Sex-linked mutations, explained
without the jargon
Sex-linked inheritance is the part of bird genetics that makes people give up. It is actually the easiest part, once you know that hens carry one copy of the relevant chromosome and cocks carry two.
- In birds the hen determines sex, the opposite of mammals
- A hen has only one Z chromosome, so she cannot be split for a sex-linked mutation
- Any sex-linked hen chick shows the mutation, which lets you sex some clutches at feathering
- Ino, cinnamon, opaline and pallid are sex-linked in lovebirds and budgerigars
Birds are the other way round
In mammals the male carries two different sex chromosomes and determines the sex of the offspring. In birds it is reversed. The cock carries two identical sex chromosomes, called ZZ. The hen carries two different ones, ZW. The hen determines the sex of the chick.
Everything about sex-linked inheritance follows from one fact: the mutations we call sex-linked sit on the Z chromosome, and the W carries no matching copy. So a cock has two chances to carry the gene and a hen has exactly one.
A hen has a single Z. She either shows the mutation or she does not carry it at all. There is no hidden copy, which means a hen can never be split for a sex-linked mutation.
Which mutations are sex-linked
In Fischer's lovebirds and in budgerigars the commonly kept sex-linked mutations are the same short list.
| Mutation | Also called | Species |
|---|---|---|
| Ino | Lutino, albino, creamino | Both |
| Cinnamon | Cinnamon, sometimes fallow-like in appearance | Both |
| Opaline | Opaline, rosy in some descriptions | Both |
| Pallid | Australian cinnamon in budgerigars | Both |
| Slate | Slate | Budgerigar |
Naming follows Terry Martin, A Guide to Colour Mutations and Genetics in Parrots (2002).
Everything else you commonly meet is autosomal: dominant, co-dominant or recessive, but not tied to sex. Dark factor, violet, pied, dilute, the fallows and parblue all behave differently and are worth learning separately.
Reading a pairing without a chart
Because hens have one copy and cocks have two, four pairings cover almost everything you will do. Work through them once and you will not need a chart again.
Normal cock to sex-linked hen. No visual chicks at all. Every cock chick is split. Every hen chick is normal and carries nothing. This is the pairing that quietly builds your split stock, and it is the one people misread as a failure.
Split cock to normal hen. Half the cock chicks are split, half are normal. Half the hen chicks are visual, half are normal. This is the pairing that lets you sex part of the clutch on sight, because any chick showing the mutation is a hen.
Split cock to sex-linked hen. Every category appears: visual cocks, split cocks, visual hens, normal hens. Roughly a quarter each.
Sex-linked cock to sex-linked hen. Every chick is visual, both sexes.
A visual sex-linked chick from a normal-looking hen is a hen chick. That is not a rule of thumb, it is a consequence of the hen having only one Z to give.
What this is worth in practice
Three practical uses, in rough order of value.
First, early sexing. In pairings where visual chicks can only be hens, you know the sex of part of the clutch at feathering rather than at maturity or by DNA test. For a breeder planning next season's cages that is months of lead time.
Second, split value. A split cock carries breeding potential that is invisible on the bird. That value only exists if you can prove it, which means recorded parents. An undocumented claim of split is worth nothing to a careful buyer, and should be.
Third, avoiding a common waste. Pairing a normal cock to a sex-linked hen produces no visual chicks whatsoever. Breeders who expected visuals conclude the pairing failed and split it up, when in fact they just produced a cage full of valuable split cocks.
Free lovebird genetics calculator, sex-linked aware. No account, no sign-up.
The mistakes that cost the most
Recording a hen as split. It is not possible. If your records show a split hen for ino, cinnamon, opaline or pallid, either the hen is actually visual and was written down wrong, or the mutation is not the one you think it is.
Assuming a visual chick proves the cock is visual. A split cock produces visual hen chicks. Seeing a lutino chick tells you the father carries it, not that he shows it.
Treating cinnamon and fallow as interchangeable. They can look similar in a young bird and they inherit completely differently. Cinnamon is sex-linked, the fallows are recessive. Confusing them will make every prediction downstream wrong.
Losing the parents. Sex-linked reasoning is entirely pedigree reasoning. A bird with no recorded parents carries no provable splits, whatever it actually has in its genome.
Common questions
Can a hen be split for lutino?
No. Lutino is a sex-linked mutation and a hen carries only one Z chromosome, so she either shows the mutation or does not carry it. A record showing a split hen for any sex-linked mutation is an error.
How can I tell chick sex from a sex-linked pairing?
In a pairing of a split cock to a normal hen, every chick that visually shows the mutation is a hen. The cock chicks in that pairing are either split or normal, never visual. This lets you sex part of the clutch at feathering.
Which lovebird mutations are sex-linked?
Ino (which produces lutino, albino and creamino depending on the base), cinnamon, opaline and pallid. Dark factor, violet, pied, dilute, the fallows and parblue are not sex-linked and follow different rules.
Why did my normal cock to lutino hen pairing produce no lutinos?
That is the expected result, not a failure. The hen passes her single Z, carrying the mutation, only to her sons, who become split. The daughters receive their Z from the normal father. Every cock chick from that pairing is split for lutino.
Is sex-linked inheritance the same in budgerigars?
Yes. The mechanism is identical across parrot species. Ino, cinnamon, opaline and pallid behave the same way in budgerigars, with slate additionally sex-linked in that species.