What is an Albino in Birds?
• An Albino is an absence of colour. It can be across the
whole body including pink (pigment free) eyes or just part of the body.
What is Leucism in Birds?
• A bird that displays leucism has white feathers and pigmented eyes (black). It can be partial or incomplete and exhibits bilateral symmetry (will be the same on both sides of the body)
• Albino = a pigmentless
"white" phenotype determined by a mutation in a gene coding for a
pigment synthesizing enzyme. Albinism is a rare condition usually inherited in
an autosomal recessive Mendelian pattern in many animals. The
white phenotype results from the body's inability to make melanin. Albinos are generally homozygous recessive "aa" with white
skin & hair and pink eye pupils (this is due to the unmasking of the red
hemoglobin pigment in the blood vessels of the retina).
• The albino gene in rabbits - Variants of the "C" gene in rabbits can lead to 4 genotypes: normal/full colour (CC, Ccch, Cch, or Cc), chinchilla (cchcch, cch, ch, or cchc), himalayan
(chch or chc), and albino (cc). As you can see, the "C" gene is
involved in a dominance hierarchy. The "Full Colour" gene "C" is
dominant to the Chinchilla gene "cch" which is dominant to the
Himalayan (albino with black extremities) gene "ch" which is in turn
dominant to the Albino gene "c".
Q: What can cause the albino phenotype? A: Mutations - mutations occur in either the somatic tissue (tissue not associated with the gonads and germ cell production) or the germinal tissue (tissue associated with the gonads and germ cell production)
• Somatic Mutations - these occur in single cells during the
development of somatic tissue. Somatic cells reproduce by dividing in two so
the original cell with the mutation produces identical clones
of itself by splitting. This process occurs in every cell
of our body and when this cell splitting becomes accelerated by a mutation, it is
called cancer. Eventually a population of identical cells is produced from the
original cell, which occur in a patch. If the mutation occurs early in development, then the patch will be larger than if it occurred later in development. E.g. If a mutation occurred at the 4 cell stage of development, the mutation will be in 1/4 of
the cells when the animal is fully developed. Compare this to a mutation that
occurs at the 64 cell stage. When the animal is fully developed, the mutation
will be present in only 1/64th of all of the animal's cells! If the mutation
has to do with colour, the animal will appear splotchy due to different areas
being different colours.
• Germinal Mutations - these occur in the germ line (tissue
that will eventually form the sex cells - eggs in females and sperm in males).
If a mutation occurs in a cell that becomes a gamete (a sex cell) and
the gamete is involved in the fertilization process (uniting of 2 different
gametes), then the mutation will be passed on to the next generation). If the mutation is recessive, it will remain hidden, until an
individual occurs with 2 copies of that mutation. If the mutation is dominant, it
will be visible in the offspring/next generation.
What types of mutations can occur?
• Loss of function mutations - this is a mutation event that destroys and/or changes critical functional regions of a gene (the
gene loses its normal/wild type function). A loss of function mutation can be either
complete or incomplete. If it is complete then it is called a null mutation
because there is nothing left of the normal/wild type function. If it is incomplete,
then it is called a leaky mutation because some of the normal/wild type function
"leaks" through into the phenotype. This may produce an intermediate
phenotype or a gradient of phenotypes depending on the "leakiness".
Loss of function mutations are generally recessive.
• Gain of function mutations - this is a mutation that causes
a receptor for the production of a protein to be constitutively active (the switch for the production of a protein is turned ON and can not be shut off). This is sufficient to cause an abnormal or new phenotype
to develop. Gain of function mutations are dominant.
Types of Mutations known to cause albinism in various
animals:
(1) Dominant Mutations
(2) Autosomal Recessive Mutations
(3) Sex-linked Recessive Mutations
(4) plus several others
• Example - Albinism in Japanese Quail
Types:
(1) Complete Albinism - autosomal recessive; completely white bodies with bright red eyes
(2) Incomplete Albinism - sex-linked recessive; pink eyes with white to light yellow bodies, light barring may occur on some feathers
(3) Dominant White - incomplete autosomal dominant; white bird with black skin; partially lethal; few birds survive to adulthood
(4) Recessive White - autosomal recessive; white bird with a few wild type feathers splashed on the head, neck, and back)
(5) White Feathered Down - autosomal recessive; lethal in the homozygous form; white feathers and down and black eyes
(6) Brown Splashed White - autosomal recessive; white splashed with WT feathers predominantly on the head, back, wings, and breast
(7) Panda - autosomal recessive; wild type splashed with white; breast feathers and primaries are always white
(8) White Breasted - autosomal recessive; white fronted birds-from eyes to vent
(9) White Crescent - possibly autosomal recessive; white crescent on the chest of a wild type colored bird
(10) White Bib - possibly autosomal recessive; white bib on the chest of a wild type colored bird
(11) White Primaries - autosomal recessive; white primary feathers only
• Why can some of these mutations be classified as albinos?
Albinism is the complete lack of pigment in the feathers,
eyes, or skin and all of these mutations exhibit at least one of those
criteria. Some have white feathers, but normal skin and eyes - they are albinos
for feather colour. The mutations that exhibit a mix of feather colours are
partial albinos for feather colour. Several genes (maybe more) are responsible
for feather colour in birds and a mutation in at least one of these genes is
leading to white feathers. These birds are an albino for the gene(s) causing
the white feathers and wild type at the genes where there has been no mutation.
• Example - Albinism in Pheasants
Types:
(1) Pied Spotting - autosomal recessive; pied with white feathers; dirty blue eyes
(2) Blue Eyed White - autosomal recessive; completely white feathers and blue eyes; pearl white shanks and beak
(3) Brown Eyed White - autosomal recessive; completely white feathers and brown eyes; buff beaks and shanks
• This is a classic example of albinism that affects
everything except the eyes (the feathers, skin, legs, and beak are without
pigment). The male in this picture appears to have black specks on it, but it
is just mud.
• These are 5 female White Ringneck Pheasants with a wild type female in the back. It is hard to tell, but all of the birds have wild type coloured eyes (which is why this autosomal recessive mutation was called white and not albino).
(4) Incomplete Albinism - sex-linked recessive; white bird and pink eyes; reduced viability
(5) Albino - inheritance unknown; white feathers and red eyes
Albino Chukar
(6) White Skin - autosomal dominant; white skin [pinky hue]
• Everest is a Blood Parrot with a large pure white
patch on his head. It is not an injury or rubbed off scales so it is most
likely a mutation. A mutation such as this would have occurred while he was
developing as an embryo. A single cell (one that would lead to all of the cells
that have produced the white patch on his head) underwent a mutation in the
colour gene which blocked the production of pigment. This lead to the albino
phenotype exhibited by those cells. As he developed, this cell would have
multiplied and has resulted in the large patch. It is an odd and rare example
of albinism that affects only part of the body.
• Albino catfish - white scales with red eyes
References
• Cooke, F. and Buckley, P.A. 1987. Avian Genetics A Population and Ecological Approach. Academic Press.
• Crawford, R. D., 1990. Poultry Breeding and Genetics. Elsevier Amsterdam, The Netherlands.