2.5 Incomplete Dominance

We have so far seen that a single dominant gene completely dominates the other gene such that it is impossible to tell if the bird carries a single or two copies of the dominant gene. A GB bird looks exactly the same as a GG bird. In some mutations, there is a visual difference between a bird carrying a single copy of the dominant gene and a bird carrying two copies of the same dominant gene. We take the spangle mutation in budgerigars as an example. Suppose we pair a normal green bird to a spangle bird. We can write this as 

Green GG X Spangle GS
The offsprings we get are
50% GG Green
50% GS Spangle 

Now lets suppose we pair two spangle birds
Spangle GS X Spangle GS
The offsprings we get are
25% GG Green
50% GS Spangle 
25% SS Spangle 

A spangle bird with genes GS looks like the bird on the right in fig 4. This is called a single factor (SF) spangle. A bird with two spangle genes SS looks like the bird on the left. This is called a double factor (DF) spangle. We see that they look very different. A single spangle gene is dominant over the normal and a bird shows a spangle pattern with only one spangle gene. However the addition of a second spangle gene has a further compounding effect on the bird. Such a mutation is called Incomplete dominant. To generalize, when one copy of the gene visibly affects the appearance of the bird and a second copy of the same gene further increases that effect the gene is called incomplete dominant.

Other common examples of incomplete dominance are the dark factor, violet factor and the dominant pied genes.

What would you get if you paired a double factor spangle to a normal green bird?


  1. Beautiful writeup.....
    So, we can say that if we pair a single factor spangle with a normal budgerigar, we will 50% spangle and 50% normal......and the normal will have no gene of spangle ie, we can't get spangle from it, unless paired with another spangle. Is my understanding right?
    Another question, what colors not to pair with spangles.....I think inos and pieds are to avoid...
    Thank You

  2. Waiting for a reply Saudbhai.....


  3. That's fantastic I had no clue about this until the moment I got to know this article of yours over here. And there is one thing which I would like to ask you. Do you have any helpful data on how to protect your personal posts from being used without you knowing it?

  4. As far as i learned by the topic we will get single factor spangles in result 100%.


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