Written by the Sellers family of Brookings, South Dakota

The second part of this series - if you haven't already, please read the first part to cover the basics

The sex of your chicks:

The sex of a chick is determined even before the egg is fertilized. Each pair of chromosomes in the fertilized egg has one chromosome from each parent. The father always contributes a long sex chromosome (the Z chromosome) to the fertilized egg.

Before the egg is fertilized, it has only those chromosomes from the mother. If the mother contributes a long sex chromosome, Z, to the unfertilized egg, the chick from that egg will be male because it will have two long sex chromosomes after fertilization, since it always gets a long sex chromosome from the father. If the mother contributes a short sex chromosome to the unfertilized egg, then the chick will be female because it will have one long and one short sex chromosome after fertilization. So, in this way the egg can be thought of as already having a sex (gender) even before it is fertilized.

The sex ratio of baby chicks:

On the basis of extensive research, it is now accepted as fact that female chicks are equally probable as male chicks. There is no bias toward one sex or the other. Given good incubation techniques, one should hatch equal numbers of male and female chicks if a statistically valid (large enough) sample of eggs is incubated. However, it is believed that female embryos are preferentially killed by fluctuations in incubation conditions.

Feather sexing baby chicks:

In order for rate of feathering to be an indicator of chick sex, the mothers of the chicks have to have a slow feathering gene (see the table) while the fathers have normal feathering or rapid feathering genes. A cross between these males and females will give pullets with rapid feathering and cockerels with slow feathering. This is a sex-linked trait that can be a sex-indicating trait in the same way that sex-linked barring can.

How to breed for a trait for sexing day-old chicks:

In the gene table in Part III the first listing is a set of sex-linked genes. Some common sex-linked traits are Cuckoo barring, gold, silver, slow feathering and dwarfism. Gold, s+, and silver, S, are allelic,which means that they are found at the same locus (on the long, Z, sex chromosome). In order to breed for a trait that will useful for sexing day-old chicks, the trait must be visible in the hatchling. The brown eye trait is not a good choice because chickens don't get their final eye colour until they reach sexual maturity.

To breed a trait that is present in male chicks and absent in female chicks, the trait must be dominant and on the Z sex chromosome (sex-linked), the female parent must have the trait and the male should be lacking the trait. Please see the sex-linked genes in the table in Part III. Any of the dominant sex-linked genes listed there can be exploited to give birds that are sexable at a very young age. The silver gene, S, is often expoited in varieties like Red Sex-Links for sexing day-old chicks. For example, we might choose to cross a red Rhode Island Red male (s+, s+) with a silver Delaware female (S, _) where this means that her long Z chromosome has the silver gene, S, and her short W chromosome is lacking that locus and is represented by an underscore or dash. The four possible gene combinations of the parent genes from this cross are: (S, s+), (S, s+), (s+,_), (s+,_). Here the dominant gene is written first and any gene is written before the underscore.

In this example of the red male mated to the silver female, there are really only two unique gene combinations since two of the four gene combinations are identical to the other two. The 50% of the chicks that inherit the gene combination, (S, s+), are silver males (male because they inhereted two copies of the long Z sex chromosome) and are essentially white birds with some possible colouration because silver can be a leaky gene. The other half of the chicks that inherit the (s+,_) genes are red females (female because she inherited the short W sex chromosome). So the pullets are red and the males are primarily white (yellow down). It is common that Delaware dams and Rhode Island Red sires are used in a cross like this to obtain a Red Sex-Link. This cross is sometimes called Sil-Go-Link for 'silver-gold-sex-link'. The silver gene used this way (the female parent having the dominant gene and the male parent having the recessive genes), will always give sons that have the dominant gene and daughters that do not.

If the cross is carried out the other way, a silver male on a red female, all the chicks will be essentially white if the male has two copies of the silver gene (homozygous for silver). In this case it is not possible to determine the sex of the day-old chicks by their colour.

Any dominant sex-linked trait can be used in this way for the purpose of sexing day-old chicks so long as that trait is visible in the chicks. The slow feathering trait can be a good choice because it does not change the basic colour or pattern characteristics of the birds (see Part III under Cuckoo barring) so that their appearance (phenotype) can be maintained.

Auto-sexing breeds:

An auto-sexing breed is a breed in which the male and female day-old chicks can be distinguished. Some physical characteristic must be observable that is different in males than females. The important difference between a sex-link hybrid and an auto-sexing breed is that the auto-sexing breed is a pure, true-breeding strain and not a hybrid. Hybrids don't breed true in the sense that phenotypes of male individuals are similar to each other and female phenotypes are similar to each other.

An example of an auto-sexing breed is the Barred Plymouth Rock. In this breed, the auto-sexing property arises from the dose effect that the barring gene, B, exhibits. The B gene is on the Z choromosome so the male Barred Rocks have two B genes while the females have one. The males and females hatch with white spots on top of their heads with the male spot being larger and less sharply defined. Also, the females tend to have darker shanks because the B gene is an efficient inhibitor of shank colour. These two traits, based on the B gene dose effect, allow sexing of day-old chicks with a high accuracy rate.

Lethal genes:

Some genes are lethal. A dominant gene that is lethal when a bird has only one of that gene (heterozygous for that gene) is immediately taken out of the gene pool, since no bird survives with it. Some dominant genes are letal only when the bird has two copies of the gene. The creeper gene, Cp and the ear tuft gene, Et, are lethal to a chicken with two copies (homozygous). I am aware of an exception to this in which someone claims to have a male with two ear tuft genes that has survived. This should be considered to be a rare exception. The short leg genes in other breeds are often lethal. Some traits, like frizzleness and rumplessness are known to reduce hatchability but are not explicitly lethal.

Genetics of ear lobe colour:

Most breeds have red ear lobes. The red colour is due to the blood of the bird and is visible because the skin of the ear lobes, comb and wattles has a rich blood supply that is not masked in any way. These skin areas are so highly vascularized that squeezing a comb between your thumb and forefinger will more than likely squeeze out some of the birds blood onto your fingers. Mosquito bites often leave a small amount of dried blood on the comb. Breeds of the Mediterranean Class (Leghorn, Minorca and Spanish) have 'white' ear lobes.

The white ear lobe is due to the purine pigment which is controlled by a number of genes. The trait is said to be polygenic. The red ear lobe is due to the lack of the genes that invoke the purine pigmentation. Sometimes the white ear lobe can have a greenish or yellowish tinge. The number and location of the genes responsible for white ear lobes is not presently known.

Genetics of eggshell colour:

Brown eggshell colour is a complex trait and as many as 13 genes have been proposed to account for the range in eggshell colour. The white eggshell colour is due to an absence of blue and brown, and perhaps some modifying factors (genes), since there are different shades of white. The blue eggshell gene, O, expresses if it is present which is why it is considered to be dominant. The gene symbol for the recessive, wild-type gene is o or o+. My understanding at present is that the locations of the brown eggshell genes are not known and it is not known how many brown modifying genes there are or where they are in relationship to the genes of known locations. Brown may itself be just an array of white modifiers. There is a recessive sex-linked gene, pr, that inhibits the expression of brown eggshell genes and can be used to help remove the brown tint from white eggs, for example.

The brown pigment, ooporphyrin, is deposited primarily on the outside of the eggshell and is a chemical compound resulting from hemoglobin metabolism. In fact, much of the brown pigment can be buffed off with a common kitchen (plastic) scrubbing sponge and warm soapy water. The blue eggshell pigment, oocyanin, is a byproduct of bile formation and is present throughout the eggshell.

The eggshell colour genes interact in the following way. The effect of the blue gene is dominant over white. The effect of the brown gene is dominant over white. When blue and brown genes are both present, both genes contribute to the eggshell colour making the eggs appear green. In this case, the inside surface of the eggshell will be significantly less green and more blue than the outside surface, which is where most of the brown pigment is.

Since the blue and brown eggshell colour genes should be at different locations, we need at least two pairs of genes to describe the genotypes of the blue, white, green and brown layers. For the purposes of this discussion, I use the fictitious symbol, Br, to indicate a brown eggshell colour gene. I represent the complementary recessive gene that takes the place of Br when it is absent as "br" (lack of brown gene). We can represent the genotype of a blue eggshell layer as (O, O) with (br, br). Blue and white genes, (O, o) with (br, br) also yields a blue egg, but perhaps a lighter blue. The pair of eggshell colour genes, (O, O) with (Br, Br), are the genes for producing a green egg, (o, o) with (Br, Br) produces a brown egg and (o, o) with (br, br) yields a white egg. Females having one blue gene and one or more brown genes will lay eggs having a greenish colour. My personal experience with eggshell colour makes me believe that this genetics picture of eggshell colour is oversimplified (there are certainly more than one gene for brown eggshell colour. In order to account for the wide range of shades of brown eggs we see in our Sil-Go-Link line, there must be a relatively large number of eggshell colour modifying genes that are not yet known. Most people accept a rule of thumb to the effect that a daughter will lay eggs that are a colour between that of the parent lines.

To explore the genetics of eggshell colour, let’s cross a green egg layer (faux-Araucana or Easter Egg Chicken) with a white egg layer (Leghorn). Here as before, I will use the fictitous symbol "Br" to represent brown eggshell genes. The genes of the green egg layer are (O, O) with (Br, Br) assuming the locations of the blue and brown genes are not the same. The Leghorn is (o, o) with (br, br) for eggshell colour (white). In this example, the daughters will all have one gene for blue eggshell colour and one gene for brown. They will all be green egg layers! My personal experience with eggshell colour genetics leads me to believe it is more complex than this. There certainly must be a number of brown eggshell genes and once you have them, it is difficult to breed them out completely.

Continue to the next page for comb, leg colour and feathering

 

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