Sex-linked

To understand sex-linked inheritance, you will need to know four facts:


  1. Cocks have a pair of identical chromosomes (often called chromosome X) which determine their sex as male, i.e. XX denotes a cock. The two X chromosomes are pretty much the same and just like the other chromosome pairs they have matching pairs of genes.

  1. Hens have two different chromosomes (X and Y) that makes them female, i.e. XY denotes a hen. The X and Y chromosomes are so different from each other that they do not have any matching pairs of genes.

  1. Sex-linked factors are recessive.

  1. The final thing that you need to know before starting is that the only known sex-linked factors (in birds) appear on the X chromosome (so far).


Now to put these things together:

  • Recessive genes means that a cock must have the factor on both genes for it to appear visually (in its phenotype - have I explained that word before?). This is because it has two matching genes on the two matching X chromosomes.

Hence, if a cock only has one gene with factor, it will be split for it - just like any recessive.

  • Hens have only one copy of the X chromosome, so they only need one gene with the factor for it to show in its appearance (since it has only one X chromosome, hens have only one copy of each gene held on the X chromosome so it does not have another gene on another X chromosome to over-ride it and prevent it changing hen's appearance).

This means that a hen cannot be split for any sex-linked factor – if it has it, then it must always appear visually (in its phenotype).

The following shows all the possible combinations for hens of cocks with and without sex-linked chromosomes, (X* denotes a X chromosome with a factor and X denotes one without).

Hens:

  
   

X*Y

 

A hen which shows a sex-linked factor in its appearance is.

   

XY

 

A normal hen.

    

Cocks:

  
          

X*X*

 

A cock which shows a sex-linked factor in its appearance.

   

XX*

 

A cock which is split for a sex-linked factor (it does not show in its appearance).

   

XX

 

A normal cock.


As you can see, there are only two possible combinations for a hen, with or without the sex-linked factor, instead of the usual three for a recessive gene. The cock does have three, which is exactly the same as any recessive factor.

This means that where the sex-linked factors lie, on the hen or the cock, makes a difference. Having a sex-linked factor hen in a pairing can produce different to having the sex-linked factor on the cock.

Tables to show the differences between ordinary and sex-linked recessive inheritance:

Where:

G

 

gene with Greywing factor

g

 

same gene, but without it

    

Xi

 

X chromosome with Ino factor

X

 

X chromosome without it

(The Greywing gene exists on one of the other chromosome pairs, which one has not been identified so we can only indicate the gene and not the chromosome. We definitely know which chromosomes the sex-linked factors are on and we must show which chromosome they are on, otherwise we cannot determine the outcome from a pairing successfully as the expectations for hens will be different from those for cocks in some matings.)

Ordinary

  

Sex-linked

Normal/Greywing x Normal
 
Normal/Ino Cock x Normal Hen

50% of the offspring are split for Greywing (gG).

The offspring in each possible combination can be either cocks or hens.

 

50% of the cocks are split for Ino (XXi) and 50% are normal (XX), but 50% of the hens are visually Ino (XiY) and 50% are normal (XY).

 

Difference: Visual factor birds (XiY - hens only) are produced when only a single recessive sex-linked gene is present (but only in the parent cock – see third pairing below).


 

Greywing x Normal
 
Ino Cock x Normal Hen
   

100% of the offspring are split for Greywing (gG).

 

100% of the cocks are split for Ino (XXi), but all the hens are visually Ino (XiY).

 

Difference: Some visually recessive birds are produced (but they are all hens).


 

Greywing x Normal
 
Normal/Ino Cock x Normal Hen

100% of the offspring are split for Greywing (gG).

(This is identical to the above pairing as it makes no difference whether it is the hen or the cock that is the Greywing.)

 

100% of cocks are split for Ino (XXi).

100% of hens are normals (XY).

(In a non-sex-linked recessive factor, this pairing and the one above it would give identical results.)

 

Difference: All the hens are normal (XY) from the sex-linked pairing and are not split.


 

Normal/Greywing x Normal/Greywing
 

Not possible

 

Difference: A similar Ino pairing is not possible as hens cannot be split for a sex-linked factor.


 

Greywing x Normal/Greywing

Normal/Ino x Ino Hen

50% of the offspring will be split for Greywing (gG) and the rest will be visually Greywings (GG).

 

50% of the cocks will be split for Ino (XXi) and the rest will be visually Ino (XiXi).

50% of the hens will be normals (XY) and the rest will be visually Ino (XiY).

 

Difference: Normal hens are produced in the sex-linked pairing, whereas in the ordinary recessive these hens would be splits.


 

Greywing x Greywing

Ino Cock x Ino Hen

100% visual Greywings (GG) produced.

 

100% visual Inos (XiXi and XiY) produced.

 

No difference.


You may have noticed a pattern, the expectations for cocks in a sex-linked recessive pairing is identical to that for any offspring in a non-sex-linked recessive pairing, but the expectations for hens are different – where a split factor bird is produced in an ordinary recessive pairing, visual factor hens are produced in a sex-linked mating.

This all boils down to a simple set of rules that you can learn to produce simple expectations:

Parents

Offspring
 

Cock

x

Hen

 

Cocks

 

Hens

 

Normal

 

Normal

 = 

 Normal

 

Normal

 
      
 

Normal/Sex-linked

 

Normal

 = 

 50% Normal/Sex-linked

 

50% Sex-linked

 
   

 50% Normal

 

50% Normal

 
      
 

Sex-linked

 

Normal

 = 

 100% Normal/Sex-linked

 

100% Sex-linked

 
      
 

Normal

 

Sex-linked

 = 

 100% Normal/Sex-linked

 

100% Normal

 
      
 

Sex-linked

 

Sex-linked

 = 

 100% Sex-linked

 

100% Sex-linked

However, simple rules do not help you when in difficult cases occur.

For instance, how can pairing an Opaline to a Cinnamon produce Opaline-Cinnamon offspring? Here are two possible pairings that do yield Opaline-Cinnamons (hens only) XoXo x XcY or XcXc x XoY, where o indicates the Opaline factor and c the Cinnamon – but from the rules you know so far you won’t get any. Convince yourself - try them!

Sexlinked Factors

The sexlinked factors that have occurred to date are:

Cinnamon

Clearbody* (Texas)

Ino

Lacewing**

Opaline

Slate

* Clearbodys are believed to be a version of Ino, Ino is believed to be a group of factors on different genes but very close together on the same the X chromosome. The difference between the two being that the Clearbody factor is the same as for Ino except that one of the comprising factors is incomplete and this leaves the wing marks unchanged from their Normal appearance.

** Lacewings are actually a composite of two sexlinked factors, Cinnamon and Ino. Therefore, a small percentage of Inos and Cinnamons can result from some pairings involving Lacewing as well as a small percentage of Lacewings can be produced from mating Cinnamons and Inos.