Yellow of the 1920s and 30s, which were often known by the alternative names of Buttercups or Buttercup Yellows. The popularity of the variety declined after the Lutino became available in the late 1930s. In 1896, George Keartland of the Calvert Expedition to the Great Sandy Desert of Western Australia, observed a yellow budgerigar flying wild in a flock on three occasions. This suggests the Dilute mutation is relatively common in heterozygous form among the wild populations, as two such heterozygous individuals would need to mate in order to produce a visible Dilute. Although the Blue mutation was first seen soon after the first Dilutes, in 1878, and had become established by 1890 in Europe, the first combination of the Blue and Dilute mutations in double homozygous form did not appear until around 1920, some 30 to 40 years later. This combination was the White (known as Silver in Australia), and it was first bred in England by H D Astley in September 1920 from a pair of Skyblues. A White was also reported in Paris in the same year. Whites were never as popular as the Yellows, as it was much harder to approach the exhibition ideal of a pure white bird. Genetics The Dilute mutation is recessive to its wild-type allele, so a bird possessing a single Dilute allele (the heterozygote) is identical in appearance to the wild-type Light Green. That is, the presence of a single wild-type allele is sufficient to permit the production of the normal number of melanin granules. Among the budgerigar fancy such a bird is said to be a Light Green split dilute, usually written Light Green/dilute, although Light Green/yellow has been used. In a bird which has two Dilute alleles (the homozygote) the number of melanin granules is greatly reduced, to around 5% of the normal amount. This results in a much reduced intensity of the black markings, and less absorption of light which passes through the cloudy layer in the medulla of barbs. As this absorption of light is a necessary part of the process which generates the blue colouration the intensity of blue is also greatly reduced. The Dilute mutation is one of a series of multiple alleles at the same locus, called dil+ in the wild-type. The others are the Clearwing (dilcw) and Greywing (dilgw) mutations. The Dilute allele (dild) is recessive to all other alleles at this locus,