An allele is one of two or more versions of a gene. An individual inherits two alleles for each gene, one from each parent. If the two alleles are the same, the individual is homozygous for that gene. If the alleles are different, the individual is heterozygous.
Autosomal means that the gene in question is located on one of the numbered, or non-sex, chromosomes. The sex chromosomes are referred to as X and Y.
Chromosomes are thread-like structures located inside the nucleus of animal and plant cells. Each chromosome is made of protein and a single molecule of deoxyribonucleic acid (DNA). Passed from parents to offspring, DNA contains the specific instructions that make each type of living creature unique.
DNA is the abbreviation of deoxyribonucleic acid, an organic chemical of complex molecular structure that is found in all cells including many viruses. DNA codes genetic information for the transmission of inherited traits.
The chemical DNA was first discovered in 1869, but its role in genetic inheritance was not demonstrated until 1943.
The resultant young in terms of colour and variety can be calculated. These are known as breeding expectations.
Genes carry the information that determines traits, which are features or characteristics that are passed on to the offspring.
Genetics and Colour Breeding of Budgerigars
by Ghalib Al-Nasser
Many fanciers consider the subject of genetics too complicated and shy away from it, but as breeders of budgerigars, and with the number of varieties and colours available to us, it is important that we have some knowledge about colour production. This article only touches on the basis of genetics with its colour and variety production and is aimed mostly for the newcomers to the fancy rather than established fanciers.
A greater depth of knowledge was obtained when Gregor Mendel, an Austrian monk, published his scientific observations during 1866. After experimenting for many years by cross-pollinating Pea plants and noting the characteristics of the resulting seedlings, he established the first laws for the science of heredity which is now known as genetics.
Mendel’s Theory of Inheritance was the basis which many scientists followed during the late nineteenth century and early part of this century, to apply both to plant and animal production. It was not until Dr H Duncker and C H Cremer of Bremen, Germany, applied Mendel’s theories to budgerigars, in about 1920, that these laws became universally accepted by the fancy. These laws are used for budgerigar production to predict the colour characteristics of the offspring from any particular pairing.
Each bird has its own genetic code contained in its own unique set of 26 microscopic bodies known as “Chromosomes”. This set of chromosomes is duplicated in each cell of the bird. Each chromosome of the set consists of a different string of “Genes” or “Factors” (which was Mendel’s term) which controls the various hereditary characters of the bird. These hereditary characters include size of spots and head, shape, type, colour, sex, bone structure, length and texture of feather, etc. The 26 chromosomes are associated in 13 pairs of equal length (except for that chromosome pair that controls the sex of the bird). Corresponding (in position) genes on each chromosome, in a pair are called “Allelmorphs” or “Alleles”.
An allele pair may be identical or different and how they interact controls one of the characters of the bird. If a pair of alleles are identical the bird is said to be “Homozygous” for the particular gene (or the gene is said to be present as a “Double Factor”); if these are different, the bird is said to be “Heterozygous”. for each gene (or each gene is said to be present as a “Single” Factor”)
A “mutation” is a genetic accident where a gene or a set of genes changed. However, a viable mutation is a rare event; which is why in the wild budgerigar population, the original gene is most common. Thus the original gene is called the “Wild-Type” gene and any departure from the wild-type is called a mutation. A mutant can differ in some major ways or minor ways from the wild-type. Obviously, some mutations can occur more easily (and hence more often) than others.
On mating, (hopefully), the sperm from the cock fertilises the ovum of the hen to produce the egg. The sperm and the ovum are single cells which contain only one chromosome of each chromosome pair (which half of a chromosome pair that gets included in the sperm or ovum is a matter of chance). Thus the fertilised egg has a full set of chromosomes, with each chromosome pair having a chromosome from each parent. Thus each parent’s genes contribute towards every characteristic of the chick.
There is a genetic complication to this process called “Crossing-over”. This occurs during that part of the production of the sperm and the ovum when the chromosomes pair up and lie parallel to each other. At this stage, a pair of chromosomes can become entangled at certain points rather like a pair of long balloons twisted together. The segments between these points can then exchange or “Cross-over”. Thus a chromosome in a sperm or an ovum can be a mixture of the chromosome pairs of either parent. Crossing-over, for a particular pair of chromosomes, tends to occur at the same locations. This means that genes found on the same segment will always be associated or “linked” with each other.
The Sex Character
As mentioned earlier, the pair of chromosomes that control the sex are not of equal lengths. The sex-chromosomes of the hen, denoted by the letters X and Y, are of different lengths, Y being the shorter member of that pair and carrying no sex genes. The cock will have a pair of sex-chromosomes of the same length, referred to as XX. Whenever a cock and a hen are paired together they should always produce equal numbers of the two sexes, on average. This is because, on mating, the cock’s sperm cell carrying half a set of chromosome pairs, combines with the hen’s egg cell also containing half a set of chromosome pairs, forming a completely new whole set of chromosomes.
The Split Character
A pair of birds of one colour “Phenotype” may produce other colours if their genetic make-up “Genotype” differs from their actual phenotypic appearance. These are impure birds and commonly known as “splits” indicated by an oblique line”/”.
The Dominant and Recessive Character
The colour genes are either “dominant” (e.g. green) or “recessive” (e.g., blue). A bird carrying the dominant gene on one half of the chromosome pair will be coloured as if it was carried on both halves. The recessive colours will only show themselves if they are carried on both halves of the chromosome pairs. The colour genes can be carried in a number of different chromosome pairs. A bird can then be one dominant colour and carry in its genetic make-up one or more recessive colours in a hidden form, but not vice-versa. Thus, one can say that in the simplest form of interaction of two dissimilar alleles, one is dominant and the other recessive, that is, the dominant allele controls the character.
For instance, when the green gene (i.e., the gene with the code for green feathers) and the blue gene are on allele pair, the bird is green because the green gene is dominant with respect to the blue gene. Because of the interaction of dissimilar alleles, a bird’s physical make-up (its phenotype) may be different from its genetic make-up (its genotype). In the colour inheritance, the following groupings can be made:
The dominant mutations are:
Greens (All Forms)
Yellow Faces (to the blue series)
The recessive mutations are:
Blues (All Forms)
The gene of a dominant character may be present as a single or double factor, 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” for a dominant character.
The various rules that govern the inheritance of the dominant character irrespective of the actual colour are:
|Pairings and Expectations – Dominant
|Dominant (Single factor) × Normal
|50% Dominant (sf)
|Dominant (Double Factor) × Normal
|100% Dominant (sf)
|Dominant (sf) × Dominant (sf)
|25% Dominant (df)
50% Dominant (sf)
|Dominant (sf) × Dominant (df)
|50% Dominant (sf)
50% Dominant (df)
|Dominant (df) × Dominant (df)
|100% Dominant (df)
The production of any of the recessive characters act as a simple “autosomal recessive gene”and the rules of their reproduction are as follows:
|Pairings and Expectations – Recessive
|Recessive × Normal
|Recessive × Normal/Recessive
|Recessive × Recessive
|Normal/Recessive × 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 colour gene being dominant or recessive, there is the inherited depth-of-colour gene call the “Dark Factor” and denoted by the letter “D”. The dark gene is not responsible for colour in itself but will alter the depth of colour. It works independently of any other colour gene. The theory used to establish different shades of colour is known as the “Incomplete Dominance Theorem”. The absence of the dark gene is denoted by “dd”, it’s presence as a single factor by “Dd”and in double factor by “DD”.
|Presence of the Dark Factor
|No Dark Factor
|One Dark Factor
|Two Dark Factors
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 realise 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 summarised as follows:
|Pairings and Expectations – Dark Factor
|DD × DD
|DD × Dd
|DD × dd
|Dd × Dd
|Dd × dd
|dd × 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 colour character, the other half determines the actual sex.
The varieties that obey the Sex-Linkage Theory are:
Lutinos and Albinos
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
|Pairings with Sex-Linkage Theory
|SL cock × SL hen
|50% SL cocks
50% SL hens
|SL cock × NL hen
|50% NL/SL cocks
50% SL hens
|NL cock × SL hen
|50% NL/SL cocks
50% NL hens
|NL/SL cock × SL hen
|25% SL cocks
25% NL/SL cocks
25% SL hens
25% NL hens
|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 the various colours and varieties.
Genetic linkage is the tendency of DNA sequences that are close together on a chromosome to be inherited together during the reproductive dividing of cells. Two genetic markers that are physically near to each other are unlikely to be separated and are said to be more linked than markers that are far apart.
German speaking Gregor Johann Mendel (1822-1884) was a meteorologist, mathematician, biologist, Augustinian friar and Abbot of St. Thomas’ Abbey in Brno, Margraviate of Moravia.
Through his work on pea plants, he discovered the fundamental laws of inheritance, deducting that genes come in pairs and are inherited as distinct units, one from each parent. Mendel tracked the segregation of parental genes and their appearance in the offspring as dominant or recessive traits. He recognized the mathematical patterns of inheritance from one generation to the next. Mendel’s Laws of Heredity are usually stated as:
- The Law of Segregation: Each inherited trait is defined by a gene pair. Parental genes are randomly separated to the sex cells so that sex cells contain only one gene of the pair. Offspring therefore inherit one genetic allele from each parent when sex cells unite in fertilization.
- The Law of Independent Assortment: Genes for different traits are sorted separately from one another so that the inheritance of one trait is not dependent on the inheritance of another.
- The Law of Dominance: An organism with alternate forms of a gene will express the form that is dominant.
The genetic experiments Mendel did with pea plants took him eight years (1856-1863) and he published his results in 1865. During this time, Mendel grew over 10,000 pea plants, keeping track of progeny number and type. Mendel’s work and his Laws of Inheritance were not appreciated in his time. It wasn’t until 1900, after the rediscovery of his Laws, that his experimental results were understood.
Split or / is the term used to indicate that a bird is carrying a variety or colour in a genetically hidden format that requires it in a double dose to make it visible. The exception would be sex-linked splits which is only possible with cock birds.
Human and most mammal male cells have one X, and a Y chromosome. Birds also have sex chromosomes, but they act in completely the opposite way. Male birds have X and Y chromosomes, and females have a single Y chromosome.
A zygote is a fertilised egg cell that results from the union of a female gamete (egg, or ovum) with a male gamete (sperm). In the embryonic development, the zygote stage is brief and is followed by cleavage, when the single cell becomes subdivided into smaller cells.