PUNNETT SQUARES FOR BOURKE COLORATION: Bird Genetics 101 ... Explaining it to Me.

I received a nice email from Diana challenging what I'd written in my January 4 post, "Bourke Coloration, A Question and Answer."

She gave a long and detailed explanation why I was mistaken in my calculations regarding transmission of coloration in Bourkes. To make a long story short, I’ve learned that humans and birds do not share the same genetics. Humans, other mammals, some insects and plants all follow what is known as the XY sex determination system. In this system, the male is heterozygous (XY) and the female is homozygous (XX). Thus the male determines the sex of the offspring.

Birds, it turns out, operate on a ZW system with the male being homozygous (ZZ) and the female heterozygous (ZW). So, unlike humans whose sex is determined by the father, in birds it is the mama bird that determines the sex of the offspring. However, papa determines the color since color is sex-linked to the male chromosome. Let’s hope the following clears the water.

Here is a Punnett Square for coloration of Bourke parakeets generously provided to us by Su Yin (neversink7). If you replace the rosey with lutino, it also works the same way.

My husband put the following together for me before I received Su Yin's Punnett Square. It says the same thing, but is visually different. Please consider it copyrighted as I will include it in a book on small exotic birds in the future, so don't steal or repost his illustrations. Thank you.

Quote from E.G. Lewis: "Many people use what is known as the Punnett square to predict an outcome of a particular cross or breeding experiment. Named after Reginald C. Punnett who devised the approach, it is used to determine the probability of an offspring's having a particular genotype. The Punnett square provides a visual representation of the possible combinations of one maternal allele with one paternal allele for the gene being studied in the cross. The following chart illustrates this method by simply looking at a typical mating, which we know should result in a 50-50 mix of male and female offspring.

Sex Determination

Keep three things in mind as we go forward:

First, we are discussing only the genetic relationship between the normal Bourke and the pink, or rosey, Bourke. There are other color mutations in the Bourke family and some of these appear to behave in a similar manner.

Secondly, the coloration is sex-linked — carried on the male chromosome. Therefore, while the female determines the sex of the offspring, the male chromosome determines its color. 

And, thirdly, the color genes are recessive to the normal color. However, since they’re carried on the male sex chromosome and a female only has one male chromosome; she will express the pink color if she inherits a pink male chromosome from her father. For our purposes, the male chromosome for pink color will be written as (Z_p) and the male chromosome for normal color will be written as (Z_n)

Remember also that a bird can appear normal, but actually be a split. A split results from mixed parentage and the offspring can carry one unexpressed gene for coloration. Depending upon what bird they are mated with, this color gene can be passed to their offspring resulting in what appears to be a normal parent producing pink babies. Of course, two full normal parents will only produce normal offspring and two full pink parents will only produce pink offspring. Now let’s examine possible combinations between normals, splits, and rosies. Keep in mind, all splits are normal males.

Pink Male - Normal Female

Our first sample mating is between a Pink Male and a Normal Female. In this case all of the males will be splits and all of the females will be pink.

 Normal Male - Pink Female

Next we look at a Normal Male and a Pink Female. In this case all of males will be splits and all the females will be normal.

 Split Normal Male - Pink Female

Now we’ll pair a Split Normal Male with a Pink Female. Half of both males and females will be pink. The rest of the males will be split normal whereas the other females will be normal.

 Split Normal Male - N Female

Lastly, we’ll pair a Split Normal Male with a Normal Female. Here half of the males will be normal and half will be splits. Likewise, half of the females will be pink and half will be normal."

My (Gail's) personal experience has been when combining a rosy male with a normal female, the females are always rosy and the males always normal. Further, when I combined one of their normal males with a normal female, the same pattern prevailed that all their rosies were hens and all males were normals, so he was split to rosy. I've been telling people on this blog to expect their young Bourkes to be the color of the opposite-sexed parent, unless one is split, which creates another possibility. This has proven true for me and for a friend with a lutino and a normal too. However, our small sampling is not enough to be completely correct.

Please refer to the Punnett squares above for a more accurate percentage of how your clutches may end up.

For more on genetics:  Su Yin on Bourke Genetics

Peace and Blessings,

Gail