I find that many breeders of budgerigars, although fascinated by the subject of hereditary, seem to be at a loss to understand how the different characters operate. Even breeders specialising in one colour phase are puzzled by the continued, if erratic, appearance of opalines, whites, cinnamons, albinos etc, turning up in their clutches of young. To help breeders understand hereditary as governed by the Mendelian Laws I will in the following paragraphs endeavour to explain simply the meaning of these Laws.
It all began well over 100 years ago when an Austrian monk, Gregor Mendel, kept records of experiments with the crossing of different strains of peas and other forms of garden produce. As his records grew it became clear to him that by crossing and re-crossing his plants the results formed definite patterns that were consistent when taken over a considerable number of the same kind of crosses. Gregor Mendel published his first scientific observations on his vast number of experiments during 1866 but they did not appear to make an impression on other scientific observers of that time. At the beginning of the 1900s Professor DeVries of Amsterdam reviewed Mendels old writings and realised the importance of his discovery. Shortly after this, in 1905, Professor R C Punnitt of Cambridge University produced his book "Mendelium" which was followed a few years later by Professor W Batesons (Cambridge University) book entitled "Mendels Principles of Hereditary", thereby making this valuable information available to all plant and animal breeders.
It was not until about 1924 that the importance of Mendelism to breeders of budgerigars was realised through the efforts of Dr H Duncker and General Consul C Cremer, whose work culminated in the publication of the Book of Budgerigar Matings. Prior to the publication of the material that went into this book the colour breeding of budgerigars had been a matter of chance. Today whilst many breeders take for granted the knowledge of Mendelism it was then a matter of great disagreement and argument.
What Mendel discovered was that all colours, shape, sizes etc in living things were controlled and handed down by a special mechanism and this Dr Duncker applied to budgerigars. This is briefly how it works - each egg produced by a hen carries a complete half set of the hens chromosomes and the sperm of the cock by which the egg is fertilized carries a complete half set of his chromosomes thereby making a complete full new set and a new bird. On each chromosome pair are smaller bodies (genes) which control all aspects of a bird, its colour, shape, bone structure, length of feather, size etc. These genes are of a definite hereditary pattern and can be dominant, recessive, sex-linked or intermediate. Once the hereditary nature of a character has been established it is quite simple to work out the expectations of that colour when paired to other colours.
A pure dominant character is one that paired to a different character will produce young just like itself in outward appearance. These young will have the dominant character only on one half of a chromosome pair but because it is dominant it shows in the plumage. Such birds are generally known as single character birds and are represented in budgerigars by the following colours - grey (grey green), Australian pied, violet and clear flight (both continental and Australian). Any birds that have a dominant colour character in their genetical make-up in either a single or double quantity must show the colour in their plumage.
There is however, an exception to this rule where a masking character like the albino (lutino) is also involved. In such cases a bird could be an Australian pied blue, a grey cobalt or a violet mauve, but because they have the albino character as well which has a masking effect they will all look to be albinos. As a point of interest the dominant colours already mentioned in this paragraph cannot appear in the nest from any ordinary pairs. Should any dominant colours appear then one of the parent birds must have that character.
An error of identification that is frequently made is when it is said that greys have appeared in nests of cobalt pairs or from a dark green/blue and cobalt pair. Such mistakes are brought about by confusing mauves with greys. Many young mauves have quite a leaden grey body colour and to the uninformed do appear to be actually grey birds. A good method of identification is by the long tail feathers, which are black in greys (grey greens) and dark blue in mauves (olive greens). No mauves (olive greens) ever have a black tail and no grey (grey greens) ever have a dark blue tail. There are of course other distinguishing features but the colour of the long tail feathers is the simplest and most sure.
A pure recessive character is one that when paired to a different character will produce young that are unlike itself. When two different recessive characters are mated together one may be dominant to the other or they may produce an entirely different colour shade. For instance, a recessive pied paired to a recessive white will give all normally coloured young, on the other hand if a pure recessive clearwing is paired to a recessive white all their young will be clearwings showing the recessive clearwing is dominant to the recessive white.
The recessive group of colours is white (yellow), fallow, greywing, Danish pied and clearwing. When any of these colours are paired to a pure normal all the young they produce will be normal in colour and "split" for the recessive colour. For example, if a Danish pied (which has the character on both halves of a chromosome pair) is paired to a normal (which has a normal character on both halves of a chromosome pair) all the young they produce will inherit half a chromosome pair from each parent. So, because the normal character is dominant to the recessive one all the young will have the normal character in single quantity and show that colour in their plumage. The other half chromosome pair will have the recessive character and when such a bird is given a correct mate they will reproduce that recessive character.
From these examples it will be seen how easy it is to unknowingly introduce a recessive character into a normal strain. A breeder of say light greens wishes to add a special quality to his stud and buys the necessary cock (or hen) which is paired to some of his own birds. This new bird could be "split" for say yellow and therefore half of its progeny could likewise be "split" yellow. In due course related birds are matched together and should two happen to be "split" yellow then a percentage of actual yellow birds will arise much to the breeders surprise.
Sex Linked Character
The sex-linked characters also have a habit of turning up to mystify the breeder because cock birds alone can carry a sex-linked character and can produce sex-linked hens when mated to a normal. The sex-linked colours are cinnamon, albino (lutino), opaline, lacewing and slate, and are all inherited by the same method. All sex-linked characters are recessive but because they are carried on the special sex forming chromosome pair they reproduce quite differently to the ordinary recessive characters.
Cock birds of the sex-linked colours must have the character on both of their sex chromosome pair but hen birds only have one half of their sex chromosome pair that can carry colour characters, the other half only determines their sex. Any coloured cock birds can therefore be "split" for a sex-linked character but hen birds just cannot, as if a sex-linked character is on the colour carrying half of their sex chromosome pair it must show in their plumage. When two different sex-linked characters are paired together one appears to act as though it were a normal. For example, a cinnamon cock to an opaline hen will give normal/cinnamon opaline cocks and cinnamon hens or an opaline cock to a cinnamon hen will give normal/cinnamon opaline cocks and opaline hens.
The only intermediate character as yet known to budgerigars is the dark character which in its single state gives the dark green and cobalt shades and in its double form the olive green and mauve shades. No bird can be "split" for the dark character and a bird carrying it in either single or double quantity must show it in its plumage in its appropriate depth of shade. The dark character is inherited quite independently of any other colour characters a bird may carry.