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.
Sex-Linked Traits​
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.​​
A Note on Sex Chromosomes
In humans and other mammals, 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). (Chickens and other birds are a bit different, and you can read about them here.)
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X-Linked Traits
Most sex-linked traits are X-linked because the X chromosome contains many more genes than the Y chromosome.
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Recessive X-Linked Traits are often more commonly expressed in males because they have only one X chromosome. If a male inherits a recessive allele for a trait on the X chromosome, he will express that trait since he doesn't have a second X chromosome to potentially carry a dominant allele.
For females to express a recessive X-linked trait, they must inherit two copies of the recessive allele (one from each parent). If they inherit only one recessive allele, they are carriers but do not express the trait because the other X chromosome likely has a dominant allele that masks the recessive one.
Y-Linked Traits
​These traits are linked to genes found on the Y chromosome. Since only males have a Y chromosome, Y-linked traits are passed directly from father to son. These traits are rare because the Y chromosome has relatively few genes.
Examples of Sex-Linked Traits
Red-Green Colorblindness
X-Linked Recessive
Red-Green Colorblindness is a common example of an X-linked recessive trait in humans. Males with a single allele for colorblindness on their X chromosome will be colorblind, while females must have two such alleles to be colorblind. If a mother is a carrier, there is a 50% chance that a son will inherit colorblindness.
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Tortoiseshell & Calico Cats
X-Linked
The gene for orange coat color in cats is located on the X chromosome. Female cats can be tortoiseshell or calico (a mix of orange and black) if they inherit different alleles on each X chromosome (X*O X*B). Males, having only one X chromosome, will be either orange (X*O Y) or black (X*B Y), but not a mix.
Tortoiseshell and calico patterns are almost exclusively found in female cats because they require two X chromosomes with different alleles for coat color.
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Hemophilia
X-Linked Recessive
Hemophilia is a disorder where blood doesn't clot properly. Males with the hemophilia gene on their X chromosome will have the disorder, while females with one hemophilia allele are typically carriers. This sex-linked hemophilia is seen in both dogs and people.
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X-Linked Muscular Hypertrophy
X-Linked Recessive
In some cattle breeds, there is an X-linked trait that causes muscular hypertrophy, or increased muscle mass. This is more commonly observed in males (XY) who inherit the recessive allele, while females (XX) would need two copies of the recessive allele to express the trait.
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Fast vs, Slow Feathering & Barred Feathering Pattern
Z-Linked
Wait.... Z-linked? Looks like there's a bit more we need to talk about. Leave it to chickens to throw a wrench into things. (Wrench throwing is an autosomal dominant trait in chickens, by the way, with all alleles leading to non-wrench throwing variants having gone extinct long ago.) For the purposes of this page, both fast/slow feathering and the barred pattern are sex-linked traits that can be used to sex chicks just after birth.
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