Alex Golden

 

College Biology Research Paper

 

January 20, 2002

 

 

 

The Genetics of Cockatiel Mutations

 

 

               The cockatiel, Nymphicus hollandicus, is a medium-sized Australian parrot with an erectile crest on the top of its head.  It has so very readily taken to captivity that it is now extremely well-known as both a cage and aviary bird.  Although it has been captive-bred for more than a century, it is only in very recent times that the cockatiel has come to exist in various distinctive color varieties giving them quite a different appearance from the wild-type grey.  By the usual definition, such changes of color and the multiple generations of captive breeding can be said to have made the cockatiel into a fully domesticated bird (Smith, 9).

 

               As with humans and other animals, the individual characteristics of any bird are determined by the formation of the first cell in the reproductive process resulting from the combination of a male cell (sperm) and female cell (ovum).  Each of these cells, specific to reproduction, carries only half the genetic material found in the cells of most living organisms.  Their combination results in a cell carrying a full complement of genetic instructions, half from the cock and half from the hen.  This cell divides and multiplies as the embryo grows.  Within the nucleus of each cell, this genetic information is carried on strands of DNA.  These strands, referred to as chromosomes, are linked together in pairs, one from the hen and one from the cock.  Situated along the length of the chromosomes are genes in corresponding positions forming pairs (Anderson, Cross, 50).

 

               Cockatiel mutations determine the color and color pattern of the bird’s feathers.  It’s parents’ own mutations, as stated above, determine the combination of a cockatiel’s mutation.  There are two types of mutations: sex-linked, and autosomal recessive.

Autosomal chromosomes carry autosomal recessive mutations.  The wild color of most birds (including cockatiels) is dominant, while most mutations are recessive.  A single dominant gene will always visually manifest itself.  The recessive mutation gene must be with another of the same type to manifest itself.  A single dominant and a single recessive produce a bird that will be split to the recessive.  The following mutations are autosomal.

 

Fig 4: Autosomal Mutations.

Symbol

Name

Pd

Pied

F

Fallow

S

Silver

Wf

White Face

Ss

Silver Spangle

 

Punnet squares can be used to predict the outcomes of a pairing.  A very simple example is a normal cock bred to a normal hen.  All offspring would be visually normal.

 

Fig 5: Normal x Normal, NN x NN

 

N

N

N

NN

NN

N

NN

NN

 

 

Fig 6: Normal x autosomal recessive mutation Pied,  NN x Pd Pd

 

Pd

Pd

N

N Pd

N Pd

N

N Pd

N Pd

All offspring would be visually normal and split to pied.

 

Fig 7: Pied x Normal split Pied – Pd Pd x N Pd

 

N

Pd

Pd

N Pd

Pd Pd

Pd

N Pd

Pd Pd

50% offspring are visual pied, Pd Pd

50% are visual normal and split to pied, N Pd

 

               In the case of autosomal recessive, we can see that it does not matter if it is the male or the female which carries the gene, the result is the same either way. 

 

               One pair of chromosomes is different from the others.  This pair is responsible for determining sex.  Male cockatiels have two ‘X’ chromosomes, while female birds have a single ‘X’ and a ‘Y’.  This is the exact opposite from mammals. 

 

               The ‘X’ chromosome is able to carry mutation information, while the shorter ‘Y’ chromosome does not.  Therefore a female cannot mask the effects of a mutant gene.  The male, on the other hand, has two ‘X’ genes; so if the mutation is present on one ‘X’ only, the male would be ‘split to’ or ‘split for’ the recessive sex-linked trait.  The female may not be split to a sex-linked mutation.  The following are the sex-linked mutations and their symbols.

 

Fig 1: sex-linked mutations

Symbol

Name

C

Cinnamon

Pl

Pearl

L

Lutino

Pt

Platinum

Yf

yellowface

 

 

 

 

               One uses superscript to show the mutation(s) carried by the ‘X’ chromosome like this:  a visual lutino cock is -  XL XL, a split lutino cock bird is -  XL X, a lutino hen is -  XL Y.

 

               One can use what is called a Punnit Square to plot the results of a pairing.  Here are some examples.

 

Fig 2: Split lutino cock x Normal hen - XLX  x  XY

                             

 

XL ¯

X ¯

X®

X XL

X X

Y®

XL Y

X Y

 

25% of offspring are Split lutino cocks- X XL

25% are normal cocks X X

25% are lutino hens XL Y

25% are normal hens X Y

 

 

This becomes more complicated but uses the same principle for multiple sex-linked mutations.  In the case of the cock that is split for two sex-linked colors, the mutant genes can be carried on the same X chromosome, or he can carry one on each.  In the next example two charts should (but need not) be used, to cover both possibilities (Andersen, Cross, 57).

 

Fig 3: Normal cock split Lutino and Pearl x Normal hen -  XL XPl  x  X Y

 

XL ¯

XPl ¯

X®

X XL

X X Pl

Y®

XL Y

XPl Y

 

Fig 3a : Normal cock split Lutino and Pearl x Normal hen- X XLPl  x X Y

 

XLpl ¯

X ¯

X®

X XLpl

X X

Y®

XLpl Y

X Y

 

                                                                X XL Normal split lutino cocks    

X XPl Normal split pearl cocks

XL Y Lutino hens

XPl Y Pearl hens

XLPl X Normal split lutino and pearl cocks

X X Normal cocks

XLPl Y Lutino Pearl hens

X Y normal hens

 

 

              

 

               To calculate the possible combinations between sex-linked and autosomal mutations one must simply plot all possibilities on the Punnet Square using as many squares as needed, depending upon the number of possible outcomes.

 

 

 

 

               Cockatiels are excellent subjects for the study of genetics.  They are relatively easy to breed and one can, as seen above, plot the possible outcomes of any pairing.  There are many mutations and many more combinations that we have discovered over the last fifty years, and even now new mutations are being discovered.  An artificial albino was even created by combining whiteface with lutino; this yields an all white bird with red eyes and colorless feet, and lacking the orange cheek patch of the normal lutino. 

 

 

 

Bibliography

 

 

 

Andersen, Diana, Peggy Cross.  A Guide to… Cockatiels.  South Tweed Heads, Australia: Australian Bird keeper, 1994.

 

Descriptions of Cockatiel Mutations.  NCS. 2000 <http://www.cockatiels.org/mutations.html>.  

 

Nita’s Nest, Cockatiel Breeding Genetics. Nita Golden.  January 20, 2002 <http://nitasnest.homestead.com/tielgenetics.html>.

 

North American Cockatiel Society Genetics Page.  Cynthia Kiesewetter. 2000 <http://www.cockatiel.org/genetics/index.html>.

 

Smith, George.  Encyclopedia of Cockatiels.  Neptune City, NJ:  T.F.H Publications, 1978.