Zebrafinch genetics : Instructions


1. Introduction

The breeding and competition of zebrafinch has grown considerably over the past fifteen years. In order to improve the size of the new mutations, breeders also have recourse to conventional "split" birds.
Some manage to combine several mutations. All this made it essential to know a minimum of applied genetics. It is this minimum that I would like to present to novice breeders.
This is not a complete course in genetics, but a simple presentation of the method I use preceded by some basics.

2. The zebrafinch and its mutations

A zebrafinch has a number of visible characters (size, shape, designs, color, sex) that constitute its phenotype. It can have, in addition to other unexpressed traits (it is said to be a split). The set of traits, expressed or not, is called the genotype.

A young zebrafinch grows out of an egg cell, the result of the fusion of the nucleus of a father's sperm and the nucleus of the female's egg. The bird's genetic program is already there: A series of cell divisions and coded information will (or not) trigger the appearance of the characters. The encoded information is carried by genes located on long filaments contained in the nucleus: chromosomes.
All chromosomes go in pairs: each chromosome therefore has its counterpart.

There are two categories of chromosomes :

- Sex chromosomes :
• XX in the male
• XY in the female

- Autosome chromosomes.

The gray zebrafinch living in Australia is the source of all of our farmed zebra finches. It has a whole set of genes distributed in its chromosomes.Whenever a new mutation has appeared, there has been a change in an original gene (and it has been shown to be hereditary). The original gene and the mutated gene are located in the same place called a locus on each of the homologous chromosomes.
Both genes are alleles.

Genetics of zebrafinch

A bird is pure (homozygous) when all of its alleles carry identical information.
A bird is heterozygous when at least one pair of alleles carries different information about the same trait.

We currently know about twenty different mutations of the gray zebrafinch.


We distinguish :

a) Dominant mutations

Pastel, crested, cheeks (grey, brown), black face, light cheeks.
A mutation is dominant when it is expressed when the mutated gene exists in only one copy. This gene is located on an autosome chromosome.
There are therefore no zebrafinch split these mutations.
Note: When the two homologous chromosomes each carry a dominant gene, the young are not viable. This is called a lethal factor.

b) Recessive mutations

White, variegated, saddled, white breast, black breast, orange breast, black cheeks, isabelle, agate, yellow beak, eumo, charcoal.
A mutation is recessive when it is expressed only if the two autosome chromosomes each have the mutated gene.
If there is only one mutated gene, the trait is not expressed. The bird is simply a "split" of the mutation.

c) Sex-Related Mutation

Brown, pale back, old type masked, new type masked.
A mutation is linked to sex when the genes responsible for this mutation are located on the X chromosome (s) of the bird (the Y chromosome of the female being empty of genes).
The mutation is expressed in females as they receive the mutated X chromosome from their father. For it to be expressed in males, the mutated gene must be carried by each of the two X chromosomes. Otherwise, the male is only a carrier of the mutation; however, he will be able to pass it on to half of his daughters.
Notes: The “Pale Back” and “Masked” genes are alleles of the same non-mutated gene. A Gray male can carry pale back and Masked backs.

Genetics of zebrafinch

A Pale back male can carry Masked, but not the other way around. In this case, even in a single copy, it is he who speaks.
The same factor (Pale Back) can be recessive with respect to Gray, but dominant with respect to Masked.
The "Brown" gene, also on an X chromosome, does not have the same locus as the previous genes.

Genetics of zebrafinch

Anterior to the other two, it is found on a different chromosome.
For these genes to be linked (Pale Brown, Masked Brown), a phenomenon had to appear, which is the subject of another article: The crossing-over.

d) Combined mutations

Many mutations as well as grey can be combined with each other. In theory, we can combine a lot of them, but in practice, it is better to remain cautious: In addition to the many necessary crosses, the bird obtained must remain typed and meet the criteria of the standards.

The most famous are :

• The Brown Pastel
• Gray or Brown Cheeks
• The Isabelle Black Chest

Brown Face Black Breast Black or Pastel Brown Breast White combine, for example, a mutation linked to sex, a dominant free mutation and a recessive free mutation. It is therefore necessary to know how to choose the best crosses to achieve this.

3. Crossing technique

a) Assign a symbol to each mutation

We start by assigning a symbol to each mutation: By analogy with atomic symbols, we can choose one or two letters of the name of the mutation. Dominant mutations are in upper case, others in lower case.
Scientists follow the symbol of the unmutated gene with a + sign.
Example: C (Crested); C + (not crested); wb (White Breast); wb + (not White Breast).

Personally, I find it more logical to write: C + (Crested); C - (not crested); wb + (White Breast); wb - (not White Breast). At the end of the day, the results will be the same.

b) Write the genetic formula for each bird

On either side of a fraction bar, the symbols of the genes carried by each homologous chromosome are shown, starting with the sex chromosomes.
Examples :

c) Sex chromosomes

Gray male : XN/XN ; Gray female XN/Y
In this case, N means Normal

d) Sex-Related Mutation

Brown male : Xbr+/Xbr+ ; Brown female : Xbr+/Y
Same formulas with pb + (pale back), ma + (masked), mn + (masked new type).

e) Dominant free mutation

Male pastel gray : XN/XN PA+/pa- ; Gray pastel female : XN/Y pa-/PA+
Same formulas with C+, BF + (black face), LH + (light cheeks).
Non-mutated recessive factors are written in lowercase.

f) Recessive free mutation

Gray male black chest XN/XN bc+/bc+ ; Gray female black breast XN/Y bc+/bc+
Same formulas with pb+ (white breast), po+ (orange chest), jn+ (black cheeks), pa+ (variegated), se+ (saddled), is+ (isabelle), etc.

g) Combined mutations

Brown male black face black cheeks : XN br+/ XN br+ Bf+/bf- bc+/bc+
Male pale back pastel yellow beak : XN pb+/XN pb+ Pa+/pa- yb+/yb+

h) Split

Gray male / (/ means split) Pale back : XN pb+/XN pb-
Brown female/Black cheeks : XN br+/Y bc+/bc-
Grey female black face/black chest : XN/Y Bf+/bf- bc+/bc-
Grey male, black face/black chest : XN/Y Bf+/bf- bc+/bc-
Male Pale Back Grey/Masked NT (new type) : Xpb+/Xmn+. In this case, we could write PB+, since the Pale Back dominates its allele, the Masked NT.

4. Place these forms in a cross table

We must first recall :

• That each parent transmits to his young only one of the two chromosomes of each pair.
• That the grouping in each gamete (sperm or egg) of these chromosomes is by chance: it is genetic mixing.

The more mutated genes the parent has on different chromosomes, the more possible combinations will be. This is the only difficulty in this method, but it is inevitable.

Let's start with a simple cross:

a)Brown male : (XN br+/XN br+) X Grey female XN br-/Y

  XN br+ XN br+
XN br- XN br+/XN br- XN br+/XN br-
Y XN br+/Y XN br+/Y

Each chromosome of the male (in this case, the sex chromosomes) finds its homologous chromosome supplied by the female. All that remains is to translate each formula.
Results : XN br+/XN br- Grey/brown male (50%) ; XN br+/Y Brown female (50%)

b) Grey/brown male : (XN br+/XN br-) X Brown female : XNbr+/Y

  XN br+ XN br-
XN br+ XN br+/XN br+ XN br-/XN br+
Y XN br+/Y XN br+/Y

Results : XN br+/XN br+ Brown male (25%) ; XN br-/XN br+ (25%) ; XN br+/Y brown female (25%) ; XN br-/Y gray female (25%).
Once the method is acquired, it is possible to find the result of any crossing. It takes time, logic and patience (or a computer).

c) Black-faced male brown : (XN br+/XN br+ Bf+/bf- pa-/pa-) X Female pastel grey : XN br-/Y bf-/bf- pa-:Pa+

  XN br+ Bf+ pa- Génotype XN br+ bf- pa- Génotype
XN br- bf- Pa+ (XN br+ Bf+ pa-)/(XN br- bf- Pa+) Male black face pastel grey/brown (XN br+ bf- pa-)/(XN br- bf- Pa+) Pastel male grey/brown
XN br- bf- pa- (XN br+ Bf+ pa-)/(XN br- bf- pa-) Mâle black face gris/brun (XN br+ bf- pa-)/(XN br- bf- pa-) Grey/brown male
Y bf- Pa+ (XN br+ bf+ pa-)/(Y bf- Pa+) Female black face pastel brown (XN br+ bf- pa-)/(Y bf- Pa+) Female pastel brown
Y bf- pa- (XN br+ Bf+ pa-)/(Y bf- pa-) Female black face brown (XN br+ bf- pa-)/(Y bf- pa-) Brown female

This crossing involves a mutation linked to sex and two dominant free mutations. Eight different phenotypes (12.5% each) are obtained.

It seems important to me:

• Not to go down to lower percentages (6.25% or 3.125%). The chances of obtaining the desired bird are too low: One in 16 or 32 birds. Rare are the pairs which provide 16 young in the year !
• To be able to distinguish carrier birds from those, of the same phenotype, which are not.

5. Conclusion

This method should allow any breeder:

• To predict the theoretical results of each couple it forms.
• To analyse the results obtained and to learn from them (some birds prove to be split of a mutation that was unknown).
• Check for possible errors in the crossing results offered by books and magazines.

However, it is essential to know perfectly the genotype of each of its birds.

This involves :

• A breeding in individual cages.
• A strictly kept breeding notebook and an individual record (pedigree) for each bird.
• Let the breeder who gives you a bird provide you with his individual record (See: Zebrafinch Certificate of Sale).

Hoping to have been able to help all those who are starting or who are engaged in the breeding of combinations of mutations, I stand at the entire disposal of the breeders who would like clarification or additional information. The easiest way is to contact me by email: renedruais@orange.fr. I will be happy to answer you.

René DRUAIS, judge CNJF-OMJ Exotic straight beaks.
Article published in 2020.

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heterozygous Genetics Chromosome genes dominant homozygous split Mutation sex-related recessive