Why the father decides his daughters' colour
Two things about parrot breeding run opposite to what people expect, and both come from the same fact. The mother determines whether each chick is a son or a daughter. The father determines every daughter's cinnamon and yellow-sided colour, completely and by himself. Neither is a rule of thumb — they are consequences of how bird chromosomes are arranged, and the colour calculator applies both to whatever pairing you give it.
Predictions are statistical. Percentages describe a theoretical infinite clutch, not the four eggs in front of you, and mutation names vary between breeders and countries. Why this matters more than it sounds.
Birds are ZW, not XY
In mammals a female is XX and a male is XY, so the father's sperm decides the sex. Almost every article you will read about genetics uses that as the worked example, and almost every misconception in parrot forums comes from carrying it across unchanged.
Birds are the other way round. The cock has two matching sex chromosomes — ZZ — and the hen has two different ones, ZW. So it is the hen who has a choice to make: whichever chromosome she puts into an egg decides what hatches. Z gives a son, W gives a daughter.
Nothing the cock does affects the ratio. He has only Zs to give. If a clutch comes out four hens and no cocks, that is chance, not the father.
The W chromosome carries no colour
A daughter is ZW: one Z from her father, the W from her mother. Cinnamon and yellow-sided both live on the Z, and the W has nothing to say about either of them.
So a daughter's sex-linked colour comes from her father, and from nobody else. Put the other way round, and this is the single most useful sentence in green-cheek genetics: a hen's own cinnamon or yellow-sided colouring has no effect whatsoever on her daughters. You can swap the mother for any colour you like and the daughters come out the same. That is one of the very few statements here that needs no hedging.
The pairing that disappoints people
Take a cinnamon hen and pair her with a plain green cock who is split to nothing. A reasonable person expects some cinnamon chicks. There are none.
- Daughters: normal green. Their single Z came from the plain father, so there is no cinnamon in them at all.
- Sons: normal green, every one of them split to cinnamon. They took one Z from their mother, and hers carries it.
The cinnamon has not been lost. It has gone sideways into the sons, invisibly, and it will come back out through them in the next generation. Breeders who understand this make the pairing on purpose; breeders who do not make it once and conclude the hen is not really cinnamon.
The same two colours, the other way round
Now give the cinnamon to the cock instead, and pair him with a plain green hen:
- Daughters: every one cinnamon.
- Sons: every one normal green, split to cinnamon.
Identical colours in the aviary, opposite sexes, and a completely different nest. That is the practical rule worth keeping: sex-linked hens come from the father. If you want cinnamon hens, you need a cinnamon or split cock. A cinnamon hen will never give you one.
Two things you can read off a bird for free
Because a hen cannot hide a sex-linked gene — she has only one Z, so there is nothing to hide it behind — the arithmetic runs backwards as well:
- Every cinnamon cock had a cinnamon mother. He needs the gene on both of his Zs, one of which came from her, and a hen carrying it always shows it.
- Every cinnamon hen had a father who was cinnamon or split to cinnamon. Her one Z came from him.
Both hold for yellow-sided too, and for any other sex-linked mutation. They are useful when a pedigree is missing: the bird in front of you is telling you something about a parent you never saw.
Why "split cinnamon hen" is a contradiction
Same reason, stated once more because it is sold so often. Being split means carrying a hidden second copy, and a hen has no second Z to carry it on. There is a fuller article on what split means, including the two genes where hens can be split perfectly well.
Try it on your own pair
Describe both birds the way you see them and the calculator separates sons from daughters, because for these colours they are genuinely different questions.