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BIRD INFORMATION:
 Getting Started

General Info
 Bird Care
 Taming & Training
 Health & Nutrition
*Breeding & Genetics
     - Basic Genetics
     - Sex-Linked Mutations
     - Crossovers
     - Allelic Mutations
     - Coloration Mechanics Pt 1
     - Coloration Mechanics Pt 2
     - Genetic Calculators
     - Oddities: Tricolor Tiel
     - Oddities: The Spot Gene
     - Cockatiel Split Signs

     - Hormone Control
     - Nestboxes
     - Egg candling
     - Egg binding

Breeding & Genetics

Allelic Mutations

Note: this article was written specifically for cockatiels. Similar principles will apply to other psittacine species.

Most color mutations are independent of each other and occur in entirely different gene pairs. But there are some color genes that have mutated more than once, resulting in different mutations that are related to each other and are classified as belonging to the same gene family. These related mutations are called alleles, which means that they are different variations of the same gene and can occur in the same gene pair. This article will use the words 'gene' and 'allele' somewhat interchangeably.  In many cases 'allele' is the proper technical terminology, but 'gene' is more familiar to most people and easier to understand.

Every color mutation gene is located on a specific chromosome, and each of these genes sits in a very specific location on its chromosome. This location is called the locus for that gene (because Latin sounds spiffier than English after all). As far as we know, there are only two alleles for most color mutations – the original wildtype gene for that locus and the color mutation gene that was developed later due to a spontaneous change in the wildtype gene. It's possible that other alleles may exist at that locus but they don't produce a noticeable effect, and for practical purposes we can disregard them.  For example, you would expect to find either the cinnamon gene or the related wildtype gene at the cinnamon locus, but at present there aren’t any other known genes that could be at that locus. Cinnamon is not considered to be a member of a mutation-gene family because there aren’t any other mutation genes that can occur at the cinnamon locus.

With a gene family, there ARE several mutation genes that can occur at the same locus, and they can be paired with each other in “mix and match” style, as well as being paired with the related wildtype gene that was the ancestor of all the mutations. Notice though that these genes are being paired not mixed together in a big group, so two is the maximum number of related genes that can be present in one bird.

The Blue Locus

The biggest gene family in cockatiels consists of whiteface alleles. Due to the quirks of nature, the location of these genes is called the blue locus not the whiteface locus. Blue-locus mutations occur in many different parrot species, and many of these species are normally green. Blue-locus mutations remove or reduce the yellow/orange/red coloring on a bird, and when you subtract yellow from a green bird, you get a blue bird; hence the name. It isn’t an actual color pigment that makes these birds look green; it’s a special structure in the feathers that makes melanin reflect light in a way that looks green to our eyes. Cockatiels normally have melanin in most of their feathers but they don’t have the special feather structure that makes it look green, so they look grey to us. The yellow-removal process is the same in all species, but the original visual effect is different so the visual effect of the mutation gene is different too. You do NOT get a blue bird when you remove the yellow from a cockatiel, so the word “whiteface” was invented to describe the bird that you do get. The name of the locus wasn’t changed though, so it’s called the blue locus even for cockatiels.

At present there are three cockatiel mutation genes that can occur at the blue locus: whiteface, pastelface, and creamface. It was previously believed that goldcheek was an allele, but it has now been determined that it is actually sex-linked dominant - not even on the same chromosome as the others. Creamface is rare. Pastelface is still somewhat rare but it’s becoming more common, and whiteface is very common.  Pastelface and creamface are classified as parblue mutations.   'Parblue' essentially means partial blue; it removes some of the yellow pigment from the bird but not all of it.

In every bird, there will be two genes from this family at the blue locus, one inherited from the mother and one inherited from the father. The bird will either have two of the same gene at this locus (the homozygous state) or it will have two different genes (the heterozygous state). There aren’t any convenient abbreviations for homozygous and heterozygous in bird-genetics lingo, so this article will borrow from reptile-genetics terminology and call them hom and het, or PF-PF to indicate a bird that is homozygous PF (meaning that it has two PF genes) and PF-WF for one that is heterozygous PF (meaning one PF gene and one WF gene).

The blue-locus mutation alleles are all recessive to the wildtype gene. In the general cockatiel community there is disagreement about whether the parblue mutations are co-dominant or dominant to whiteface.  However the evidence from other parrot species with the same or similar mutation indicates that the parblue mutation alleles are co-dominant to each other in these species, so it's expected that pastelface and whiteface should be co-dominant to each other. This means that there is a blended effect when a bird has both genes, and a heterozygous PF bird (one with one PF gene and one WF gene) is expected to have paler coloring than one that is homozygous PF (two PF genes). Avian genetics expert Terry Martin estimates that hom PF reduces the intensity of the yellow/orange color to 50% of normal. There is no information on how much additional color reduction there is in het PF; but the mathematical average of hom PF (50% of normal intensity) and hom WF (0% of normal intensity) is 25% of normal intensity which might not be an unreasonable estimate for a PF-WF bird.  That's the theory anyway; in the real world, the bird's other splits and visual mutations have an influence on the cheek color, which makes it difficult to determine what's really going on.

It is a common practice to pair PF birds with WF. In Europe they use a special term to identify heterozygous (PF-WF) birds, calling them palefacewhiteface. In the U.S. there seems to be a general tendency to think that PF is dominant to WF, because there is still some color present when the genes are paired together (het PF), even though the whiteface gene may have washed out some of the color and the result may be paler than hom PF. There are other mutations that can have an influence on face color so it's not always easy to identify cause and effect. Because of this thinking, there is no word in the US to distinguish het PF (PF-WF) from hom PF (PF-PF); both types are called pastelface, or sometimes pastelface split whiteface for heterozygous birds. This imprecise terminology has the potential to cause confusion about a bird's genetic makeup and how these genes interact with each other.

The way that the two online genetics calculators deal with pastelface reflects this dual thinking. With the simpler genetic calculator, a het PF bird must be entered as split to both mutations and visual for neither, and the results will tell you that het PF chicks are split to both mutations and visual for neither, but will look like a visual pastelface bird.  This approach is flawed since a het PF bird is certainly visual for something; but at least it reflects the reality that the bird has one gene for each of two different recessive mutations. 

With the more complicated calculator, het PF birds must be entered as visual pastelface split to whiteface.  Although this is the way many people think of these birds, there are major theoretical problems with this approach.  Since PF and WF are both autosomal recessive mutations, it implies that a het PF bird has two genes for pastelface and one for whiteface at a locus where it is only possible to have two genes.  The results also look strange; data entry for two het PF parents will show 25% WF babies in the results. In general, when both parents are visual for a recessive mutation all offspring should be visual for that mutation, and people who don't understand the mechanics of allelic mutations will be left wondering how two visual PF parents could have non-PF babies.

Creamface is so rare that it isn’t known whether it has a co-dominant relationship with pastelface and whiteface. But it’s likely that it does; it has been established that blue-locus alleles in other psittacine species all have co-dominant relationships. As creamface becomes more easily available, it is theoretically possible that it could be indiscriminately combined with pastelface. This would not be a good idea though because it would result in a variety of blended visual colors, making it very difficult to figure out what genes the bird has. At present, it’s acceptable to breed these mutations with whiteface birds but not to mix the rarer alleles with each other.

Some examples are given below showing the outcome of different combinations of parent genes. The examples show outcomes only for pastelface and whiteface, but the same principles are expected to apply to the other whiteface alleles. The examples use Punnett square diagrams which are a standard method for displaying genetic outcomes. There is a row for each of the mother's blue-locus genes and a column for each of the father's blue-locus genes, and each intersection of a row and a column shows the characteristics of the offspring who received these genes. There are four offspring squares in each diagram, and each square represents 25% of the total offspring. The article on sex-linked recessive mutations used little cockatiel icons to represent the birds, but those are too small to clearly show subtle color differences. So the diagrams here use colored circles to represent the color of the cheek spot. It should be noted that the blue-locus alleles affect the intensity of all yellow/orange color on the body, not just the cheek spot.

The examples are meant to illustrate the general principle, not to show every possible combination.  The blue-locus genes are carried on a pair of autosomal chromosomes, so it doesn't really matter which specific parent has a particular pair of genes; a hom PF male and a WF female will have the same results as a WF male and a hom PF female.

The following icons are used in the charts to illustrate the genetic characteristics and visual appearance of the offspring:

  Normal face color (wildtype) (N)

  hom PF (homozygous pastelface) (PF-PF)

  het PF (heterozygous pastelface-whiteface)(PF-WF)

  WF (homozygous whiteface) (WF-WF)

 

Example 1: Father is het PF (PF-WF), mother is het PF (PF-WF)

     Father
PF WF
 
M
o
t
h
e
r
PF PF-PF
hom PF

WF-PF
het PF
WF PF-WF
het PF


WF-WF
Whiteface

 

Example 2: Father is hom PF (PF-PF), mother is normal split WF (N/WF)

     Father
PF PF
 

M
o
t
h
e
r

N N/PF
Normal split PF
N/PF
Normal split PF
WF PF-WF
het PF

PF-WF
het PF

 

Example 3: Father is WF (WF-WF), mother is het PF (PF-WF)

  Father
WF WF
 

M
o
t
h
e
r

PF WF-PF
het PF

WF-PF
het PF

WF WF-WF
Whiteface

WF-WF
Whiteface

 

Example 4: Father is normal split whiteface (N/WF), mother is het PF (PF-WF)

  Father
N WF
 

M
o
t
h
e
r

PF N/PF
Normal split PF

WF-PF
het PF

WF N/WF
Normal split WF

WF-WF
Whiteface

 

The Ino Locus

There's more than one color mutation family in cockatiels with multiple mutation alleles. Platinum is an allele of lutino, and the location of these genes is called the ino locus. At present the platinum mutation is only available in Australia.

Similar to the blue-locus alleles, lutino and platinum are recessive to the wildtype gene and co-dominant to each other. Terry Martin estimates that the platinum gene reduces the melanin to 50% of normal intensity while lutino reduces it to near zero, and the two genes combined have an intermediate effect.

But unlike the blue-locus alleles, these genes are sex-linked.  The inheritance rules are different for males and females, and it DOES matter which parent has which gene(s). For detailed information on sex-linked inheritance, see the article on sex-linked recessive mutations

The ino locus is on the X chromosome, and with birds males are XX and females are XY. Because hens have only one X, they can have only one ino-locus gene. They can be platinum or lutino but they can’t have both genes, and they also can’t be split to either gene; if they have one of these genes they will be visual for the color. Males have two X’s so they CAN have both genes, and it’s also possible for them to be split to one or the other. A male who has platinum on one X paired with lutino on the other X is called a platino; there is no need for het and hom designations because there is a unique word for each state.

The following icons are used in the charts to illustrate the genetic characteristics:

  Normal grey (NG)

  Platinum (PL)

  Platino (PL-L)

  Lutino (L)

Here are some examples showing the results for different combinations of parent genes. Colored circles are used to indicate the gene on the parent's X chromosome and the visual color of the offspring, and a “Y” is used to designate the hen's empty Y chromosome.

 

Example 1: Father is platinum, mother is lutino

     Father
 
M
o
t
h
e
r
PL-L
Male
Platino

PL-L
Male
Platino

Y PL-Y
Female
Platinum
PL-Y
Female
Platinum

 

Example 2: Father is lutino, mother is platinum

     Father
 
M
o
t
h
e
r
L-PL
Male
Platino

L-PL
Male
Platino

Y L-Y
Female
Lutino
L-Y
Female
Lutino

 

Example 3: Father is platino, mother is platinum

     Father
 
M
o
t
h
e
r
PL-PL
Male
Platinum

L-PL
Male
Platino

Y PL-Y
Female
Platinum
L-Y
Female
Lutino

 

Example 4: Father is platino, mother is lutino

     Father
 
M
o
t
h
e
r
PL-L
Male
Platino

L-L
Male
Lutino

Y PL-Y
Female
Platinum
L-Y
Female
Lutino

 

Example 5: Father is platino, mother is normal grey

     Father
 
M
o
t
h
e
r
NG/PL
Male
Normal grey
split platinum

NG/L
Male
Normal grey
split lutino

Y PL-Y
Female
Platinum
L-Y
Female
Lutino

 

Non Sex Linked Lutino (NSL Ino)

It isn't clear whether this mutation actually still exists in cockatiels or not. It originated in Europe but was difficult to establish, and the whereabouts of the only known breeder are currently unknown.  However the mutation has been established in other species including budgies and Indian ringnecks.

Personally I dislike the name NSL ino, because using the word "lutino" to describe two completely unrelated mutations has the potential to cause vast amounts of confusion. It's confusing enough that an NSL ino bird looks similar to a sex-linked lutino. NSL ino and bronze fallow (usually called fallow in the US without including the word 'bronze') are different versions of the same gene (alleles of the same locus). A mutation name that included the word 'fallow' instead of 'lutino' might help make the distinction from sex-linked ino clearer, but the mutation was named before the nature of the gene was fully understood. 

The fallow/NSL ino  gene family follows the inheritance rules for ordinary recessive mutations, and the mutation alleles are co-dominant to each other.  Because of the rarity or outright nonexistence of the NSL ino mutation in cockatiels, diagrams are not presented here to show the interaction between these genes.  This gene family follows the same inheritance rules as the blue-locus alleles described earlier in this article, and the blue-locus diagrams would show accurate results for NSL ino and fallow simply by changing the names and colors on the charts.

Copyright 2014 Carolyn Tielfan all rights reserved