The sex chromosome pair for birds are labeled Z & W (for mammals X & Y). The Z chromosome is longer than the W, and therefore has genes that have no counterpart on the W. In birds the males have a like pair (ZZ) and females have an unlike pair (ZW).
This means that
1) females determine gender of offspring &
2) that females have only one copy of some genes while males have 2 copies.
Male:
Z ______________________
Z ______________________
Female:
Z ______________________
W ______________
Some of these genes can be used to determine gender at a very young age (not all of the sex-linked genes cause a visible difference, and even of those that do, not all are obvious in young chicks.
Two of the most useful genes for creating a bird that is sexable at hatching are the silver/gold gene and the barring gene.
Silver and gold are the same gene--just different alleles, or variations of it. Silver (S) is dominant; gold (s) is the recessive allele. Barred (B) and not-barred (b)are the alleles for the barring gene.
As you can see, the lower case symbol is used to indicate a recessive allele whereas an uppercase one represents a dominant allele.
With the sex-linked genes, if you pair a dominant allele hen with a recessive allele cock, the male offspring will inherit the dominant allele from their mother and the recessive allele from their father. Because the allele inherited from the mother is dominant, their appearance will match her variety.
The female offspring, however will only inherit from their father, and their appearance will match his variety.
For barring here is how it works:
Hen--> B -
C | b B,b b,-
o | b B,b b,-
c V
k
Those offspring who are B,b are, by definition sons because they have two copies of the Z chromosome; the offspring who are b,- are daughters because they only have the one Z chromosome--the W chromosome does not have a coupterpart for the B gene. Silver works the same way.
This means that
1) females determine gender of offspring &
2) that females have only one copy of some genes while males have 2 copies.
Male:
Z ______________________
Z ______________________
Female:
Z ______________________
W ______________
Some of these genes can be used to determine gender at a very young age (not all of the sex-linked genes cause a visible difference, and even of those that do, not all are obvious in young chicks.
Two of the most useful genes for creating a bird that is sexable at hatching are the silver/gold gene and the barring gene.
Silver and gold are the same gene--just different alleles, or variations of it. Silver (S) is dominant; gold (s) is the recessive allele. Barred (B) and not-barred (b)are the alleles for the barring gene.
As you can see, the lower case symbol is used to indicate a recessive allele whereas an uppercase one represents a dominant allele.
With the sex-linked genes, if you pair a dominant allele hen with a recessive allele cock, the male offspring will inherit the dominant allele from their mother and the recessive allele from their father. Because the allele inherited from the mother is dominant, their appearance will match her variety.
The female offspring, however will only inherit from their father, and their appearance will match his variety.
For barring here is how it works:
Hen--> B -
C | b B,b b,-
o | b B,b b,-
c V
k
Those offspring who are B,b are, by definition sons because they have two copies of the Z chromosome; the offspring who are b,- are daughters because they only have the one Z chromosome--the W chromosome does not have a coupterpart for the B gene. Silver works the same way.
