Definitions of Terms

Here is a brief lesson in those genetic concepts which pertain to understanding how different traits are inherited in finches, followed by a list of terms and their definitions:

Chromosome pair

This is a chromosome pair or homologous pair, consisting of two chromosomes (numbers 1 and 2) which are similar in form and function. There are two classifications for the chromosome pairs studied in finch genetics, the sex chromosomes and the autosomes. The sex chromosomes are a single pair of chromosomes which, as the name implies, determine the bird's sex. They are denoted by Z (the avian equivalent to the mammalian "X") and W (the avian equivalent to the mammalian "Y"). Unlike mammals, the cock carries two Z chromosomes (ZZ) and hen carries one Z and one W (ZW). Note that this is the opposite of mammalian genetics where the male is XY and the female is XX. Interestingly, of the two sex chromosomes, only the Z chromosome seems to carry information for color; for our purposes, the W only functions to turn the bird into a hen, and does not seem to carry information for color. So when a specific color is carried by the Z chromosome, that color is said to be sex-linked, or carried by a sex chromosome.

The other pairs of chromosomes are called autosomes or autosomal pairs, they include every pair except the sex chromosomes. We will consider our pair of chromosomes 1 and 2 autosomal so that for this example, either sex can carry them.

Gene loci

The area circled on each chromosome is called a locus (we'll call this locus "A"). The locus is the position that a given gene occupies on a chromosome. Each locus is represented twice in the genome (one "locus A" will be found on each chromosome).

Homozygous genes

Here is the chromosome pair sporting identical genes (highlighted in purple) at each locus A. Because the locus on each chromosome contains the same gene, this pair is said to be homozygous or "double-factor" for the purple gene. If the purple gene affects the breast color of the bird, the bird with this chromosome pair would have a purple chest.

Heterozygous genes

Here is the chromosome pair again, only this time the chromosomes do not contain the exact same gene on locus "A." When this occurs, the gene pair is called heterozygous, and the bird is said to be "single-factor" for the white gene (or, respectively, single-factor for the purple gene). Locus "A" on chromosome 1 contains a purple gene and locus "A" of chromosome 2 contains its allele, a variation of the purple gene. This variation also affects the breast color of the bird, but it contains information to turn the breast white instead of purple. The genotype or genetic makeup for this bird, therefore is single-factor for white breast, single-factor for purple breast. But which trait will show up physically on the bird? In other words, which of the two alleles will "win" and become the phenotype, or physical manifestation? The answer is whichever of the two alleles is dominant. If the purple allele is dominant, the bird will have a purple chest, even if it is still carrying the white gene. In this case, the white gene is recessive and the bird is said to be "split" for white breast (it carries the gene for white breast, but because it is not carrying two copies of the gene, and because white is recessive to purple [which is also present], the bird does not express the white breast color, and so the chest appears purple). In some cases, a gene will be neither recessive nor dominant, but incompletely dominant, and the color will be shared equally by both genes; if white and purple were incompletely dominant, the chest would appear to be a shade in between white and purple.

Now, some genetics terms defined:

One of one or more possible forms of a gene, each affecting the inherited trait somewhat differently.
"Of or relating to any chromosome other than the sex chromosomes," a characteristic inherited on any gene pair other than the sex chromosomes.
Chromosome Pair:
A pair of chromosomes that are similar in form and function, but may vary in genetic composition due to allelic differences at matching loci on the pair; "homologous pair."
There is no dominant allele, rather both alleles are expressed at the same time. An example of co-dominance is blood group types (A, B, O, and AB). Co dominance can also cause a heterozygous bird to appear as a "visual split." For example, if rump color is codominant, a chick which inherits a gene for black rump from one parent, and a gene for white rump from the other, will have a rump which is not gray nor all-black nor all-white, but rather a mix of solid black feathers and solid white feathers.
A genetic factor that will be expressed in the phenotype even when only one copy is present (heterozygous or single-factor). E.g.: A is dominant over a because the phenotypes of AA and Aa are the same.
Two identical "copies" for a gene are present in the gene pair (one copy per chromosome); abbreviated as DF or DBF and sometimes used interchangeably with "homozygous" when referring to autosomal traits or sex-linked traits in a cock bird.
The genetic makeup of an organism, including those traits which are not physically expressed (such as a gouldian who is split for white breast or split for blue).
Having two different alleles for a given trait at corresponding loci on a chromosome pair.
Having identical genes at the corresponding loci on a chromosome pair. Usually applied to autosomal chromosomes, but can also be used to describe the sex-linked inheritance in cock birds. Usually interchangeable with "double-factor."
Incomplete dominance:
A genetic factor that will be "partially expressed" due to the sharing of its controlling influence over the phenotype with the other genetic factor(s) which are present. In other words, the dominant allele (R) is not completely dominant over the recessive allele (r), giving the heterozygote (Rr) a phenotype which is intermediate between the homozygotes (RR and rr, respectively). E.g.: In flowers, red is incompletely dominant with white, so any flower containing information for both red and white will not have red petals nor white petals, but rather petals which are a shade of pink. Some people refer to offspring carrying incompletely dominanant genes as "visual splits."
Locus (pl. loci):
The position of a particular gene (or its allele) on a chromosome; chromosomal pairs have matching loci (plural of locus).
The expression of a genotype in the appearance of an organism; the observed trait.
A genetic character or factor that will only be phenotypically expressed when present on both loci of a homologous pair (homozygous or double factor). E.g.: a is recessive to A because the phenotype for Aa is like AA and not like aa.
Those genetic characteristics that are present on a sex chromosome, in this case, the male's Z chromosome (female's W chromosome determines sex only and not color). May be inherited as single or double factor in cocks, and only single factor in hens.
Only one gene is present for a particular characteristic in a homologous pair. Abbreviated SF and occasionally used interchangeably with heterozygous with reference to autosomal traits.
Usually used to describe those traits which are "hidden" (recessive) in a heterozygous situation. Abbreviated as "/".
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