The Brahma and Cochin Club of Australia Inc
Poultry Breeding and Genetics Information
By Luke C. Price B.Sc. (Hons)
The following text will include some words which, at first, may leave you with somewhat of a headache. I have included a glossary of terms at the bottom of the page to help relieve the pain. If the pain persists please read the glossary of terms again, take a deep breath and read the paragraph of interest once more.
What colours can I breed together?
This is an important question which is regularly asked. Several colour varieties may be mated together and still produce birds with exhibition quality colour or colours which are close to the standard. In fact, to regularly produce some exhibition colours you are required to mate two different colours together. The following is a guide to aid breeders of not only Brahma and Cochin but all poultry breeds, when crossing of different colours is required in order to produce a particular colour, or to improve breed characteristics in a colour variety.
Black, Blue and Splash
Black
Photo: Andy Vardy
Blue
Photo: Andy Vardy
Splash
Photo: Andy Vardy
We will look at Black, Blue and Splash first. The colours blue and splash are based on a black bird. What makes them look different is the presence of an incompletely dominant, autosomal gene, called Blue (Bl). A bird which is heterozygous for the gene Bl will be blue and a bird which is homozygous (pure) for the Bl gene will be splash. The colours Blue and Splash can vary dramatically in shade and pattern; this is due partly to the variation in the expression of the Bl gene and also the presence of other genes which may interact to modify the colour.
Ignoring variation in shades and patterns of Blue and Splash there are always some certainties when breeding the colours. The expected outcomes from mating these three colours together are listed below.
Black x Black = All Black birds
Black x Blue = 50% Black and 50% Blue
Black x Splash = 100% Blue
Blue x Blue = 25% Black, 50% Blue and 25% Splash
Blue x Splash = 50% Blue and 50% Splash
Splash x Splash = 100% Splash
The expected percentage of colours hatched is not affected by the sex of the birds mated together as the Bl gene is autosomal (located on chromosomes not involved in specifying sex). E.g. a black rooster can be mated with a blue hen and vice versa and the expected ratios of colours in the hatch is always the same.
Black and Barred
Black
Photo: Andy Vardy
Barred
Photo: Andy Vardy
Now we will look at Black and Barred in relation to sex-linked barring. For sex-linked barring, a barred bird is essentially a black bird which possesses a dominant, sex linked, gene called Barring (B).
A bird which is heterozygous for the B gene will be barred and a bird which is homozygous (pure) for the B gene will also be barred, however the homozygous bird will have wider white bands because of a gene dosage effect (i.e. two copies of the B gene will increase size of white banding on the black bird). Now the tricky thing to remember is males can possess two copies of the B gene whilst females can only possess one. Because females can only posses one B gene they are called hemizygous and not heterozygous.
This is because the B gene is located on chromosomes which determine the sex of an individual, termed sex chromosomes. The two sex chromosomes in chooks are called Z and W. Females have a Z and W (ZW) chromosome and males have two Z chromosomes (ZZ). The W chromosome in females is very small and does not carry the same genetic information as the larger Z chromosome. In humans we call our sex chromosomes X and Y and the pairing of these chromosomes is reversed between the sexes. So human females have two X chromosomes (XX) and males have an X and a Y chromosome (XY), with the Y chromosome being small and carrying less genetic information.
We call the sex possessing two copies of the same sex chromosome the homogametic sex and the sex possessing one of each type of sex chromosome the heterogametic sex.
Not all sex-linked barred birds have the same barring pattern. Many combinations of genes can go into making a barred bird differ in its appearance. For example, the barring on an exhibition quality Plymouth Rock is clean and regular, whilst the barring on a cuckoo bird is generally indistinct and irregular. The variability is caused by differences in the expression of the same genetic mutation on different genetic backgrounds.
To simplify the terminology I will refer to homozygous Barred males as twice Barred males, heterozygous Barred males as single Barred males and hemizygous Barred females as Barred females. The expected outcomes from mating these colours together are listed below:
Black x Black = 100% Black males and females
Black male x Barred female = 100% single Barred males x 100% Black females
Single Barred male x Black female = 50% single Barred males and Barred females and 50% Black males and females
Single Barred male x Barred female = 25% single Barred males, 25% twice Barred males, 25% Barred females and 25% Black females
Twice Barred male x Black female = 100% single Barred males and 100% Barred females
Twice Barred male x Barred female = 100% twice Barred males and 100% Barred females
Light, Blue light, Buff Columbian, Blue Buff Columbian
Light
Photo: Andy Vardy
Buff Columbian
Photo: Luke Price
Blue Light
Photo: Andy Vardy
Blue Buff Columbian
Photo: Andy Vardy
The colour differences between Lights, Blue Lights, Buff Columbians and Blue Buff columbians are produced by the presence or absence of two dominant genes; the autosomal, incompletely dominant gene Blue (Bl) and the sex-linked gene Silver (S). The alternatives to Bl and S are bl (black) and s (gold) respectively. The expected outcomes from mating Light and Blue Light and Buff Columbian and Blue Buff Columbian are the same as you would expect from mating Blue to Black. These outcomes are listed below.
Light x Light = 100% Light
Light x Blue Light = 50% Light and 50% Blue Light
Light x Splash Light = 100% Blue Light
Blue Light x Blue Light = 25% Light, 50% Blue Light and 25% Splash Light
Blue Light x Splash Light = 50% Blue Light and 50% Splash Light
Splash Light x Splash Light = 100% Splash Light
Splash Lights will appear almost white, generally you will see some light blue feathers or parts of feathers around the neck hackle and tail which will provide a clue as to their genetic makeup.
Buff Columbian x Buff Columbian = 100% Buff Columbian
Buff Columbian x Blue Buff Columbian = 50% Buff Columbian and 50% Blue Buff Columbian
Buff Columbian x Splash Buff Columbian = 100% Blue Buff Columbian
Blue Buff Columbian x Blue Buff Columbian = 25% Buff Columbian, 50% Blue Buff Columbian and 25% Splash Buff Columbian
Blue Buff Columbian x Splash Buff Columbian = 50% Blue Buff Columbian and 50% Splash Buff Columbian
Splash Buff Columbian x Splash Buff Columbian = 100% Splash Buff Columbian
The neck hackle, tail and portions of the primary flight feathers of Splash Buff Columbians will appear white, however there will generally be some feathers or sections of feathers which are blue.
The expected percentage of colours hatched is not affected by the sex of the birds mated together as the Bl gene is autosomal (located on chromosomes not involved in specifying sex). E.g. a black rooster can be mated with a blue hen and vice versa and the expected ratios of colours in the hatch is always the same.
Where expected percentages of colours hatched will be affected by the sex of the birds mated together is when birds carrying the sex-linked gene Silver (S) and its counterpart gold (s) are involved. E.g. when Buff Columbians are crossed to Lights and vice versa. This also applies to Blue Buff Columbians, Blue lights and other combinations. The expected outcomes from such crosses are listed below.
Light male x Buff Columbian female = 100% Light (hiding Buff) males x 100% Light females
Buff Columbian male x Light female = 100% Light (hiding Buff) males and 100% Buff Columbian females
Light male (hiding Buff) x Light female = 25% Light males, 25% Light males (hiding Buff), 25% Light females and 25% Buff Columbian females
Light male (hiding Buff) x Buff Columbian female = 25% Buff Columbian males, 25% Light males (hiding Buff), 25% Light females and 25% Buff Columbian females
Light males (hiding Buff) will look like a Light male because the Silver gene is dominant over its alternative the Gold gene. So heterozygotes for Silver and Gold look like a Light. However, the gold can sometimes 'leak' through in the plumage, particularly around the neck and saddle hackle and on the wing . This is not always a certainty and often Light males (hiding Buff) can appear to be a pure Light male. In this instance the males genetic makeup is only revealed when some of his daughters turn out to be Buff Columbian even though he was mated to a Light female. At first, the results of these crosses may seem odd but if we think about the inheritance of sex-linked genes it makes sense. Remember males can possess two copies of either the Silver or Gold gene or a combination of both genes whilst females can only possess one of the alternatives. Because females can only posses one Silver or one Gold gene they are called hemizygous and not heterozygous.
This is because the Silver and Gold genes are located on chromosomes which determine the sex of an individual, termed sex chromosomes. The two sex chromosomes in chooks are called Z and W. Females have a Z and W (ZW) chromosome and males have two Z chromosomes (ZZ). The W chromosome in females is very small and does not carry the same genetic information as the larger Z chromosome. In humans we call our sex chromosomes X and Y and the pairing of these chromosomes is reversed between the sexes. So human females have two X chromosomes (XX) and males have an X and a Y chromosome (XY), with the Y chromosome being small and carrying less genetic information.
We call the sex possessing two copies of the same sex chromosome the homogametic sex and the sex possessing one of each type of sex chromosome the heterogametic sex.
Dark, Blue Dark, Partridge and Blue Partridge
Dark (female pencilling)
Photo: Luke Price
Partridge (Gold)
Photo: Luke Price
Blue Dark
Photo: Luke Price
Blue Partridge
Photo: Luke Price
The colour differences between Darks, Blue Darks, Partridge and Blue Partridge are produced by the presence or absence of two dominant genes; the autosomal, incompletely dominant gene Blue (Bl) and the sex-linked gene Silver (S). The alternatives to Bl and S are bl (black) and s (gold) respectively. The expected outcomes from mating Dark and Blue Dark and Partridge and Blue Partridge are the same as you would expect from mating Blue to Black. These outcomes are listed below.
Dark x Dark = 100% Dark
Dark x Blue Dark = 50% Dark and 50% Blue Dark
Dark x Splash Dark = 100% Blue Dark
Blue Dark x Blue Dark = 25% Dark, 50% Blue Dark and 25% Splash Dark
Blue Dark x Splash Dark = 50% Blue Dark and 50% Splash Dark
Splash Dark x Splash Dark = 100% Splash Dark
Splash Dark males will look simialr to a Splash (produced from a Blue), generally you will see some light blue feathers or parts of feathers on a mostly white bird.
Partridge x Partridge = 100% Partridge
Partridge x Blue Partridge = 50% Partridge and 50% Blue Partridge
Partridge x Splash Partridge = 100% Blue Partridge
Blue Partridge x Blue Partridge = 25% Partridge, 50% Blue Partridge and 25% Splash Partridge
Blue Partridge x Splash Partridge = 50% Blue Partidge and 50% Splash Partidge
Splash Partridge x Splash Partridge = 100% Splash Partridge
Males of Splash Partridge will look like a Pile male but some feathers or parts of feathers will have light blue or blue instead of being white. Female Splash Partridge will appear mostly white but with red patterning in the feathers and patches of light blue or blue in some feathers.
The expected percentage of colours hatched is not affected by the sex of the birds mated together as the Bl gene is autosomal (located on chromosomes not involved in specifying sex). E.g. a black rooster can be mated with a blue hen and vice versa and the expected ratios of colours in the hatch is always the same.
Where expected percentages of colours hatched will be affected by the sex of the birds mated together is when birds carrying the sex-linked gene Silver (S) and its counterpart gold (s) are involved. E.g. when a Partridge is crossed to a Dark and vice versa. This also applies to Blue Partridge, Blue Darks and other combinations. The expected outcomes from such crosses are listed below.
Dark male x Partridge female = 100% Dark (hiding Partridge) males x 100% Dark females
Partridge male x Dark female = 100% Dark (hiding Partridge) males and 100% Partridge females
Dark male (hiding Partridge) x Dark female = 25% Dark males, 25% Dark males (hiding Partridge), 25% Dark females and 25% Partridge females
Dark male (hiding Partridge) x Partridge female = 25% Partridge males, 25% Dark males (hiding Partridge), 25% Dark females and 25% Partridge females
Dark males (hiding Partridge) will look like a Dark male because the Silver gene is dominant over its alternative the Gold gene. So heterozygotes for Silver and Gold look like a Dark. However, the gold can sometimes 'leak' through in the plumage, particularly on the wing and around the neck and saddle hackle. This is not always a certainty and sometimes Dark males (hiding Partridge) can appear to be a pure Dark male. In this instance the males genetic makeup is only revealed when some of his daughters turn out to be Partridge even though he was mated to a Dark female. At first, the results of these crosses may seem odd but if we think about the inheritance of sex-linked genes it makes sense. Remember males can possess two copies of either the Silver or Gold gene or a combination of both genes whilst females can only possess one of the alternatives. Because females can only posses one Silver or one Gold gene they are called hemizygous and not heterozygous.
This is because the Silver and Gold genes are located on chromosomes which determine the sex of an individual, termed sex chromosomes. The two sex chromosomes in chooks are called Z and W. Females have a Z and W (ZW) chromosome and males have two Z chromosomes (ZZ). The W chromosome in females is very small and does not carry the same genetic information as the larger Z chromosome. In humans we call our sex chromosomes X and Y and the pairing of these chromosomes is reversed between the sexes. So human females have two X chromosomes (XX) and males have an X and a Y chromosome (XY), with the Y chromosome being small and carrying less genetic information.
We call the sex possessing two copies of the same sex chromosome the homogametic sex and the sex possessing one of each type of sex chromosome the heterogametic sex.
Golden Crele and Partridge
Partridge
Photo: Luke Price
Golden Creel (Bantam)
Photo: Luke Price
Now we will look at Partridge and Golden Crele in relation to sex-linked barring. For sex-linked barring, a Golden Crele bird is essentially a Partridge bird which possesses a dominant, sex linked, gene called Barring (B).
A bird which is heterozygous for the B gene will be Golden Crele and a bird which is homozygous (pure) for the B gene will also be Golden Crele, however the homozygous bird will have wider white bands because of a gene dosage effect (i.e. two copies of the B gene will increase size of white banding on the partridge background colour). Now the tricky thing to remember is males can possess two copies of the B gene whilst females can only possess one. Because females can only posses one B gene they are called hemizygous and not heterozygous.
This is because the B gene is located on chromosomes which determine the sex of an individual, termed sex chromosomes. The two sex chromosomes in chooks are called Z and W. Females have a Z and W (ZW) chromosome and males have two Z chromosomes (ZZ). The W chromosome in females is very small and does not carry the same genetic information as the larger Z chromosome. In humans we call our sex chromosomes X and Y and the pairing of these chromosomes is reversed between the sexes. So human females have two X chromosomes (XX) and males have an X and a Y chromosome (XY), with the Y chromosome being small and carrying less genetic information.
We call the sex possessing two copies of the same sex chromosome the homogametic sex and the sex possessing one of each type of sex chromosome the heterogametic sex.
Not all sex-linked barred Golden Crele birds have the same barring pattern. Many combinations of genes can go into making the barring pattern differ in its appearance. For example, the barring on an exhibition quality Plymouth Rock is clean and regular, whilst the barring on a cuckoo bird is generally indistinct and irregular. The variability is caused by differences in the expression of the same genetic mutation on different genetic backgrounds.
To simplify the terminology I will refer to homozygous Golden Crele males as twice barred males, heterozygous Golden Crele males as single Barred males and hemizygous Golden Crele females as Golden Crele females. Twice barred Golden Crele males, whilst useful for breeding, are not suitable for exhibition as the wider barring makes their colour deviate from the required colour described in the standard. The expected outcomes from mating these colours together are listed below:
Partridge x Partridge = 100% Partridge males and females
Partridge male x Golden Crele female = 100% single Barred Golden Crele males x 100% Partridge females
Single Barred Golden Crele male x Partridge female = 50% single Barred Golden Crele males and Golden Crele females and 50% Partridge males and females
Single Barred Golden Crele male x Golden Crele female = 25% single Barred Golden Crele males, 25% twice Barred Golden Crele males, 25% Golden Crele females and 25% Partridge females
Twice Barred Golden Crele male x Partridge female = 100% single Barred Golden Crele males and 100% Golden Crele females
Twice Barred Golden Crele male x Golden Crele female = 100% twice Barred Golden Crele males and 100% Golden Crele females
Silver Crele and Dark
Now we will look at Dark and Silver Crele in relation to sex-linked barring. For sex-linked barring, a Silver Crele bird is essentially a Dark bird which possesses a dominant, sex linked, gene called Barring (B).
A bird which is heterozygous for the B gene will be Silver Crele and a bird which is homozygous (pure) for the B gene will also be Silver Crele, however the homozygous bird will have wider white bands because of a gene dosage effect (i.e. two copies of the B gene will increase size of white banding on the partridge background colour). Now the tricky thing to remember is males can possess two copies of the B gene whilst females can only possess one. Because females can only posses one B gene they are called hemizygous and not heterozygous.
This is because the B gene is located on chromosomes which determine the sex of an individual, termed sex chromosomes. The two sex chromosomes in chooks are called Z and W. Females have a Z and W (ZW) chromosome and males have two Z chromosomes (ZZ). The W chromosome in females is very small and does not carry the same genetic information as the larger Z chromosome. In humans we call our sex chromosomes X and Y and the pairing of these chromosomes is reversed between the sexes. So human females have two X chromosomes (XX) and males have an X and a Y chromosome (XY), with the Y chromosome being small and carrying less genetic information.
We call the sex possessing two copies of the same sex chromosome the homogametic sex and the sex possessing one of each type of sex chromosome the heterogametic sex.
Not all sex-linked barred Silver Crele birds have the same barring pattern. Many combinations of genes can go into making the barring pattern differ in its appearance. For example, the barring on an exhibition quality Plymouth Rock is clean and regular, whilst the barring on a cuckoo bird is generally indistinct and irregular. The variability is caused by differences in the expression of the same genetic mutation on different genetic backgrounds.
To simplify the terminology I will refer to homozygous Silver Crele males as twice barred males, heterozygous Silver Crele males as single Barred males and hemizygous Silver Crele females as Silver Crele females. Twice barred Silver Crele males, whilst useful for breeding, are not suitable for exhibition as the wider barring makes their colour deviate from the required colour described in the standard. The expected outcomes from mating these colours together are listed below:
Dark x Dark = 100% Partridge males and females
Dark male x Silver Crele female = 100% single Barred Silver Crele males x 100% Dark females
Single Barred Silver Crele male x Dark female = 50% single Barred Silver Crele males and Silver Crele females and 50% Dark males and females
Single Barred Silver Crele male x Silver Crele female = 25% single Barred Silver Crele males, 25% twice Barred Silver Crele males, 25% Silver Crele females and 25% Dark females
Twice Barred Silver Crele male x Dark female = 100% single Barred Silver Crele males and 100% Silver Crele females
Twice Barred Silver Crele male x Silver Crele female = 100% twice Barred Silver Crele males and 100% Silver Crele females
White Laced Gold (Buff Laced) and Gold Laced
Now we will look at Buff Laced and Gold Laced. The Buff Laced colour is based on a gold laced bird. Both colours require purity of the sex-linked gene gold (s). What makes these two colour patterns look different is the presence of an incompletely dominant, autosomal gene, called Dominant White (I). A bird which is heterozygous for the gene I, will be have reduced eumelanin (black pigment) and a bird which is homozygous (pure) for the I gene will generally display no eumelanin and faded pheomelanin (red pigment). The colours Buff Laced and Buff Laced homozygous for the gene I can vary dramatically in pattern; this is due partly to the variation in the expression of the I gene and also the presence of other genes which may interact to modify the colour such as the presence of Blue (Bl) which when pure can help to remove black pigment that 'leaks' through when the I gene is heterozygous. Buff Laced birds homozygous for the I gene (i.e. 'pure I') are not suitable for showing because of the fading of red pigment which destroys the colour pattern.
Ignoring variation in the expression of black pigment of Buff Laced there are always some certainties when breeding the Gold Laced to Buff Laced. The expected outcomes from mating these two colours together are listed below.
Gold Laced x Gold Laced = All Gold Laced birds
Gold Laced x Buff Laced = 50% Gold Laced and 50% Buff Laced
Gold laced x Buff Laced (pure I) = 100% Buff Laced
Buff Laced x Buff Laced = 25% Gold Laced, 50% Buff Laced and 25% Buff Laced (pure I)
Buff Laced x Buff Laced (pure I) = 50% Buff Laced and 50% Buff Laced (pure I)
Buff Laced (pure I) x Buff Laced (pure I) = 100% Buff Laced (pure I)
The expected percentage of colours hatched is not affected by the sex of the birds mated together as the I gene is autosomal (located on chromosomes not involved in specifying sex). E.g. a Buff Laced rooster can be mated with a Gold Laced hen and vice versa and the expected ratios of colours in the hatch is always the same.
Silver Laced and Blue Laced Silver, Gold Laced and Blue Laced gold
The colour differences between Silver Laced, Blue Laced Silver, Gold Laced and Blue Laced Gold are produced by the presence or absence of two dominant genes; the autosomal, incompletely dominant gene Blue (Bl) and the sex-linked gene Silver (S). The alternatives to Bl and S are bl (black) and s (gold) respectively. The expected outcomes from mating Silver Laced and Blue Laced Silver and Gold Laced and Blue Laced Gold are the same as you would expect from mating Blue to Black. These outcomes are listed below.
Silver Laced x Silver Laced = 100% Silver Laced
Silver Laced x Blue Laced Silver = 50% Silver Laced and 50% Blue Laced Silver
Silver Laced x Splash Silver Laced = 100% Blue Laced Silver
Blue Laced Silver x Blue Laced Silver = 25% Silver Laced, 50% Blue Laced Silver and 25% Splash Silver Laced
Blue Laced Silver x Splash Silver Laced = 50% Blue Laced Silver and 50% Splash Silver Laced
Splash Silver Laced x Splash Silver Laced = 100% Splash Silver Laced.
Splash Silver Laced will appear almost white, generally you will see some blue feathers or parts of feathers around the neck hackle and tail and flecks of blue which will provide a clue as to their genetic makeup.
Gold Laced x Gold Laced = 100% Gold Laced
Gold Laced x Blue Laced Gold = 50% Gold Laced and 50% Blue Laced Gold
Gold Laced x Splash Gold Laced = 100% Blue Laced Gold
Blue Laced Gold x Blue Laced Gold = 25% Gold Laced, 50% Blue Laced Gold and 25% Splash Gold Laced
Blue Laced Gold x Splash Gold Laced = 50% Blue Laced Gold and 50% Splash Gold Laced
Splash Gold Laced x Splash Gold Laced = 100% Splash Gold Laced
The neck hackle, tail and lacing of Splash Gold Laced will appear white, similar to a Buff Laced, however there will generally be some feathers or sections of feathers which are blue.
The expected percentage of colours hatched is not affected by the sex of the birds mated together as the Bl gene is autosomal (located on chromosomes not involved in specifying sex). E.g. a black rooster can be mated with a blue hen and vice versa and the expected ratios of colours in the hatch is always the same.
Where expected percentages of colours hatched will be affected by the sex of the birds mated together is when birds carrying the sex-linked gene Silver (S) and its counterpart gold (s) are involved. E.g. when Gold Laced are crossed to Silver Laced and vice versa. This also applies to Blue Laced Gold, Blue Laced Silver and other combinations. The expected outcomes from such crosses are listed below.
Silver Laced male x Gold Laced female = 100% Silver Laced (hiding Gold) males x 100% Silver Laced females
Gold Laced male x Silver Laced female = 100% Silver Laced (hiding Gold) males and 100% Gold Laced females
Silver Laced male (hiding Gold) x Silver Laced female = 25% Silver Laced males, 25% Silver Laced males (hiding Gold), 25% Silver Laced females and 25% Gold Laced females
Silver Laced male (hiding Gold) x Gold Laced female = 25% Gold Laced males, 25% Silver Laced males (hiding Gold), 25% Silver Laced females and 25% Gold Laced females
Silver Laced males (hiding Gold) will look like a Silver Laced male because the Silver gene is dominant over its alternative the Gold gene. So heterozygotes for Silver and Gold look like a Silver Laced. However, in Laced varieties the gold generally 'leaks' through in the plumage, particularly around the neck and saddle hackle and on the back and wing. This is not always a certainty and sometimes Silver Laced males (hiding Gold) can appear to be a pure Silver Laced male. In this instance the males genetic makeup is only revealed when some of his daughters turn out to be Gold Laced even though he was mated to a Silver Laced female. At first, the results of these crosses may seem odd but if we think about the inheritance of sex-linked genes it makes sense. Remember males can possess two copies of either the Silver or Gold gene or a combination of both genes whilst females can only possess one of the alternatives. Because females can only posses one Silver or one Gold gene they are called hemizygous and not heterozygous.
This is because the Silver and Gold genes are located on chromosomes which determine the sex of an individual, termed sex chromosomes. The two sex chromosomes in chooks are called Z and W. Females have a Z and W (ZW) chromosome and males have two Z chromosomes (ZZ). The W chromosome in females is very small and does not carry the same genetic information as the larger Z chromosome. In humans we call our sex chromosomes X and Y and the pairing of these chromosomes is reversed between the sexes. So human females have two X chromosomes (XX) and males have an X and a Y chromosome (XY), with the Y chromosome being small and carrying less genetic information.
We call the sex possessing two copies of the same sex chromosome the homogametic sex and the sex possessing one of each type of sex chromosome the heterogametic sex.
Birchin (Silver Birchin) and Brown Red (Gold Birchin)
Now we will look at Birchin and Brown Red in relation to sex-linked Silver and Gold. The expected percentages of colours hatched will be affected by the sex of the birds mated together when birds carrying the sex-linked gene Silver (S) and its counterpart gold (s) are involved. E.g. when Birchin are crossed to Brown red and vice versa. This also applies to Blue Birchin, Blue Brown Red and other combinations. The expected outcomes from such crosses are listed below.
Birchin male x Brown Red female = 100% Birchin (hiding Gold) males x 100% Birchin females
Brown Red male x Birchin female = 100% Birchin (hiding Gold) males and 100%
Brown Red females
Birchin male (hiding Gold) x Birchin female = 25% Birchin males, 25% Birchin males (hiding Gold), 25% Birchin females and 25% Brown Red females
Birchin male (hiding Gold) x Brown Red female = 25% Brown Red males, 25% Birchin males (hiding Gold), 25% Birchin females and 25% Brown Red females
Birchin males (hiding Gold) will look like a Birchin male because the Silver gene is dominant over its alternative the Gold gene. So heterozygotes for Silver and Gold look like a Birchin. However, the gold can sometimes 'leak' through in the plumage, particularly around the neck and saddle hackle and on the wing .This is not always a certainty and often Birchin males (hiding Gold) can appear to be a pure Birchin
male. In this instance the males genetic makeup is only revealed when some of his daughters turn out to be Brown Red even though he was mated to a Birchin female. At first, the results of these crosses may seem odd but if we think about the inheritance of sex-linked genes it makes sense. Remember males can possess two copies of either the Silver or Gold gene or a combination of both genes whilst females can only possess one of the alternatives. Because females can only posses one Silver or one Gold gene they are called hemizygous and not heterozygous.
This is because the Silver and Gold genes are located on chromosomes which determine the sex of an individual, termed sex chromosomes. The two sex chromosomes in chooks are called Z and W. Females have a Z and W (ZW) chromosome and males have two Z chromosomes (ZZ). The W chromosome in females is very small and does not carry the same genetic information as the larger Z chromosome. In humans we call our sex chromosomes X and Y and the pairing of these chromosomes is reversed between the sexes. So human females have two X chromosomes (XX) and males have an X and a Y chromosome (XY), with the Y chromosome being small and carrying less genetic information.
We call the sex possessing two copies of the same sex chromosome the homogametic sex and the sex possessing one of each type of sex chromosome the heterogametic
sex.
Yet to be completed- stayed tuned. Last updated 2/07/2015
Glossary
Autosomes: chromosomes not involved in specifying sex.
Allele: an alternative form of a gene.
Chromosome: a physically organised form of DNA in a cell. The fowl has 38 pairs of autosomes and a pair of sex chromosomes.
Diploid: having a pair of each type of chromosome.
Dominant: the allele that generates the phenotype in a heterozygous organism.
Expression: the process by which the information in a gene is made into a functional gene product.
Gametes: the haploid germ cells; sperm in males, eggs (ova) in females.
Genotype: the genetic composition of an organism.
Haploid: having a single set of chromosomes.
Heterozygote: has two different alleles at a given locus (heterozygous).
Homozygote: has two identical alleles at a given locus (homozygous).
Incomplete dominance: in a heterozygote both alleles at a locus are partially expressed, resulting in an intermediate phenotype.
Linkage: measure of the probability of two genes being transmitted together to offspring.
Locus: the particular point on a chromosome where a gene is located.
Meiosis: the process of cell division by which gametes are formed.
Mendel’s laws: all alleles for a character segregate independently of one another and each is represented in 50% of the gametes and all gametes have an equal chance of fertilisation.
Phalanges: the bones of a finger or toes.
Phenotype: the observed physical and physiological traits of an organism, produced by the genotype in conjunction with the environment.
Recessive: an allele that only generates a phenotype if it is homozygous.
Sex chromosomes: chromosomes which determine the sex of an individual. ZW in females and ZZ in males. The W chromosome in females is very small and does not carry the same genetic information as the Z chromosome.
Sex linked: carried by a sex chromosome.
Wildtype: allele(s) responsible for the ‘normal’ (non-mutant) phenotype.