The English Grey budgerigar mutation is one of approximately 30 mutations affecting the colour of budgerigars. It appeared briefly in the 1930s but was lost shortly after and until recently was believed to be no longer extant. However, the appearance of the Anthracite budgerigar mutation in 1998 with a seemingly identical appearance and identical genetic behaviour, insofar as can now be determined, suggests the mutation may have been regained.
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Appearance
The English Grey variety is very similar in appearance to the Dominant Grey. The original reports described the English Grey in the Blue series as being slate grey with a dark grey to black rump, a white mask carrying black or almost black spots and dark grey, bordering on black, cheek patches, black and white wings and an intense black tail. When compared side by side with the Dominant Grey the English Grey was seen to be generally darker, with the cheek patches being particularly so. Both Dominant and English Greys were produced in the three shades of Dark, with a noticeable though slight difference in shade. The English Grey Skyblue was stated to be darker than the Australian Grey Mauve.
The English Grey of the Green series was grey-green in colour and very like today's Grey-Green of the Dominant Grey variety.
Genetics
Due to the similar appearance of the English and Dominant Greys they were first thought to be the same variety, although the reported breeding results showed clearly that Mrs Harrison's birds were the result of a simple dominant gene whereas Mr Brooks' Greys could not be so easily explained. The evidence for the different breeding behaviour gradually built up, and by 1938 there was no doubt that the two mutations were distinct.
Mr Brooks published clear and detailed breeding records of all his English Grey pairings between 1933 and 1938 in the Budgerigar Bulletin. The first English Grey was bred from a pair of Cobalts, and a similar pairing of two Cobalts later bred further English Greys. This shows that the mutation cannot be dominant, and led many, initially including Mr Brooks himself, to assume that the mutation was therefore recessive. Indeed, the mutation is often called the Recessive Grey, and both Taylor and Warner and Elliot and Brooks give breeding expectations based on the assumption of simple recessive inheritance.
However, the breeding results reported by Mr Brooks show that the mutation cannot be a simple recessive. For example, he reports that a pair of English Greys bred a Skyblue and a Cobalt, and an English Grey paired to an unrelated bird bred two English Greys. A further anomaly to be explained is the lack of Mauves from the Cobalt x Cobalt pairings.
As the mutation discovered by Mr Brooks is no longer extant its genetic behaviour cannot be investigated further, but the extensive breeding results of Mr Brooks do permit some speculation about its nature. The breeding behaviour was initially considered by Dr H Steiner and was later examined in detail by both Prof F A E Crewe and Dr T Daniels. All these writers come to a similar conclusion, namely that the reported breeding results can only be explained by assuming the English Grey gene produces a similar effect to, and interacts with, the Dark gene. Following Dr Steiner, Dr Daniels proposed that the English Grey gene is autosomal and incompletely dominant to its wild-type allele. In the visual English Grey the depth of the cloudy zone is known to be very small, just 1-2μ. He suggested that the English Grey gene reduces the depth of the cloudy zone rather as the Dark gene does, although perhaps to a slightly greater degree. A blue series bird heterozygous for English Grey would have an appearance very similar to a Cobalt, maybe a little darker, and a bird homozygous for English Grey would appear grey, as described in Appearance above. Also, on this hypothesis, a bird with one English Grey allele and one or two Dark alleles would also appear grey. Dr Daniels states that this hypothesis satisfactorily explains all the breeding results reported by Mr Brooks, including the lack of Mauves. Interestingly, the behaviour and appearance of the recently discovered Anthracite mutation (see Historical notes above) appears to follow exactly this hypothesis.