and 25% gray females [ZW].
In reality, avian genetics is much more complicated, but you can use some of the general guidelines I have outlined to help you in your matings. For example, the genetics of budgies is mind-boggling, but many traits are well documented and understood. 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. Just look a lutino cockatiels or the many Gouldian finch mutations. It takes experienced breeders years to establish a vigorous mutation, which is usually accomplished by outcrossing to normal individuals then breeding back for the desirable trait. But, it is hard to resist the temptation to sell that desirable mutation for a good bit of money rather than outcross.
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.
We also are faced with the problem of trying to find suitable mates for rare specimens. Again, avoid inbreeding if you can, and wait on a baby being produced by a fellow aviculturist. Remember, with parrots we have 20 and perhaps 40 years of breeding time, so we must be patient. Waiting a few years for an unrelated bird to mature is not too much to ask when you need to produce quality specimens.