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 dis-similar 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 determinnation 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 cock). 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.

Putting it all together

Two tables are included with this article. Table 1 is a Phenotype/Genotype table, showing all possible genotypes of head coloration and the corresponding phenotypes. Table 1 treats the absence of an “R” gene at the Red locus as being the same as the presence of a black head gene at the same locus. Of nine possible combinations, only six can be females; the three genotypes having two genes for red
are always males. Each genotype is assigned a number 1-9. Reading across is TYPE NUMBER (our assigned number), PHENOTYPE (visual description), SEX (the sexes that may possess that particular genotype), CHROMOSOME COUNT (“R” and “Y” representing the individual genes of the specific genotype), and GENOTYPE (the genetic  description).

Table 2 is a breeding outcomes chart using the Genotype numbers from Table 1 to show all possible breeding results by genotype/phenotype. Nine male genotypes are listed along the left margin and six female genotypes across the top. Reading across are two lines of boxes for each male genotype, the upper representing male offspring and the lower representing female offspring. The boxes at the intersections of the horizontal lines and vertical columns contain numbers which correspond to the numbers in Table 1 and represent all possible genotypes that can be produced by that specific pairing. This chart does not attempt to address the probability of any specific outcomes.

                       Table 1.

 Table 2.

Using Table 2
Known Genotypes: When the genotypes of a pair are known, possible outcomes may be read at the intersection of the male and female genotypes.

Unknown Genotypes: If genotypes are unknown it is still possible to know something about the possible outcomes. Phenotypes are grouped in both tables allowing easy location of possible outcomes by phenotype. Example: Red headed cock (6,7,8,9) X red headed hen (7,9) would include all outcomes listed in the 16 boxes representing the intersection of these phenotypes.

Determining parents genotype by outcome: Phenotypes of offspring can be used in with Table 2 to refine what is known about the parents genotype. In the example above (RH X RH) production of any yellow heads (4,5) mean that the cock is either a #6 or #7 and the hen a #7. Only boxes at intersections of these genotypes contain
the correct outcomes. Should a #3 BHYTB also be among the offspring the exact genotypes of the pair are identified. The cock and hen are both #7 genotype.

Hopefully this article has provided a fresh approach to the inheritance of head coloration in Gouldians as well as helping clarify sex-linked inheritance, especially as it relates to head coloration.

We wish to extend our thanks to Dr. Luis Baptista for his assistance in locating materials for this article.

References:

1. Brush AH, Seifried H: Pigmentation and feather structure in genetic variants of the Gouldian Finch, Peophila Gouldiae. The Auk 85:416-430, 1968.