Updated December 29, 2020
Extract from my artikcle published in the BVA-International journal from April 2019
In 2017 I was contacted by the owners of ‘Aviario Gonzalez Pradas’. They had a pair of two blue Agapornis personatus. In the first nest there were to their surprise three blue and two green chicks. When they announced this on the internet, almost everybody was laughing. For most breeders this is impossible.
And indeed in 99% of all cases the offspring of blue x blue will be blue, but there are some situation where the progeny phenotypes do not match the Mendelian principles. We know now that genetics is a lot more complicated than Mendel’s laws (genes and alleles), some cases of genetic inheritance can be far more complex than simple Mendelian inheritance.
So this is indeed possible. It is rare but it is possible. It is no joke!!
How is it possible? Well the answer must be sought in what we currently call epigenetics. In epigenetics the scientist researches the question why a gene gets a different expression, without a change in the sequence of the bases. In other words, the same DNA but a different type –.
I definitely do not want to complicate it too much, but one of the most likely trains of thought is that genetically the male is probably green/blue. Remains the question how it is possible that he is phenotypically blue?
The answer is probably: epigenetic gene regulation. This can occur through various mechanisms, but the most likely are methyl labels . Simply put: during the methylation process (methylation takes care of the maintenance and repair of the DNA in all cells and tissue in the body). During this process a methyl group can bind to the DNA. This can affect, or even stop the operation of the gene concerned. Methyl labels are very stable and usually remain during the cell divisions. Only during the meiosis (in the male the creation of sperm cells and in the female the creation of egg cells) will these methyl labels be removed and will the unmutated blue gene again be transmitted in the reproductive cells.
Complicated? Not really, just keep in mind that this is a green split blue male. An extra factor (methyl label) eliminates the functioning of the unmutated bl allele in the DNA of the bird (heterozygotic for blue) and the bird actually becomes homozygotic blue. Because of this the blue bird cannot produce psittacine and he is therefore blue. But his DNA (genome) is just a normal green split blue. During the creation of the sperm cells the label is removed and as a result an unmutated bl gene is passed to the young.
How and why this happens and what is the cause remains a mystery for now. Daily new research is conducted, new answers are found and definitely new questions. It never stops.
I hope you now realize that in genetics nothing is as simple as it seems or that the aviculturists would want and foremost there are still a lot of questions. Genetics is much more than just genes, alleles and loci.
 A. Petronis, J. L. Kennedy, en A. D. Paterson, “Genetic anticipation: fact or artifact, genetics or epigenetics?”, The Lancet, vol. 350, nr. 9088, pp. 1403–1404, 1997.
 R. Jaenisch en A. Bird, “Epigenetic regulation of gene expression: how the genome integrates intrinsic and environmental signals”, Nat. Genet., vol. 33, pp. 245–254, mrt. 2003.
 Richard C. Francis, Epigenetics: How environment shapes our genes. WW Norton & Company, 2011.