Basics


A little knowledge we must have about heredity to breed better Silkies.

This text is intended as a basic color-genetic guide for the beginner chicken color breeder to become a better vieuw on what  will happen when he want to do color crosses with a simple genetic approach.



Basic color genetics.

Genotype = the genetic makeup of the organism.
Phenotype = the physical appearance of the organism.

There are 18 color genes loci
: e+e+  co+co+ db+db+  pg+pg+ ml+ml+  Cha+Cha+ mh+mh+ di+di+ Ig+Ig+ cb+cb+ i+i+  bl+bl+ Lav+Lav+  C+C+ Mo+Mo+ b+b+*b+/-  s+s+*s+/-  Choc+Choc+*Choc+/-

Gene
= a unit of hereditary , a section of DNA found on a chromosome that codes for a particular protein.
Allele = one of two alternate forms of a gene that has the same locus on homologous chromosomes.
Locus (plural=Loci) = the location of an Allele on the Chromosome.


You can see a gene contain always 2 alleles, ex. : gene = "e+e+", allele = "e+".
You can see ALL the alleles are written with a "+" behind it, this means the allel is NOT mutated, in other words she is original.
You can see the 3 last genes are written a little different, there is a "*" in between = sexlinked genes. The cock have 2 alleles, the hen have 1 allele and a "-"


A gene can take 3 different forms:

Homozygote = where alleles of a locus on homologous chromosomes are the same. Ex. : e+e+
Heterozygote = where alleles of a locus on homologous chromosomes are different. Ex. : Ee+

Hemizygote = a genetic locus present in one copy only. Of females, in reference to the only one allele at each locus on the single Z chromosome (only one allele possible, as only one Z chromosome in females, therefore not heterozygote or homozygote as no chromosome pair). Ex. : s+/-


A gene can inheritance in 2 different ways:

In a Dominant way = producing the same phenotype whether its allele is identical or dissimilar

In a recessive way = producing its characteristic phenotype only when its allele is identical


You can see I wrote "Dominant" with a capital ex. : EE
You can see I wrote "recessive" with a little letter ex. : ebeb


The 2 MOST important genes loci in breeding :

E-locus : can be expressed in 15 different ways:
(I write in the gene with a "/" to be more clear between the 2 alleles)


homozygote: E/E = Extended black
heterozygote: E/ER, E/EWh, E/eb, E/e+ (same fenotype)

homezygote: ER/ER = Birchen crowwing
heterozygote: ER/EWh, ER/eb, ER/e+ (same fenotype)

homozygote: EWh/EWh = Wheaten
heterozygote: EWh/eb, EWh/e+ (same fenotype)

homozygote: eb/eb = Partridge/brown
heterozygote: eb/e+ (same fenotype)

original gene: e+/e+ = duckwing/lightbrown



S-locus : can be expressed in different ways :
(is a sex-linked gene, the cock have 2 alleles, the hen have 1 allele and a "-")

For a cock:

S/S = Silver (= Dominant, homozygote)
S/s+ = golden (= Dominant, heterozygote)
s+/s+ = gold/wildtype (= recessive, homozygote)

For a hen:

S/- = Silver (= Dominant, hemizygote)
s+/- = gold/wildtype (= recessive, hemizygote)


There are also other genes ('tranformers") and combinations of genes.

These "transformer genes" can give a complete different fenotype to the original "Bankiva" (e+/e+) as also to his 4 mutations (E/E - ER/ER - EWh/EWh - eb/eb)

Also here there are Dominant and recessive genes, also they can be homozygote, heterozygote or hemizygote (as the sex-linked genes S - B - choc)

These genes can be "diluters" : Splash: Bl/Bl, Blue: Bl/bl+, pearlgray: lav/lav, Dilute: Di/Di or Di/di+, lemon: ig/ig, Blonde: Cb/Cb or Cb/cb+
These genes can be "modelators" : Cuckoo: B/B or B/b+ and B/-, Pattern: Pg/Pg or Pg/pg, mottled: mo/mo, Blacktail: Db/Db or Db/db+, Columbia: Co/Co or Co/co+
These genes can be "transformers" : White: I/I or I/i+ or I/Id or I/Is, Dun: Id/Id or Id/i+, Smokey: Is/Is or Is/Id or Is/i+, chocolate: choc/choc and choc/-, white: c/c, zwart: mz/mz, charbone: cha/cha
These genes can be "reinforcers" : (pheomelanine) of Mahagony (RED) Mh/Mh or Mh/mh+, (eumelanine) of Melanotic (BLACK) Ml/Ml or Ml/ml+



There are several ways in which the color-genes act and interact with eachother:
Dominant, Epistasis, incomplete Dominant, recessivity are the most common.

For examples and explanations about these most common ways see :

MAIN PAGE SILKIES


Since there are only 2 color-pigments, Black (eumelanin) and
Red (pheomelanin) as we can see in the original Bankiva fowl.


Bankiva partridge color is the bases on wich all color-variants are made of and is the so called genetic
  "wildcolor"
The Bankiva (Gallus Gallus)
All alleles are original (with a "+")
e+e+ co+co+ db+db+ pg+pg+ ml+ml+ Cha+Cha+ mh+mh+ di+di+ Ig+Ig+ cb+cb+ i+i+ bl+bl+ Lav+Lav+ C+C+ Mo+Mo+
b+b+ s+s+ Choc+Choc+ (males)    or    b+/- s+/- Choc+/- (females)

(the sex-linked genes)


The coloration of this chickens feathers is determinate by only two pigments. The phenotype-color of our chickens is NOT made of ONE gene but of SEVERAL genes that act and interact in different ways along their composition and along the different mutations on the different loci.

the red-pigment express itself mainly by the s+ allels (gold) it can take different forms of tonality
from the very light variant as wheaten (eWh) to the very dark variant mahogany (Mh)
the black-pigment express itself mainly by the E allels (black) it can take different forms of tonality
from the very light variant as lavender (lav) to the very dark variant melanotic (Ml)




The less common ways of inheritance are:

-A bit a special way of inheritance is the SEX-LINKED inheritance, sex-linked traits are gold/silver, cuckoo and chocolate. In sexlinked inheritance the gender that has an odd couple of sex chromosomes always shows the sexlinked traits and those traits come from the opposite gendered parent.
Example : E/E choc/choc (chocolate cock) X E/E Choc+/Choc+ ( Black hen) = Black cocks and chocolate hens.
-There is also the CO-DOMINANCE when both alleles of a genepair express but of this there is not an example available in the chicken colors.
Example :
an example is AB in our blood group.
-An other very special way of inheritance also in chickencolors is the CROSSING-OVER  when genes are far apart on the chromosome they can segregate independantly, a piece of chromosome can be interchanged then, when the genes loci are close to one another on the chromosome there is a certain chance of recombination.
Example : co+-ml+-pg+//co+-ml+-pg+ can also be added db+/db+
-Than a more experimental cross is the RECIPROCAL cross, with the phenotype of each sex reversed as compared with the original cross, to test the role of parental sex on inheritance pattern. A pair of crosses of the type genotype A(female) X genotype B(male) and genotype B(female) X genotype A(male).
Example : red hen X black cock and black hen X red cock.




Just a little extra common information about the sex-linked colors "s+" and "S".
(because we will constandly need it as it is 1 of the 2 most important loci : the S-Locus)


 
Sex-linkend colours heridity:

Most organisms, including chickens, have many autosome pairs per cell but only 1 pair of sex chromosomes.
The rooster gives half of his genetic information to a spermcell, the hen also but because she lacks a second Z chromosome, she partly donates an incomplete set which is written as "-", to an egg.
So the rooster always donates a Z and the female donates a Z or the odd partner w ("-").
That means that all offspring of the rooster always receives a Z from him.
Since hens only have one Z it always comes from their father. From their mother they get the dwarf chromosome w.
So all sex-linked traits like silver/gold and cuckoo or barred and chocolate from a hen she becamed from her father.
A hen can never give sexlinked traits to her daughters!





To be clear there are only 3 loci that are sex-linked:

S-locus : mutation is Silver = S/S * S/- or original is NOT Silver but gold = s+/s+ * s+/-
cocks can be unpure Silverish = S/s+ (because S dominant over s+)

B-locus : mutation is Barred (light cock) = B/B * B/- or original is NOT Barred but black = b+/b+ * b+/-
cocks can be unpure dark Barred = B/b+ (because B dominant over b+)

choc-locus : mutation is chocolate = choc/choc * choc/- or original is NOT chocolate but black = Choc+/Choc+ * Choc/-
cocks can be unpure Black = Choc+/choc (because Choc+ dominant over choc)



  Photo's of Tünde, Manuela, Louisa and Hans of the sex-linked colors (choc not yet in Silkies)        
  
       gold s+/-                        Silver S/-             Cucula B/- * B/b+       chocolate choc/choc



Summary of the Silkies genotypes :

white : eb/eb s+/s+ * s+/- or S/S * S/- ml+/ml+ mh+/mh+ c/c
Black : E/E s+/s+ * s+/- or S/S * S/- Ml/Ml mh+/mh+
Blue : E/E s+/s+ * s+/- or S/S * S/- Ml/Ml or ml+/ml+ mh+/mh+ Bl/bl+
lavender : E/E s+/s+ * s/- or S/S * S/- Ml/Ml or ml+/ml+ mh+/mh+ lav/lav
Red : eb/eb Co/Co s+/s+ * s+/- ml+/ml+ Mh/Mh (gray under fluff)
Red : eWh/eWh Co/Co s+/s+ * s+/- ml+/ml+ Mh/Mh (red under fluff)
Buff : eWh/eWh Co/Co s+/s+ * s+/- ml+/ml+ Mh/Mh Di/Di
gold Asiatic partridge : eb/eb s+/s+ * s+/- Pg/Pg ml+/ml+ mh+/mh+
Silver Asiatic partridge : eb/eb S/S * S/- Pg/Pg ml+/ml+ mh+/mh+
Cuckoo : E/E S/S * S/- ml+/ml+ mh+/mh+ B/B or B/b+ * B/-
out of categorie
Platenbont : E/E S/S * S/- Ml/Ml mh+/mh+ I/? (mo/mo)!
Splash : E/E s+/s+ * s+/- or S/S * S/- ml+/ml+ mh+/mh+ Bl/Bl

Alleles in red are the mutated alleles, those in white we must consider also, those in bluish are options.
In grayish written names are excisting colorations in Silkies but who are not (yet) regognized officially.
In blue written names are sex-linked colors (the chocolate was not mentioned because not yet in Silkies)

At this very moment there is being worked on bringing this coloration also in Silkies so here is genotype

chocolate : E/E s+/s+ * s+/- or S/S * S/- ml+/ml+ mh+/mh+ choc/choc * choc/-



Always a phenotype can hide a heterozygote recessive color-allele what than can result in unexpected colors in the offspring when those both hidden recessive color-alleles comes together in the chick via
the sperm of the cock (which is containing ONE allel of all genes) and the eggcell of the hen (which is also containing ONE allel of all genes *exception for the sex-linked genes*) becoming again a complete homozygote recessive color-gene that will express itself in the new phenotype of the new born chick.





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