Return to Author Return to Main PageToday’s article is a basic overview of avian genetics, terminology and a demonstration on how to use the punnett square when breeding birds. Initially the concept of genetics appears to be daunting for some, however it truly isn't that difficult for one to grasp the basics. I have always found genetics to be quiet an interesting topic along with the topic of Microbiology. Genetics has been a topic that I have had to review several times, I have been fortunate to have studied aspects of genetics in my animal technology course, bachelor of psychology course and also through general reading when I was breeding birds. This article is only intended to give the reader a generalised overview of the basics of genetics and how it can be applied when breeding birds. The reader is always encouraged to read further at their leisure in order to further enhance their understanding of this topic.
What better way to start a genetics article than to give special thanks to the monk “Mendel”. Mendel initially identified the basis of what we call genetics through growing peas. While growing his peas, Mendel noted their were changes in plant offspring when cross pollinating purple and white pea flowers, his study further lead to other areas of plants and he identified the concept of inheritance. Simply put, inheritance are the characteristics that have been derived from the parents, for example blood types in humans or even eye or hair colours. Mendel’s discovery was revolutionary for future generations and was literally the planted seed that grew into a whole field of study, research and understanding into the development of life.
With respect to keeping and breeding birds, genetics is especially relevant if we are trying to understand why some birds appear different while others may appear similar to the parents. Perhaps we may wish to understand how we intentionally continue to breed for a specific colour or other desired trait. An example of this could be if we are interested in breeding birds for a specific colour, body size or even reproductive ability. To do this we would use the process commonly known as “selective breeding”. Selective breeding in the case of birds is when we take a specific pair of birds and segregate the pair from all other birds so the offspring are only from these two birds. The reality is by segregating birds into pairs and breeding for certain attributes we are invariably playing god. This must come with a warning which must never be overlooked, because playing with genetics means we are favoring certain attributes over others and may change the status quo from a strong dominant characteristic into a prevalent weak characteristic. Therefore, by selectively breeding for specific traits over time it is also possible to weaken the integrity of any living entity by accidentally causing weaker genes to become higher in prevalence. A good example is to explore the many types of genetic issues which often plague pure bred dogs now days e.g hip dysplacia in German Shepherds, hemophilia in Irish Setters etc. Even though I have mentioned the dangers of selectively breeding, the fact remains there can also be positive aspects to selective breeding if done correctly. Below I will attempt to explain genetic terminologies and how to use punnett squares., so lets now move forward and examine some of the commonly used terminologies regarding genetics.
The first step in learning genetics is understanding the different terminologies and how each terminology applies to reproduction. Once you understand what each terminology means, you will be able to better understand and communicate the differences between your birds and why certain birds may have differing prices or look different than others of that same species. So lets get the ball rolling:
Perhaps the three most common words we first need to be familiar with when discussing our bird pedigree lines and genetics are Dominant, Recessive and Hybrid. These three words describe how prevalent a certain characteristic is, but what do we mean by Dominant, Recessive and hybrid? We can understand the meaning of the word dominant as a trait characteristic that over powers other characteristics. Thus the dominant characteristic is the most prevalent and most frequently observed. While recessive characteristics are less prominent and are overcome by dominant characteristics. Hybrids can be described as having a combination of dominant and recessive allele characteristics. Therefore to align these words with the topic of bird keeping, we can describe the grey colour characteristic in Cockateils as a dominant colour, where the lutino colour is the recessive colour and the pied Cockateils are hybrids because they have a combination of grey and white colouring.
All the traits are stored within Chromosomes, therefore an easy way that I find to conceptualise chromosomes is to imagine they are like small containers for bunch of characteristics that determine what we look like. Within chromosomes lay an information instruction set of genes and alleles. If your I.T savvy, another analogy regarding chromosomes is similar to the USB stick being the container which holds folders and within the folders lay the files that contain data. Therefore the USB stick is the chromosome, the folders are the Genes and the Alleles are the individual data files. Without any analogies in place, we can describe chromosomes are thin cylindrical structures which are long chains of DNA. Genes are the segmented area of characteristics on a chromosome and will contain many genes. Alleles on the other hand are the specific characteristic for each gene, thus in the case of the grey Cockateil, the grey body colour would be one such allele, the yellow head would be another allele, the red cheeks as another allele. Alleles are also identified as dominant and recessive. So when people talk about chromosomes, genes and alleles we can say the gene is the area on the chromosome which is like a reserved space for a set of characteristics say blue, brown or green eyes, where the allele combination will actually specify the eye colour. In humans, alleles are paired and because of Mendel’s understanding of inheritance we now know each allele from a pair comes from each parent e.g. one allele is from father and one allele from mother.
Autosomes can be described as all chromosomes excluding the sex chromosomes. While the sex chromosomes are chromosomes relating to gender, where traits relating to specific gender are referred to as Sex-Linked chromosomes or sex-linked genes. An example of sex chromosomes in humans is XX for female and XY for male. An interesting example regarding how sex linked disorders work is to consider the case of human colour blindness. We know colour blindness in males is a sex linked chromosome fault because the genetic fault lays on the X chromosome. As I recently mentioned, In humans the chromosomes are paired, thus in the case of colour blindness because males only have 1 X and 1 Y chromosome, there is no backup available should the X chromosome in the male contain a genetic fault such as colour blindness. In the case for females, should one the the X chromosomes have the recessive colour blindness fault, the other X chromosome will override the initial colour blindness fault. So we say colour blindness is a female sex linked disorder. However in this case the female with one X with the colour blindness fault is referred to s a carrier because it is possible a copy of this faulty X chromosome is passed on to her siblings.
Phenotype and Genotype are the two words used to describe how traits are expressed. Phenotype describes the external appearance of a trait such as blond hair, black hair, blue eyes, green eyes, even the yellow face in the male grey Cockateil, while Genotype describes the genetic makeup (gene instruction set) of the traits and these traits are from both parents DNA. DNA is a word many people will be familiar with which stands for Deoxyribonucleic acid, it is the body’s genetic code made out of proteins cytosine (C), guanine (G), adenine (A), or thymine (T).
Two more words you may hear when discussing genetics is Homozygous which is when both alleles are either dominant or recessive, where Heterozygous is when only one allele is dominant and the other allele is recessive which is otherwise known as a Hybrid. Therefore with the differentiation between Heterozygous and Homozygous traits it is also possible to include in the description of Dominant and Recessive e.g Heterozygous dominant and Heterozygous recessive, or Homozygous dominant GG and Homozygous recessive WW. The last three terminologies ill discuss in this article are monohybrid and dihybrid. We will see how monohybrid and dihybrid work when I show you how to use punnett squares, however ill describe a Dihybrid cross as the mating of two hybrids where each individual of the pair both have a dominant and recessive gene for a certain characteristic. While a Monohybrid cross is when one of the birds in the pair is a hybrid and the other is either dominant or recessive. Finally we have a last terminology with the word filial
which represent a generation otherwise view as the son or daughter from the parent. Thus each consecutive generation of one breeding pair is refereed to first filial (F1), second filial (F2), third filial (F3) and so on.
So how did you go with the above terminologies? I am hoping I explained them in away that you understood. If not thank god for Google or books hey! The above terminologies are the most difficult part of the process, the easiest part and perhaps the most entertaining of genetics is working out the estimated results of the chick offspring phenotypes. In order to accomplish the goal of estimating chick phenotypes we use what is called “punnett squares”.
Completing a punnett square is a relatively straight forward process. We first start off by drawing up a table as shown below and then place the labels male and female on the table. It does not matter if you place the male or female at the top of the table, just be sure the gender is not the same on both sides for obvious reasons. For demonstration purposes, I highlighted boxes pink for female alleles and blue for male alleles which is next to the labels male and female. In these coloured boxes you will see that I have split the male and female allele pairs and placed them in separate boxes. In this example we are breeding two dominant grey Cockateils, therefore the alleles have been set to G. Now that we have set up our square we can now complete the test. To start with, we take the first top blue G and pair it with the first left hand side pink G which gives us the answer GG, this answer is then entered into the green colour box. We continue this pattern taking the top blue G and then match it to the right side pink G which gives us the GG which is then entered into the box coloured orange. We have now completed matching the first male allele across two female alleles. We now do the same for the second male allele which is the bottom blue G and match it to the first left pink G giving us the answer GG in the yellow box. And finally we match the bottom blue G with the right pink G giving us the answer GG in the white box. We have now completed our first punnett square which is below and in this case we see 100% of the Cockateil chicks will be grey.
In our first example we will consider breeding two homozygous grey Cockateils that will have the alleles represented as GG. The offspring from this breeding pair will be F1, meaning the offspring will be the first generation.
| Female | ||||
|---|---|---|---|---|
| G | G | |||
| Male | G | GG | GG | |
| G | GG | GG | ||
As we can see with the above example, when we are breeding two dominant traits such as the grey Cockateil 100% of the F1 offspring will show the dominant grey colour. If we continued to breed these offspring with other GG, we will continue to get GG offspring. However, what if we have one dominant coloured bird and one recessive coloured bird as a breeding pair? We can test the results by using another punnett square, lets say we have a male grey Cockateil that is represented by the two alleles GG and a female lutino (white) Cockateil that is represented by two WW alleles.
| Female | ||||
|---|---|---|---|---|
| W | W | |||
| Male | G | GW | GW | |
| G | GW | GW | ||
This time the results show us the F1 offspring from this couple will be 100% GW, meaning they will all be pied’s because the chicks will all have the dominant and recessive Alleles. Now let us take this next breeding example by saying we get the F1 from the first punnett square and a F1 from this second punnett square and test the results. This means we will take a dominant colour grey Cockateil which is represented with the alleles GG and we will mate this with a pied Cockateil that is represented by alleles GW. Let us test the results.
| Female | ||||
|---|---|---|---|---|
| G | W | |||
| Male | G | GG | GW | |
| G | GG | GW | ||
As we can see, the results in this monohybrid cross example show us that 50% of the chicks will carry dominant grey alleles, and the other 50% will result in pieds which are hybrids because the pied chicks will carry both of the dominant and recessive alleles GW. So at this stage we have bred two dominant grey Cockateils which gave us our F1 being 100% grey chicks. We have also bred a pied with a dominant grey which resulted in another F1 being 50% grey and 50% pieds. Now lets test what our F2 results will be if we were to breed two pieds from from this same clutch together.
| Female | ||||
|---|---|---|---|---|
| G | W | |||
| Male | G | GG | GW | |
| W | GW | WW | ||
As we can see when we apply the punnet square in this dihybrid cross example, we have identified the F2 offspring have resulted in Dominant grey GG 25%, Lutino WW=25% and pied GW 50%.
In the above examples I had not discussed the concept of sex linked traits, I also wanted to clarify the above are examples of how one would proceed to understand the concept of using a punnett square, the reality is alleles may be labeled with different letters either upper and lower case or include symbols to demonstration whether dominant, recessive or even sex linked genes are present.
In order to demonstrate the concept of sex-linked genes and how they are expressed in the punnett square we will say that a long Cockateil comb allele will be represented with a + and a normal comb will carry the standard omission of the + allele. The + allele will be a recessive sex linked condition (long comb) located on the female chromosome. Therefore I will breed a female grey Cockateil with the recessive allele with a long comb (G+G) with a grey male with a normal comb (GG). Four eggs were laid and hatched, with all four chicks have been reared. Also just a quick note that I am using chromosomes XX for female and XY for male, the reality is these are human chromosomes not parrots. The actual chromosome pairs for parrots are males are ZZ and females are WZ [1].
| Female | ||||
|---|---|---|---|---|
| G+x | Gx | |||
| Male | Gx | G+xGx | GxGx | |
| Gy | G+xGy | GxGy | ||
| G+x Gx = 25% Female is a Carrier for long comb |
| G+x Gy = 25% Male will have a long comb |
| Gx Gx = 25% Female will have a short comb |
| Gx Gy = 25% Male will have a short comb |
Through the above example we are able to now understand how a female recessive allele is expressed in males.
When I started this article I initially felt that it would be easy for me to write, in fact trying to take information in my head to be express so it can be easily understood has been more difficult than I first envisioned. This article has sought to explain some of the common terminologies of genetics and the use of punnett squares to demonstrate to the reader how they can estimate possible genotypes or phenotypes in future offspring. This article used the basic alleles GG to represent dominant grey Cockateils, WW to represent recessive white/lutino Cockateils and the GW or WG to represent Cockateils that are hybrid such pied Cockateils. I had also briefly discussed the concept of sex linked traits.
When applying the punnett square technique to determine estimated future outcomes, we must remember there are two factors that will impact on the final results not previously discussed in this article. These two factors are clutch sizes and chick deaths. Clutch sizes are rarely a static number, I.E birds will never 100% of the time lay only 4 eggs, nor are they likely to 100% lay the same number of eggs each time. In fact some will lay more, while others may lay less. Thus its difficult to say 25% of chicks will have a GG allele when only two eggs are laid. In stead we view it as the % of chance. The only true way of determining Genotypes is by sending the results to a laboratory for genetic testing. The second factor that will impact on the final predictive results will be the number of chicks that have died during the raring stage. Even though these chicks did not survive it is always possible the chick that died contained the traits displayed in the punnett square are not accounted for by the other chicks that lived. So four Cockateil chicks may of hatched and three lived, if we had a 25% chance of carrying the WW allele, that may have gone with the chick that died.
Until next time...
Darren Hamburger
Page Last update:17/07/2017
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