Practice: Codominance and Incomplete Dominance - KEY

1. Practice setting up keys for the phenotypes listed in each set. Remember that the "medium" trait must always be heterozygous.

a) Birds can be blue, white, or white with blue-tipped feathers.

BB = blue; BW = blue tips; WW = white

OR you can use other way to set up problems using a single letter

BB = blue, Bb = tips, bb = white

b) Flowers can be white, pink, or red. RR = red, RW = pink, WW=white

c) A Hoo can have curly hair, spiked hair, or a mix of both curly and spiked. CC = curly, CS = mixed, SS = spiked

d) A Sneech can be tall, medium, or short. TT= tall, TS = medium, SS = short

e) A Bleexo can be spotted, black, or white. BB = black, BW = spotted, WW = white

2. Now, can you figure out in the above list, which of the letters represent codominant traits and which are incomplete.

if it *BLENDS* then it is incomplete dominance, if both traits are expressed, it is codominant. Students can become overly anxious about this difference, when in fact the problem solving is no different and the difference is largely a matter of semantics.

Codominant __a, c, e__________ Incompletely Dominant _____b, d_______

3. In Smileys, eye shape can be starred, circular, or a circle with a star. Write the genotypes for the pictured phenotypes

SS = star, SC = circle-star, CC = circle

4. Show the cross between a star-eyed and a circle eyed. SS x CC
What are the phenotypes of the offspring? ___all star circle____
What are the genotypes? ____SC____

5. Show the cross between a circle-star eyed, and a circle eyed. SC x CC
How many of the offspring are circle-eyed? ___1/2______
How many of the offspring are circle-star eyed? ___1/2_________

6. Show the cross between two circle-star eyed.
How many of the offspring are circle-eyed? ___SC x SC____
How many of the offspring are circle-star eyed? ___1/2_______
How many are star eyed? ______1/4______