Sex-linked Peafowl Genetics

In peafowl, the sex-linked mutations travel only on the Z chromosome, never the W chromosome. Hens are hemizygous and so they cannot be het for a sex-linked mutation, because they only have one Z- the other is "missing." This is why you must track the sex chromosome not just the mutation. To keep track of males vs females, we will track the chromosomes (Z and W), with the gene attached in parenthesis.

• For example, the purple gene would be noted as Z(pl). Z for the chromosome, "pl" for the gene, parenthesis to attach them together.
  • Normally attachment would be indicated with a carrot (ie, Z^pl) or superscript, but my host site doesn't support superscript and we have a unique case where multiple genes are present on the same chromosome and would get confusing with the colon (for example, Z(pl:c)Z(pl:c) written as Z^pl:cZ^pl:c might look like there are three different sets of genes, or Z^pl:cZ^c:brbr might look like the brbr is part of the sex linked set of genes).
• The parenthesis cannot be separated from the Z it belongs to.
• Nothing within a single set of parenthesis may be separated from anything else within the parenthesis- a peach would be noted as Z(pl:c) but the "pl" for purple and the "c" for cameo cannot be separated from each other when they travel and should be treated as one gene when figuring out inheritance.

As of Jan 2025, the single-gene sex-linked colors are:
Cameo: A pale brown/tan body color with a faint, powder-blue tone and brown neck. Train may be pale or dark brown.
American Purple: Neck is royal blue/purple with cold blue ones, train is purple, blue and teal green. Body is brown, notably lacking in black pigment. Purple is an allele to European violet.
European violet: Neck is royal blue/purple with warm tones, train is stark purple, body is a warm, reddish brown. EV is an allele to American Purple.

• When bred together, purple x European violet does NOT produce wild type birds. The heterozygous offspring are said to resemble nicely-colored purples or poorly-colored EV, and so it is important to keep breeding stock well separated to avoid mixing the gene up.
  

Sonja's Violeta: Resembles a purple with black pigment laid over the entire body. Black-purple neck, dark brown body, black-purple train.

For morphs which include sex-linked colors but that are NOT single-gene mutations (peach, taupe, etc), please see Peafowl Genetics 203: Multiple-expression Phenotypes.

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Sex Linked x Itself

I will use purple throughout this guide, but you may replace purple with any of the mutations above and the squares will work the same way.

Any sex-linked color bred to itself will produce more of the sex-linked color (ie, all sex-linked colors "breed true"). Here is purple bred to purple.

Sex-Linked x Wild Type

The following squares will show you breeding combinations against the wild type, or het forms. We will start with a purple male bred to a wild type hen.

As you can see, unlike with autosomal recessive mutations, when only one parent is sex-linked, you can still produce mutant offspring. However, you may notice that ONLY the hen offspring are purple. This is because hens MUST inherit their Z chromosome from their father, because only their mother is capable of giving them a "w" chromosome. Since hens are hemizygous when it comes to the sex chromosome (they have 1 Z and the other is "missing"), they only need one copy of a sex-linked gene in order to express the mutant phenotype. If it helps, think of it as "all" of a bird's Z sex chromosomes must have the mutation. Since she only has one, that's "all" even though it's only one.

Let's look at what happens when we breed a purple hen to a wild type cock.

As you can see, even though we still bred a purple to a wild type like the previous example, the results are different. That is because the cock is WT, and the hens MUST inherit their Z from him still, their one Z must be wild type. In this case, the males get a purple Z from their mother, and will be het purple.

Let's see what happens if we breed a het male to a wild type hen!

As you can see, a het male only has one purple Z chromosome, which means only half of his hen offspring will be purple, and only half of his sons will be het purple. There is NO way to distinguish between wild type and het birds for any of the sex-linked colors, so male offspring from a male parent that is only het would be called "possibly het" instead. The hens, however, will only be wild type or purple, as they cannot be het for sex-linked genes.

Lastly, let's have a look at what happens when when we breed a het purple male to a purple hen!

In this case, half of both sexes will be purple! Like with the previous example, the hens still get their Z only from dad, so the other half of the hens will be plain wild type. This time, all of the males will be able to get a purple Z from their mother, so the other half of the males will be het purple for sure.

Sex-linked x Non-Same Sex-linked

While the sex-linked genes are not necessarily alleles, they all exist on the same chromosome (the Z chromosome), so in standard circumstances, a bird may only be one sex-linked color at a time.*

• *"Peach" is technically the phenotype of a purple + cameo bird, but this is a VERY unusual exception to the rule, even in broader genetics outside of peafowl. We will go more into peach and how it exists in Peafowl Genetics 203: Multiple-expression Phenotypes.
  

Being on the same chromosome also means that they don't transfer quite the same as autosomal recessives.

Let's try a full color x color cross. We'll use purple and cameo (which will hopefully also help you understand why breeding purple x cameo may never produce peach!).

In the square above, you can see that males are het purple and het cameo by genotype, and by phenotype they are blue, NOT peach. As stated at the top of this page, the genes typically cannot move between chromosomes under normal circumstances, which means each Z may ONLY have one sex-linked gene on it.

Because a bird with one purple and one cameo chromosome is a blue by phenotype, we know that purple and cameo are NOT alleles. However, because they cannot jump between chromosomes, they still cannot coexist with one another in homozygous form (without a chromosomal crossover event, discussed on the next page).

In any pairing with two different sex-linked colors, the hens will always be the same color as the dad, and the males will be split both colors.

Let's take one of the het purple/het cameo boys, and try to breed to a purple again, to show again that they can't combine.

In this case, the male donates a purple or a cameo Z to his hen offspring, meaning they will be purple or cameo, but not both. Half the male offspring will be full purple (getting a purple Z from each parent) or blue het purple+cameo like their father.

For one last sex-linked example, I will show a het sex-linked male bred to a different sex linked hen, in this case a het purple male and a cameo hen.

As you can see, all the males will be het cameo, and half of them will also be het purple. Just like with a het purple x wild type, half the hens are purple and half are wild type.

This wraps up info on the single-gene sex-linked mutations. If you'd like to learn about multiple-gene mutations, please continue to Genetics 203: Multiple-Expression Phenotypes.