written by Vincenzo Forlino and Iggino Van Bael

When dealing with mutations knowing their inheritance is essential.

Without a general knowledge on mutations and their inheritance, we will struggle in breeding the desired phenotype or it will be more difficult to succeed in establish a new mutation.

Nowadays thanks to the internet this knowledge is of easy access for everyone. We will shortly try to explain the main types of inheritance.

The main genetic law, the law of Mendel, is simple and you should not let you discourage by the mathematical formulas.

If you would like more insight, I would like to recommend to see a few video's available on YouTube about genetics, It will be very helpful if you want to understand how mutations work.

There are more videos on YouTube so you can find probably 1 in your own languages aswell.

video about punnett squares The punnett square I also explained a bit here.

video about Mendel's Law

video about inheritance

Some terms from the genetics we will need to get used to.

I'm not always use these terms, but they can be very helpful, certainly when you google or even better look on Youtube about punnett squares and/or the law of Mendel. I would say look a few and you will understand perfectly how it works. Because when you read it, it's difficult to understand but when you can see it, you will understand it much faster.


Every living being is made up of DNA. We can think of DNA as the bricks that contain all the information necessary to build a living organism. In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes. This molecules will be what will make up the Genome.


Are the carriers of the genetic material.

The number of chromosomes in birds is very difficult to determine. Birds possess apart from a number of large chromosomes (called macro-chromosomes) they also have very many tiny chromosomes (called micro-chromosomes). These micro-chromosomes are very difficult to count by their very small dimensions.

A standard bird karyotype has 16 macrochromosomes and 64 microchromosomes, giving the diploid chromosome number 80. However this number will vary a lot depending on the species in examination. In all orders investigated, there will be a tendency towards a reduction of the number of microchromosomes and an increase of the number of macrochromosomes.

Also sex is determined in birds using Sex chromosomes, but in birds it is a little different. In humans and other mammals females have two copies of the same Sex chromosome (XX), and males have one X, and one Y (XY).

In birds, it is the males who have two copies of the same Sex chromosome (here we call them ZZ), and females who have one Z chromosome and one W chromosome(ZW).


Each chromosome is composed of genes, each gene carries an inherited trait.

A gene is a piece of DNA on the chromosome. 

A gene is made up of two alternative forms called alleles.


An allele is a part of a gene. one gene is made up of two alleles. Alleles can be Dominant, co-Dominant or recessive. Sometimes, different alleles can result in different observable phenotypic traits, such as  a different colour. However, most genetic variations result in little or no observable variation.

The location of these alleles is called locus (or loci if plural). When an individual has two identical alleles, they are called homozygous. Heterozygous means having two different versions of the same allele.

In genetics we call this type of “different” alleles mutant. When more than one mutant allele exists on the same locus, different things can happen. However an allele can have only two of these at the same time.

When working with mutations it can occur that there will be no “natural” allele, but only two different mutated alleles. In this case the way they will interact with each other will determinate the final phenotype.


Gametes are an organism's reproductive cells. They are also referred to as sex cells. Female gametes are  egg cells, and male gametes are called sperm. Gametes are haploid cells, and each cell carries only one copy of each chromosome.


Is a type of cell division that reduces the number of chromosomes in the parent cell by half and produces four gamete cells. This process is required to produce egg and sperm cells for sexual reproduction. During reproduction, when the sperm and egg unite to form a single cell, the number of chromosomes is restored in the offspring.


Process which the cell divides into 2 daughter cells that exactly the same genetic information contain . In contrast with meiosis, mitosis is no bisection of the number of chromosomes of a cell.

Genotype: The genetic information as they find in the chromosomes (is your complete heritable genetic identity)

Phenotype:  is what you see - the visible or observable expression of the results of genes, combined with the environmental influence on an organism’s appearance or behavior.


When an organism has two identical alleles on the same locus, we will refer to them as homozygous for that locus. An allele represents one particular form of a gene. Alleles can exist in different forms and diploid organisms typically have two alleles for a given trait. These alleles are inherited from parents during sexual reproduction. Upon fertilization, alleles are randomly united as homologous chromosomes pair up. A human cell, for example, contains 23 pairs of chromosomes for a total of 46 chromosomes.

One chromosome in each pair is donated from the mother and the other from the father. The alleles on these chromosomes determine traits or characteristics in organisms.

Homozygous alleles may be (co-) Dominant or recessive. A homozygous Dominant allele combination contains two Dominant alleles and expresses the Dominant phenotype (expressed physical trait). A homozygous recessive allele combination contains two recessive alleles and expresses the recessive phenotype.


Heterozygous means having two different alleles on the same locus. . When recessive, a bird that has inherited a recessive allele for a genetic trait or mutation but does not display that trait or show symptoms of the disease. also called carrier or splits. Carriers however, are able to pass the allele onto their offspring, who may then express the genetic if they inherit the recessive allele from both parents.

A heterozygous Dominant allele combination contains one Dominant allele and expresses the Dominant phenotype (expressed physical trait).



When referring to the Sex chromosome there will be only a single copy of a gene instead of the customary two copies. The genes on the single Z chromosome in females, in birds, are called hemizygous.


The genotype is a set of genes in our DNA which is responsible for a particular trait. This information doesn’t always reflect what we see in the phenotype. If two organisms, have even the minutest difference in their genes are referred as having different genotypes.


With the term "phenotype" we generally refer to the physical expression, or characteristics observable physical appearance of an organism. An organism's phenotype is determined by its genotype, which is the set of genes the organism carries, as well as by environmental influences upon these genes.


With chromosomal crossover (or crossing over) is a mechanism where two homologous chromosomes can exchange a sequence of genes of the same length containing the same loci and re-assort so their combination of alleles. This occurs during meiosis - a stage where sister chromatids are formed. It occurs several times possibly producing multiple crossovers. The rate at which the crossover can occur for two given mutations is called a Recombinant frequency.

Autosomal and Sex Chromosomes

With Autosomal we intend a term used in genetic genealogy to describe DNA which is inherited from Autosomal chromosomes. Autosomal chromosomes is any of the numbered chromosomes, as opposed to the Sex chromosomes. In mammals the Sex chromosome is carried by the male of the species. In avian species it’s the opposite. It’s the female who carries the sex chromosome.

In birds these chromosomes are called Z and W. With males having a homozygous (ZZ) allele and females having a hemizygous allele (ZW).

Types of Inheritance

Autosomal recessive Mutation:

With Autosomal recessive mutations. A single mutated allele cannot overrule the information in the second allele. In order to do so, the mutation would have to be present in its homozygous form. In this case the mutation will fully express itself in the phenotype.  If the mutation will be present only in its heterozygous form, there will be no significant changes in the phenotype and we will refer to it as split to that mutation. We will refer at it as “split”. In order to produce a visual mutated bird, both parents must be at least split for that mutation.

The sex of the parents is irrelevant.

go to the page Autosomal recessive mutation

Autosomal Dominant Mutation:

Autosomal Dominant mutations, will overrule the information contained in the second allele even in its Heterozygous form. In this case, the presence of only one mutated allele is sufficient for the mutation to fully express itself in the phenotype. Depending on wherever a bird his homozygous or heterozygous for that mutated allele. We will refer to the as Single Factor (SF) and Double Factor (DF). However visually, they will have the same phenotype. If the parents, one or both, are SF or DF, The only difference will be the number of visual mutated offspring produced.

The sex of the parents is irrelevant.

Go to the page Autosomal Dominant mutation

Autosomal Incomplete Dominant Mutation:

Autosomal Incomplete Dominant mutations have the peculiarity of being capable of partially overrule the other allele even in their heterozygous form. In this case the presence of only one mutated allele is sufficient for the mutation to partially express in the phenotype. To express itself in its full potential, the mutated alleles must be present in their homozygous form. Also in this case we will refer to the homozygous form as DF and its heterozygous form as SF.

In the case of SF birds, the mutated allele won’t be able to work at its full potential and the phenotype will result in something halfway between a DF and a “normal” looking bird.

The sex of the parents is irrelevant.

Go to the page Autosomal Incomplete Dominant mutation = the same page as the Auto. Dominant mutation page.

Co-Dominant Mutation

When on the same locus there can be more than one typology of mutant alleles. We can assist at different interactions. In the case this interaction is neither Dominant or recessive, we refer to this interaction as Co-Dominant.

This means that the two different types of mutant alleles are both partially capable to work in the presence of a different mutated allele. This will produce a phenotype that will be visually halfway between the two different mutations.

We will write the name of both mutations together without no space between the two names.

This type of interaction between multiple mutated alleles on the same locus, won’t  change its inheritance towards the wild form (non-mutant phenotype), that can still be recessive, Dominant or Incomplete Dominant. And can be either on a Autosomal Loci or the Sex.

Lethal Semi Dominant

Lethal Semi Dominant mutations are often found only in its heterozygous form. Usually the homozygous form is lethal in some degrees. Depending on the mutation all DF birds will die during the embryonic development or only a small % of them will survive in some rare cases.

Sex-linked recessive Mutation

A Sex-linked mutation  is located on the sex locus, more specifically on the Z-chromosome. As seen above, in birds it is the female who has only one Z-chromosome,  with the sexual locus being ZW, with W-chromosome who determinates the sex.

In Sex-linked recessive mutations, a female cannot be heterozygous (split) for a sex-linked mutation recessive mutation. The mutated allele will either  express itself in the phenotype or it won’t. We refer at this as hemizygotus. In this case it will be sufficient a single mutated allele for the mutation to manifest in the phenotype.

However in  males we will still find the same rules that rule other Autosomal recessive mutations. A Sex-linked recessive mutation to express itself in the phenotype, will have to have two homozygous mutated alleles. If it will be heterozygous for that particular gene, the genetic information contained in that allele won’t be able to overrule the second allele.

We refer to heterozygous birds as so-called “split” for that mutation. From the above it follows that, it is necessary for males, that both parent carry at least one mutated allele to produce a visual mutated male. For females, It will be sufficient having a male that is split for that mutation.

Go to the page Sex-linked recessive mutation.

Sex-linked Incomplete Dominant Mutation

A Sex-linked mutation  is located at the Z-chromosome. As seen above, in birds it is the female who has only one Z-chromosome,  with the sexual locus being ZW, with W-chromosome who determinates the sex.

Sex-linked Incomplete Dominant mutations, can partially overrule the second allele of the pair, so the presence of only one mutated allele is sufficient for the mutation to partially express itself. However to fully express itself we will need that allele to be homozygous for that mutation.  This  will be possible only in males, And we will refer to them as Double Factor (DF).  And heterozygous birds will be referred as Single Factor (SF). In this case an individual can’t be split for that mutation.

A female  is hemizygous so she have only 1 Z chromosome, and therefore she never can be double factor or single factor. She got the colour or not. The reason she can not be called single factor is because a single factor got 2 different colours (1 on each Z chromosome), which make the Dominant mutation visible, as she got only 1 Z chromosome this is not possible.

Go to the page Sex-linked Dominant mutation.

Or go to the main page mutations, click here.

A few drawings I have made, which maybe explain it a bit.

In the first example  you see 2 homozygous who are also be the same, for example normal x normal or mutation x mutation(2 of the same mutations), you get then the same homozygous youngster.

In the second example also both are homozygous but not equal to eachother , for example in a recessive mutation blue x normal, now the chick get 2 different genes which is called heterozygous. Or as we call it, normal split blue.

Or in a Dominant mutation for example darkgreen (Single Factor) x normal you get then a chick with also both genes but the Dominant mutation will show, so no split in this case. this chick is called a Single Factor (SF).

The picture below shows 3 single birds, each bird have 2 gametes, in the homozygous birds, the gametes are the similar as the parent. A heterozygous got 2 unequal gametes, 1 normal, and the other who carry the mutation gene.

The next picture show a pairing of a heterozygous x heterozygous.

The gametes which are 2 from each parent, each gamet will go to 1 gamet of the other partner.

Which lead to the next outcome  50% heterozygous, 50% will be homozygous, but as you can see in the picture, both are not the same, 1 homozygous is clear, normal and the other homozygous is dark, = mutation.

So we have 50% heterozygous, 25% homozygous (clear) and 25% homozygous (dark)

For example with a recessive mutation:

Let's give it a name. the pairing is normal split blue X normal split blue (split means carrier for blue).

The outcome is then:

50% normal split blue (heterozygous)

25% normal (homozygous)

25% blue (homozygous)

Note that between the normal and normal split is no visible difference, only genetic.

Another example but now with a Dominant mutation.

Single Factor pied x Single Factor pied (so 1 normal gene and 1 pied gene)

50% Single Factor pied (heterozygous)

25% normal (homozygous)

25% Double Factor pied (homozygous)

You see it is not so difficult (when you are not working with many mutations in 1 pair).


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