As budgerigar breeders, it is important for us to have some knowledge about the production of color because there are so many different varieties of budgerigars and colors to choose from. However, many budgerigar fanciers avoid the topic of genetics because they believe it to be too complicated and for this reason they avoid it. This article merely touches on the fundamentals of genetics in terms of the generation of color and diversity, and it is written primarily with newbies to the hobby in mind rather than more experienced fanciers.
When Gregor Mendel, an Austrian monk, released his scientific observations in the year 1866, it was a significant step forward for the advancement of human understanding. He created the first laws for the science of heredity, which is today known as genetics, after conducting experiments involving the cross-pollination of pea plants over a period of several years and documenting the features of the seedlings that resulted from these experiments.
In the late nineteenth century and the early part of this century, Mendel’s Theory of Inheritance served as the foundation that many scientists relied on to apply to both plant and animal production. This was true for both fields of study. Around the year 1920, when Dr. H. Duncker and C. H. Cremer of Bremen, Germany, applied Mendel’s theories to budgerigars, these laws became universally recognized by scientists. Prior to that time, however, acceptance was slow to come. These principles are utilized in the development of budgerigars to make predictions about the color characteristics of the offspring that will result from any given coupling.
Each bird has its own unique set of 26 chromosomes, which are microscopic organisms. This genetic code is what distinguishes one bird from another. This particular collection of chromosomes is replicated in every cell that makes up the bird. Each of the set’s chromosomes is made up of a unique series of “Genes” or “Factors” (Mendel’s words), which are responsible for the many inherited characteristics of the bird. Characteristics that are inherited include the size of the head and spots, the form, the kind, the color, the sex, the bone structure, the length of the feathers, and their texture. The 26 chromosomes are organized into 13 pairs, and each pair is the same length (except for that chromosome pair that controls the sex of the bird). Corresponding genes on each chromosome, known as “Allelmorphs” or “Alleles,” in terms of their position on the chromosome.
The way in which two alleles interact determines one of the bird’s characteristics, and they can be the same or different from one another. It is claimed that a bird is “Homozygous” for a given gene (or that the gene is present as a “Double Factor”) if its two alleles are identical to one another. On the other hand, if the alleles are not identical to one another, it is said that the bird is “Heterozygous” for the gene. each gene individually (or each gene is said to be present as a “Single” Factor”)
A “mutation” is the alteration that occurs in a gene or collection of genes as the result of a genetic accident. However, a mutation that is viable is an extremely uncommon occurrence; this is the reason why the original gene is the one that is most prevalent in the wild budgerigar population. Therefore, the gene that was originally present is referred to as the “Wild-Type” gene, and any variation from the wild-type is referred to as a mutation. A wild-type organism and a mutant can deviate from one another in significant and in insignificant ways. It is obvious that certain mutations are more likely to take place readily (and, as a result, more frequently) than others.
During the act of mating, the ovum of the hen will hopefully be fertilized by the sperm of the cock, which will result in the production of an egg. Both the sperm and the ovum are solitary cells, and as such, they only have one copy of each chromosomal pair in their respective genomes (which half of a chromosome pair gets included in the sperm or ovum is a matter of chance). Therefore, the egg that has been fertilized has a complete set of chromosomes, with each chromosome pair consisting of a chromosome inherited from both of the parents. Therefore, the DNA of both parents contributes to the development of each and every aspect of the chick.
This process can be complicated genetically by something that is referred to as “crossing over.” This takes place during the phase of the creation of sperm and ovum in which the chromosomes couple up and lay parallel to one another. At this stage, a pair of chromosomes has the potential to become entangled at particular spots, similar to how a pair of long balloons can twist together to form a single longer balloon. The sections that lie in between these two positions are then able to “cross over” or trade places with one another. Therefore, a chromosome in an egg or sperm can include a mixture of the chromosomes from each parent. The act of crossing over from one set of chromosomes to another typically takes place at the same positions in each cell. This indicates that genes that are located on the same segment of DNA will always be connected to or “linked” with one another.
The Sex Character
As was just discussed, the pair of chromosomes that determine a person’s sex is not of equal lengths to one another. The sex chromosomes of the hen, which are denoted by the letters X and Y, are of different lengths. The Y chromosome, which is the shorter member of that pair and does not carry any sex genes, is the shorter of the two. The male and female chromosomes in the cock will both have the same length, which is denoted by the symbol XX. When a rooster and a hen are mated, they should always, on average, produce an equal number of offspring of both genders whenever they produce offspring. This is because, during the process of reproduction, the chromosomes from the cock’s sperm cell, which carries half of a set of chromosome pairs, combine with the chromosomes from the hen’s egg cell, which also contains half of a set of chromosomes pairs, resulting in the formation of an entirely new complete set of chromosomes.
The Split Character
It is possible for a pair of birds of a single color “Phenotype” to spawn offspring of a different color “Genotype” if the birds’ genetic makeup is different than their actual phenotypic appearance. In popular parlance, these birds are referred to as “splits,” and an angled line denotes their impure status.
The Dominant and Recessive Character
The genes that control coloration can either be “dominant” (for example, green) or “recessive” (e.g., blue). A bird that carries the dominant gene on only one-half of the pair of chromosomes will have the same coloration as a bird that carries the gene on both sides. The recessive colors will not be visible unless they are present on both parts of the chromosomal pair in which they are carried. There is a wide variety of possible chromosome pairings in which the color genes can be carried. If this is the case, a bird can have one dominant color and also secretly carry within its genetic makeup one or more recessive colors, but it cannot have the other combination of colors. Therefore, it is possible to say that in the most basic form of the interaction between two different alleles, one is dominant and the other is recessive, which means that the dominant allele governs the character.
Because the green gene is dominant over the blue gene, for instance, a bird will have green feathers if it carries both the green gene and the blue gene as part of an allele pair. This is due to the fact that the green gene contains the genetic code for green feathers. It’s possible for a bird’s genetic makeup and its physical makeup (its phenotype) to be completely different from one another due to the interaction of distinct alleles (its genotype). The following classifications are possible within the realm of color inheritance:
The dominant mutations are:
- Greens (All Forms)
- Dominant Pieds
- Greys
- Clear-Flights
- Violets
- Spangles
- Yellow Faces (to the blue series)
- Crests
- Easley Clearbody
The recessive mutations are:
- Blues (All Forms)
- Recessive Pieds
- Fallows
- Whites
- Yellows
- Grey wings
- Clearwings.
- Saddlebacks
The gene of a dominant character may be present as a single or double factor, the determination of which is only possible by trial pairing to a pure normal. It is not possible for any normal-looking bird to be “split” from a dominant character.
The various rules that govern the inheritance of the dominant character irrespective of the actual color are:
| Pairings | Expectations | |
|---|---|---|
| 1 | Dominant (Single factor) × Normal | 50% Dominant (sf) 50% Normals |
| 2 | Dominant (Double Factor) × Normal | 100% Dominant (sf) |
| 3 | Dominant (sf) × Dominant (sf) | 25% Dominant (df) 50% Dominant (sf) 25% Normals |
| 4 | Dominant (sf) × Dominant (df) | 50% Dominant (sf) 50% Dominant (df) |
| 5 | Dominant (df) × Dominant (df) | 100% Dominant (df) |
The production of any of the recessive characters acts as a simple “autosomal recessive gene” and the rules of their reproduction are as follows:
| Pairings | Expectations | |
|---|---|---|
| 1 | Recessive × Normal | 100% Normal/Recessive |
| 2 | Recessive × Normal/Recessive | 50% Recessive 50% Normal/Recessive |
| 3 | Recessive × Recessive | 100% Recessive |
| 4 | Normal/Recessive × Normal/Recessive | 25% Recessive 50% Normal/Recessive 25% Normal |
| 5 | Normal/Recessive × Normal | 50% Normal/Recessive 50% Normal |
From the table above, it can be deduced that there is absolutely no merit in the pairings indicated in rules 4 and 5. A lot of wastage is produced from these pairings and also it is not possible to distinguish the split progeny from the Normals.
The Dark Character
As well as the color gene is dominant or recessive, there is the inherited depth-of-color gene called the “Dark Factor” denoted by the letter “D”. The dark gene is not responsible for color in itself but will alter the depth of color. It works independently of any other color gene. The theory used to establish different shades of color is known as the “Incomplete Dominance Theorem”. The absence of the dark gene is denoted by “dd”, its presence as a single factor by “Dd” and in double factor by “DD”.
| Basic Colour | No Dark Factor (Light Factor) dd | One Dark Factor (Medium Factor) Dd | Two Dark Factors (Dark Factor) DD |
|---|---|---|---|
| Green | Light Green | Dark Green | Olive |
| Blue | Blue | Cobalt | Mauve |
The results and percentages of the mating and production of budgerigars with regard to the dark character is governed by the Mendelian Theory. It is important to realize when giving results in percentages, that the percentages are calculated over a wide number of different pairings of the same combination and not for a single nest. In doing so, the practical results will roughly agree with the theoretical expectation.
Therefore results of cross-mating with various shades of dark genes can be summarized as follows:
| Pairings | Expectations |
|---|---|
| DD × DD | 100 DD |
| DD × Dd | 50% DD 50% Dd |
| DD × dd | 100% Dd |
| Dd × Dd | 25% DD 50% Dd 25% dd |
| Dd × dd | 50% Dd 50% dd |
| dd × dd | 100% dd |
The Sex-linked Recessive Character
One further character worth mentioning is the sex-linked recessive inheritance character. With this character, the relevant genes occur only on the X sex chromosome. As mentioned before, the hen only has one X sex chromosome, hence the hen can either have a sex-linked gene or none at all; it cannot be split for sex-linked genes. Therefore its phenotype must be the same as its genotype. However, the cock can be split for sex-linked genes. This is because the cock birds of the sex-linked varieties can have this gene on either one or both of their sex chromosomes; while the sex-linked hens have only one-half of their sex chromosome pair that can carry the sex-linked color character, the other half determines the actual sex.
The varieties that obey the Sex-Linkage Theory are:
- Opalines
- Cinnamons
- Lutinos and Albinos
- Lacewings
- Slates
- Texas Clearbody (but dominant to Ino)
The five possible pairings with the Sex-Linkage Theory are, using the following abbreviations:
- SL for Sex-Linked
- NL for Non Sex-Linked
- NL/SL for Non Sex-Linked/Sex-Linked
| Pairing | Expectation | |
|---|---|---|
| 1 | SL cock × SL hen | 50% SL cocks 50% SL hens |
| 2 | SL cock × NL hen | 50% NL/SL cocks 50% SL hens |
| 3 | NL cock × SL hen | 50% NL/SL cocks 50% NL hens |
| 4 | NL/SL cock × SL hen | 25% SL cocks 25% NL/SL cocks 25% SL hens 25% NL hens |
| 5 | NL/SL cock × NL hen | 25% NL cocks 25% NL/SL cocks 25% SL hens 25% NL hens |
When two birds with different sex-linked characters are mated, one will act as if it were a non-sex-linked bird, and rule 2 applies. With this knowledge of genetics, we can now perhaps, appreciate the production of various colors and varieties.
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