These photos are in part collected here for my own records, but I hope they can also be educational when others are pondering the changes they may see between chick fluff and adult feathers. They grow up so quickly...
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Born on St. Paddy's Day, I was very tempted to keep this little one, my first successful 2020 hatchling.
CHICKENS & EGGS
Sex-Linked Traits in Chickens​
The rest of this article will assume you have a very basic working knowledge of inheritance and genetics. If you need a bit of a refresher, click here. If you'd like to learn more about genetics, my friend Jess has authored and illustrated an incredible website on genetics. She focuses on canine color genetics, but it can help improve the understanding of matters of inheritance in any species.​
A sex-linked trait is a characteristic determined by a gene located on one of the sex chromosomes, which are the X and Y chromosomes in mammals, including humans. These traits are most often related to the X chromosome because it carries more genes than the Y chromosome.​​
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A few sex-linked traits are notable in chickens because they can help producers sex chicks just after birth.
A Note on Sex Chromosomes in Chickens
In humans and many other organisms, sex is determined by two types of chromosomes: X and Y. Females typically have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
In chickens and other birds, sex chromosomes are labeled Z and W instead of X and Y, which are used in mammals. The ZW sex-determination system is different from the XY system found in mammals and some other animals.
So why are they are labeled Z and W?
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The labeling of Z and W chromosomes in birds originated as a way to distinguish the system from the mammalian XY system. Scientists wanted to avoid confusion and thus chose different letters to represent the sex chromosomes in birds.
Sex Determination in Birds
One reason scientists wanted to avoid confusion between the avian system and mammalian system was that in birds, the ZW system is the opposite of the mammalian XY system in terms of which sex is heterogametic (having two different sex chromosomes).
In birds, females are ZW, meaning they have one Z and one W chromosome. This makes female birds the heterogametic sex. Males, on the other hand, are ZZ; they have two Z chromosomes. (In mammals, the males are the heterogametic sex, as they have one X and one Y chromosome.)
Z and W Chromosomes
Z Chromosome
The Z chromosome is larger and contains more genes, similar to the X chromosome in mammals. It carries important genes for various traits, including those related to development and sex differentiation.
W Chromosome
The W chromosome is much smaller and contains fewer genes, mainly involved in determining female sex. It is similar in function to the Y chromosome in mammals but differs significantly in its genetic content.
The Significance of Sex-Linked Traits in Chickens
One of the most significant applications of sex-linked traits in chickens is the ability to determine the sex of chicks at hatching. This is particularly useful in the commercial poultry industry, where sex differentiation is important for managing egg-laying breeds (where females are preferred) and meat breeds (where males may be preferred for their growth rate). Early sex determination using sex-linked traits reduces the costs associated with raising unwanted males in egg-laying breeds. For example, if only females are needed for egg production, identifying and separating males early can save resources and improve efficiency.​
Sex-Linked Feathering Rate
The feathering rate is determined by a gene on the Z chromosome, where the slow-feathering (K) allele is dominant, and the fast-feathering (k) allele is recessive.
Females
Since females have only one Z chromosome, they express the feathering rate based on the allele they inherit from their father. If they inherit the slow-feathering allele (K), they will show slow feathering. If they inherit the fast-feathering allele (k), they will show fast feathering.
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Males
Males, with two Z chromosomes, can be either homozygous (KK, which results in slow feathering) or heterozygous (Kk, resulting in slow feathering due to the dominant K allele) or homozygous recessive (kk, resulting in fast feathering).
Sexing Chicks
This trait is used to sex chicks immediately after hatching. Breeders cross fast-feathering males (kk) with slow-feathering females (K-). As a result, female chicks will inherit one slow-feathering allele and one fast-feathering allele, showing fast feathering, while male chicks, with two slow-feathering alleles, will develop feathers more slowly. This visual difference allows breeders to quickly separate males and females.​​​
Barred Feathering Pattern
The barred feather pattern in chickens is another trait linked to the Z chromosome. The barred (B) allele is dominant, and the non-barred (b) allele is recessive.
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Females
Female chickens will display the barred pattern if they inherit the barred allele (B) on their single Z chromosome.
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Males
Males with two barred alleles (BB) will have a more pronounced barred pattern, while those with one barred and one non-barred allele (Bb) will also show barring, though it may be less distinct. Males with two non-barred alleles (bb) will not show any barring.
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Sexing Chicks
In breeding programs, a barred male (BB or Bb) crossed with a non-barred female (bW) will produce female chicks that are barred and male chicks that are non-barred or show weaker barring. This difference in feather pattern can be used to sex chicks at hatch.
Plumage Color (Silver vs. Gold)
Plumage color in chickens, particularly the Silver (S) and Gold (s) alleles, is controlled by a gene on the Z chromosome. The silver allele (S) is dominant, and the gold allele (s) is recessive.
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Females
Female chicks inherit the plumage color based on the allele on their single Z chromosome. If they inherit the silver allele (S), they will have silver plumage. If they inherit the gold allele (s), they will have golden plumage.
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Males
Males can be homozygous for either allele (SS for silver, ss for gold) or heterozygous (Ss), where the silver allele dominates, and they appear silver.
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Sexing Chicks
By crossing a silver male (SS or Ss) with a gold female (sW), the resulting chicks will have distinct plumage colors based on their sex. Male chicks (ZZ) will show a uniform color (silver if Ss or SS, gold if ss), while female chicks (ZW) will show a distinct color based on the single allele they inherit (silver or gold). This allows for sexing based on plumage color. This method is often used in breeds like the Golden Comet or Red Sex-Link, where the color difference at hatching allows for immediate sex determination, which is particularly beneficial in egg-laying operations where early separation of sexes is economically advantageous.
