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Definitions of Words used in the Articles

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Albinism: A group of inherited conditions which include a decrease in the amount of pigment in the eyes, skin and feathers.

Allele: Just a specific variation of a gene

Autosomal: Referring to a chromosome other than one of the sex (Z or W) chromosomes.

Cell: The container for chromosomes. A Budgerigar cell carries at least 30 chromosomes in a gamete cell or at least 30 chromosome pairs in all other cells.

Chromosome: A microscopic structure, made out of DNA, which carries the genes. All cells within the body have a set of chromosomes. Each chromosome contains a large number of genes except for the W-chromosome in female birds.

Co-Dominance: The lack of dominant or recessive traits allowing both alleles in a chromosome pair to express themselves.

Cross section: A cross section is a cut through something (such as a feather shaft) at an angle perpendicular to its axis in order to view its interior structure.

Crossover: When a pair of chromosomes trade genes.

DNA: Desoxyribonucleic acid, a natural substance which stores genetic information as an intertwined double chain. The body reads the code stored in this chain to learn how to assamble proteins from amino acids.

Dominant: The opposite of recessive. A gene that is visible when paired with other genes. The most expressive gene at the allele of a chromosome pair.

DOPA: Dihychoxyphenylalanine, a natural chemical which the body makes as a step in the process of making the pigment melanin.

Epistasis: The suppression of a gene by the effect of an unrelated (epistatic) gene.

Eumelanin: A black or brown form of the pigment melanin as is to be found in e.g. Budgerigars.

Featherbulb: The "root" of a bird feather, from which growth and coloration of the feather develops. Featherbulbs go through moulting cycles, and when moulting or the feather is pulled out, a new one starts to grow after three days.

Gene: A piece of information, stored in a code in DNA, which tells the body how to make a particular protein.

Hemizygous: Hens, having only one copy of the Z-chromosome or a human male having only one copy of the Y-chromosome.

Hypopigmentation: A general term for decreased or total absence of pigmentation or coloration.

Hypostasis: The inability of a gene to produce its usual effect when coupled with another gene that is epistatic toward it. Such a gene is called hypostatic.

Iris: The normally white colored part visible in the eyes of Budgerigars, which closes in and opens out around the pupil.

Locus: A location on a chromosome or chromosome pair. A specific place that defines an allele.

Melanin: A type of pigment made in the eyes, skin and feathers.

Melanocyte: A type of cell specialized to make the pigment melanin. Melanocytes are located in the skin, the featherbulbs and the eyes. Researchers have found melanocytes in the skin, featherbulbs and eyes of birds with albinism.

Melanosome: A pigmentgranule within the melanocyte and/or within the featherbarbules.

Microchromosome: A chromosome that is almost to small to positively identify as a chromosome. Therefore the exact number of chromosomes in the Budgerigar is higher than in most reference books.

Modifier: Secondary genes that modify the expressivity of a co-dominant or dominant gene.

Phaeomelanin: A reddish or foxbrown form of the pigment melanin which has never been found in psittacine species.

Phenotype: The appearance. What the dominant genes express.

Pigment: A coloring matter. Pigment absorbs light in the medullas of the featherbarbules. The eye sees the light that is not absorbed but reflected back.

Pleiotropism: The control by a single gene of several distinct and seemingly unrelated phenotypic effects.

Recessive: Referring to an altered gene which does not shows its effect if the bird carrying that gene also has an unaltered gene.

Retina: The surface on the inside of the back of the eye. Light enters the eye through the pupil, and the lens focuses the light on the retina. The retina converts the light to a message to the brain.

Sex-linked: Any gene on the Z-chromosome.

Species: A set of creatures which, in their natural setting, breed among themselves.

Transposon: Segment of DNA that moves to a new location in a chromosome, or to another chromosome, and alters theexisting genetic instructions, sometimes producing significant changes. They are able to trigger changes in gene expression by shutting of genes or causing insertion mutations.

Tyrosine: An amino acid used by the system to make melanin.

Tyrosinase: An enzyme or specialized protein substance in the pigment cell which promotes the conversion of the amino acid tyrosine to DOPA in the process of making pigment.

X-linked: Referred to a gene that is passed on with the Z-chromosome. Male birds have two Z-chromosomes, while female birds have one Z-chromosome.