Chicken Genetics Calculator

The sex-link rule everybody gets backwards

A sex-link cross only works in one direction. Not "works better" — works, or does nothing at all. Getting the parents the wrong way round is the single most common mistake in backyard breeding, and it costs you a whole hatch before you find out.

The reason is chromosomes. Hens are ZW and roosters are ZZ, which is the reverse of mammals. A pullet inherits her single Z from her father and her W from her mother — so daughters show whatever the father carries. Sons take one Z from each parent and show whichever allele is dominant.

Put those together and the requirement falls out: you need a recessive father and a dominant mother. Daughters then show his recessive form, sons show her dominant one, and the two are visibly different in the hatcher.

The same two breeds, both ways round

Same two breeds. Same pairing. One of them hands you a box you can sort on hatch day; the other hands you a box of chicks that all look the same and a four-month wait. A Rhode Island Red cockerel over a Rhode Island White hen is a red sex link — the reverse is just a cross.

Worth knowing before you plan around it: sex-links are hybrids. Breed two of them together and the trick does not carry into the next generation, because the tidy one-dominant-parent arrangement is gone.

Egg colour: one clean gene, one messy one

Blue shell is a single dominant gene, which makes it genuinely predictable. Two split parents produce 75% blue-base offspring; a split bird crossed to one with no blue gene gives 50%. Only a bird carrying two copies breeds true — which is exactly why hatchery birds sold as "Ameraucana" so often turn out to be Easter Eggers laying whatever they feel like.

Brown is a different animal. It is polygenic — several genes stacking to produce everything from a faint tint to Marans chocolate — so it blends rather than segregating into neat quarters. This calculator treats it that way and deliberately rounds the result down, because shell colour reliably underdelivers: hatchery Marans lay lighter than their parents, and every hen's eggs fade as a laying cycle goes on.

The two stack rather than mixing, because blue goes right through the shell while brown is painted on at the end. Blue base plus ordinary brown reads green; blue base plus a dark-brown line behind it reads olive. That is all an Olive Egger is.

One thing that surprises people: the rooster decides as much as the hen does. He carries and passes shell-colour genes he will never demonstrate, so a cockerel out of a blue-egg line is worth as much to a breeding project as the hens are.

Frequently asked questions

What is a sex-link chicken cross?

A cross where cockerels and pullets hatch looking visibly different, so you can sex them on day one without vent sexing. It works by using a gene carried on the Z chromosome — barring for black sex links, silver/gold for red sex links. The chicks are hybrids, so the trick works once and does not carry into the next generation.

Why does a sex-link cross only work in one direction?

Because of how chicken sex chromosomes work. Hens are ZW and roosters are ZZ, so a pullet inherits her single Z from her FATHER and her W from her mother. That means daughters show whatever the father carries. For the chicks to differ by sex you need a recessive father and a dominant mother: daughters show his recessive form, sons pick up her dominant one. Swap the parents and every chick shows the dominant phenotype — same two breeds, same pairing, completely useless for sexing.

What makes a red sex link?

A gold (red) rooster over a silver (white) hen. The classic pairing is a Rhode Island Red or New Hampshire cockerel over a Rhode Island White, Delaware or Silver Laced Wyandotte hen. Pullets hatch reddish-buff and cockerels hatch pale — different enough to sort into two boxes at hatch. The reciprocal cross, a silver rooster over a gold hen, gives you nothing.

What makes a black sex link?

A non-barred rooster over a barred hen — most often a Rhode Island Red cockerel over a Barred Rock hen. Cockerels inherit barring from their mother and hatch with a pale head spot; pullets hatch solid black. Again the direction matters: a Barred Rock cockerel over a red hen produces barred chicks of both sexes and tells you nothing.

How is blue egg colour inherited?

Blue shell is a single dominant gene, so one copy is enough to produce a blue base and it behaves as a straightforward Punnett square. Two split parents give roughly 75 percent blue-base offspring; a split parent crossed to a bird with no blue gene gives 50 percent. Only a bird with two copies breeds true, which is why hatchery "Ameraucanas" so often turn out to be Easter Eggers laying whatever they please.

How do you get green and olive eggs?

By stacking brown on top of blue. Blue pigment goes right through the shell, while brown is applied as a coating at the end of shell formation, so the two layer rather than mix. Blue base plus an ordinary brown overlay reads green; blue base plus a dark brown overlay — a Marans or Welsummer in the background — reads olive. That is exactly what an Olive Egger is.

Does the rooster affect egg colour?

Entirely, even though he never lays one. Shell-colour genes are carried and passed by both parents, so a cockerel from a blue-egg line is as valuable to a blue-egg breeding project as the hens are. This surprises people, and it is why buying a good cockerel matters if you are breeding for shell colour rather than just collecting it.

Why is the brown prediction less exact than the blue one?

Because brown is polygenic — several genes stacking to produce everything from a faint tint to Marans chocolate — while blue is a single gene. A single gene segregates predictably and can be drawn as a Punnett square; polygenic traits blend. This calculator treats brown as blending and rounds it downward, because shell colour reliably comes out lighter than the parents and fades across a laying cycle.