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Sex-linked inheritance

In sex-linked factors such as Yellow (Dilute), Ivory and Cinnamon, the gene for these factors lies on a Z-chromosome. This means that a sex-linked factor is paired with either ZR or Zr. Particularly at a crossing of a man "Red head split Black head and a sex-linked factor (split)" the outcome of the offspring depends on the location of the factor or on ZR (ZR/Zr Zfactor/Zfactor+) or on Zr (Zfactor+/Zfactor).
For example, breeding "Redhead Green SF Yellow split Black head male times Black head Green female" will give Red head SF Yellow females and Black head Green females, according to the following example:

  Males Females
Red head SF Yellow split Black headX Black head Green
ZR/Zr Zdil/Zdil+ X Zr/W Zdil+/W
Red head SF Yellow split Black head
ZR/Zr Zdil/Zdil+
Red head SF Yellow split Black head
Red head SF Yellow
ZR/W Zdil/W
Red head SF Yellow
Black head Green
Zr/Zr Zdil+/Zdil+
Black head Green
Black head Green
Zr/W Zdil+/W
Zwartkop Groen

Hereby the redhead and dilute factor are on one and the same chromosome. When the dilute factor and the allele for blackhead are on one and the same chromosome then a Red head SF Yellow split Black head male is genetically ZR/Zr Zdil+/ZP. This ZR/Zr Zdil+/ZP male can easily be obtained by, for example, breeding a Black head SF Yellow male with a Red head Green female. The male offspring is then Red head SF Yellow split Black head, but ZR/Zr Zdil+/Zdil

It is clear that these two genetically different Red head SF Yellow split Black headed males cannot be phenotypically distinguished from each other. Only a cross with (also) a Black head Green female can give the answer:

  Males Females
Red head SF Yellow split Black headX Black head Purple breast Green
ZR/Zr Zdil+/Zdil X Zr/W Zdil+/W
Black head SF Yellow
Zr/Zr Zdil/Zdil+
Black head SF Yellow
Black head SF Yellow
Zr/W Zdil/W
Black head SF Yellow
Red head Green split Black head
ZR/Zr Zdil+/Zdil+
Red head Green split Black head
Red head Green
ZR/W Zdil+/W
Red head Green

As you can see a totally "different" offspring than the first mentioned breeding. However, both breedings can be called "Red head SF Yellow split Black head male times Black head Green female". In order to be able to calculate both crossings, an extra choice is added to the sex-linked backcolors. You can now choose on which Z-chromosome the dilute factor is.