Dr John Pilkington answers questions on Genetics

Answer: Three different mutations of Normal genes have occurred to produce three different Fallow varieties, namely English, German and Scottish. Whilst the appearance (phenotype) of the three different varieties is almost identical, it must be remembered that the genetic make-up (genotype) of each of the varieties is completely different. None of the three Fallow varieties is sex-linked; they are all autosomal recessives. (An autosome is any chromosome other than the sex chromosomes]

In Budgerigars, the most common example of an autosomal recessive gene is the Blue mutation. All of the three Fallow genes act in exactly the same pattern of inheritance in relation to the Normal (non-Fallow) variety, as does the Blue gene to the Green gene, and each of the Fallow genes operate independently from the other Fallow gene systems.

If the Normal (non-Fallow) gene is represented as F (dominant), and the Fallow gene is represented as f then the breeding patterns of the Fallow are as follows:

Mating No.	Pairing	Expectation
1	FF × FF	100% FF
2	FF× Ff	50% FF 50% Ff
3	FF × ff	100% Ff
4	Ff × Ff	25% FF 50% Ff 25% ff
5	Ff × ff	50% Ff 50% ff
6	ff × ff	100% ff

Now, with specific regard to your question; the mating of your original Fallow cock could fit into mating 3, 5 or 6 in the above table. Since you say that you bred from this bird for several years without ever producing a single Fallow offspring, then I am assuming (rightly or wrongly) that the fallow cock produced in excess of offspring; it is therefore obvious that you never mated the Fallow cock to a Fallow hen as in mating 6 above. In the case of mating 5; the offspring have a 50/50 chance of being Fallow, whilst it is possible to toss a coin for more than 20 times without falling as "Head" it is an exceptionally unlikely occurrence, therefore it is very unlikely that you ever mated your Fallow cock to a Normal/Fallow hen as in mating 5. I, therefore, feel confident to say that your Fallow cock produced all offspring as in mating 3, i.e., all Normal/Fallow splits.

Although you say that you mated the offspring of the original Fallow cock to each other, you give no indication of the numbers of offspring these pairings produced. Since all the offspring of the Fallow cock have the Ff genotype, then the pairing of the offspring to each other is mating 4, as above, which would, on average, produce 25% Fallows. The fact that you failed to produce any Fallows from this pairing suggests to me that you did not persevere with these matings, nor did you produce many offspring. It would have been far better to mate the Fallow cock to several of his best daughters, as in mating 5; this system would have, on average, produced 50% Fallow offspring

If you persevere with the interbreeding of your known Fallow bloodlines then you will eventually produce some Fallows. If you are desiring to produce Fallows in quantity, then follow matings 5 and 6 as above. If you are trying to produce show quality Fallows, then mate some of your best Fallows to some of your best Normals in order to produce Normal/Fallow splits which can be used in matings 4 and 5 to produce Fallows.

I must confess that I am unaware whether all three varieties of Fallow are still in existence, but if more than one variety still exists, then the breeding patterns described in this answer, apply to a single Fallow variety only. Two Fallow birds of different Fallow varieties to each other, when mated, would produce all Normal / Fallow offspring because the two different genes act independently of each other.

Editors' note: I regret that Dr Pilkington is unable to answer any further questions.

Question to Dr John Pilkington from Robin