Head Color-Inheritance In Gouldian Finches
2nd November 2011, 1:25 pm
Head Color - Inheritance in Gouldian Finches (Chloebia gouldiae)
by: Robert Clark and John Wilson
(NFSS - May/June 1998)
Much of the difficulty in understanding the inheritance of head color in Gouldians results from the expectation of a clear hierarchy of dominance. Unfortunately such a pattern doesn't exist. The actual expression of head color is more along the lines of the children's Rock, Paper, Scissors game. Black covers yellow and red covers black, but yellow covers red. Were the relationships even this simple little if any confusion would remain and repeated efforts to clarify, such as this would not be needed. The glossary and definitions that follow reflect an effort to simplify the subject or at least cut it into bite-sized pieces. The genetics terms that follow are the same ones used in describing the inheritance of body color.
Glossary of Terms
Genotype: genetic make-up of the individual.
Phenotype: gross appearance of the individual, regardless of genetic make up.
Loci (pl) Locus (sing): a specific location of a trait (gene) on a chromosome. Gene pairs have matching loci, each of which carries a single gene.
Autosomal: Those characteristics inherited on gene pairs. These may be recessive, co-dominant, or dominant.
Sex-linked: Those characteristics inherited on the male sex chromosome. These may be inherited singly or in pairs by males and as a single copy in the female.
Homozygous: the presence of two alike genes at the two matching loci of a gene pair (2 yellow head genes are an example). A term generally applied to autosomal genes, but could also be applied to sex linked inheritance in males. However, the term double-factor (DF) for this sort of sex linked inheritance seems to be the more accurate, and clearer terminology, and is what will be used here.
Heterozygous: the presence of two dissimilar genes at the two loci of a gene pair. A term also generally applied to autosomal genes (a single yellow head gene is an example). This could also be applied to sex linked inheritance in males, but for clarity, this situation will be termed single-factor (SF).
Recessive: factors expressed in the phenotype only when present on both loci of a gene pair (DF or homozygous). These are hidden when carried as single factors.
Dominant: factors expressed in the phenotype when present on only one locus of a gene pair (SF) are considered dominant. See "Sex Linkages" for exceptions.
Split: abbreviated "/". Synonymous with heterozygous, and generally only used to refer to hidden traits. Used in describing both autosomal traits and sex-linked traits in males.
SF: single-factor. Only a single gene for a trait is present. Sometimes used interchangeably with "heterozygous" when referring to autosomal traits.
DF: Double-factor. Two genes for a trait are present. Sometimes used interchangeable with "homozygous" when referring to autosomal traits.
Sex Linkage
Because the genes for red head color are sex-linked and one of these genes is required for the expression of yellow head coloration, a clear understanding of sex linkage is necessary to understand the inheritance of both red and yellow head traits.
By definition, sex-linked traits are carried on the male sex chromosome (X-chromosome), while other traits are carried on autosomal chromosomes. This is a very important distinction. All autosomal chromosomes are carried in pairs by birds of both sexes. Male birds carry two sex (X) chromosomes and female birds carry only one sex (X) chromosome. This is exactly opposite to the situation in mammalian genetics. In Gouldian males, where two (X) sites are available , inheritance is similar to autosomal inheritance.
In females, there is a single (X) site , and any gene present will be fully expressed when present as a single copy, whether co-dominant, recessive, or dominant. As an example, a yellow hen carries a single co-dominant gene for yellow back color yet fully expresses yellow back, not dilute back.
It is simplest to describe the mechanism of Gouldian head color inheritance by referring to the black headed bird as the normal state and noting the effect on the black headed bird when it inherits one or more genes for red head or yellow head. All possible head color combinations can be described by this method without the added confusion of black headed inheritance. The actual mechanisms by which head colors are produced is described below.
Chemistry and Mechanisms
Research data has shown that melanin is the only pigment involved in head coloration in black headed Gouldians.
Canthazanthin (a beta-carotenoid) is the only pigment present in the head feathers of red headed Gouldians, while lutein epoxide (an alpha-carotenoid) is the only pigment present in the head feathers of yellow headed Gouldians. The distinction of alpha and beta carotenoids is important since it strongly implies that red and yellow headedness are truly separate traits with completely separate metabolic pathways for head pigmentation, rather than one simply being a derivative of the other.
There are additionally very distinct differences in the feather structure on the heads of black headed versus red or yellow headed birds. Specifically, the barbs are flattened and lack barbules in areas of pigment in the red or yellow headed birds, presumably allowing for better presentation of the color. This finding strongly suggests a second (structural) gene at play in the determination of head coloration. It would also suggest that either this second gene is tightly linked to the gene determining red head color, or alternatively, that this gene is suppressed by the genetic black head.
Head color
Black head is the most prevalent Gouldian head color in the wild, and probably represents the original Gouldian head color. Black head is inherited as a sex-linked recessive to red head. Many prefer to think of this as the absence of any head color modifier genes.
Red head inherits as a sex-linked trait, dominant over black head. Males may carry none (black head), one (SF red head) or two (DF red head) genes for red head. Since these genes are carried on the male (X) chromosome, the hen carries only a single copy of the gene. Hens may carry none (black head) or one (red head) gene for red head. Because the single X chromosome of a red headed hen must carry the red head gene and will be passed to every male offspring, all male offspring of a red headed hen will also be red headed, unless they also inherit two yellow head genes (yellow head). An easy means of remembering this relationship is that all black headed males have black headed mothers. The reverse of this statement is: red headed mothers produce all red headed or yellow headed sons. Red headed males can produce offspring of all three head colors depending on the genotypes involved.
Yellow head is most easily thought of as a double or combination mutation, even though that has been shown to be not precisely correct (see Chemistry & Mechanisms). The yellow head mutation inherits as an autosomal recessive trait requiring two genes for expression, but also requires the presence of at least one red head gene in order to be expressed. Offspring from yellow headed pairs may be black headed (with yellow beaks) hens only, (note this only occurs when the male is SF for red head) or yellow headed (cocks and hens) when the male is DF for red head.
From the above description it becomes apparent that either sex may carry a single yellow head gene without its expression. This is a hallmark of recessive inheritance. For a bird to be black headed, it must carry the black head gene on all its X chromosomes (1 for the hen, 2 for the male). However, a bird may be a phenotypic (visual) black head while also being a genetic yellow head. These birds carry two yellow head autosomal genes while also carrying black head genes on their X chromosomes. These birds will, however, have yellow-tipped beaks. Black headed birds that are heterozygous for yellow head will have red-tipped beaks, as will those that aren't carrying a yellow head gene. A bird which carries a single red head sex-linked gene will always be a visual red head unless also carrying two autosomal recessive yellow head genes, in which case it will be a phenotypic yellow head.
To obtain a yellow headed Gouldian, the bird must be both genetically yellow headed (carrying 2 yellow head genes), and must also be a "genetic" red head. Both sexes may be heterozygous for yellow head without expressing any visual evidence of this state.
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