Incomplete Dominance

What is incomplete dominance? With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits. Let’s imagine that Labrador retrievers, yellow, black and chocolate, demonstrate incomplete dominance when it comes to the color of their coats. (In reality, coat color in this breed is controlled by two different genes, thus this is just a fictitious example.)

How does incomplete dominance work? When the phenotype of the heterozygous is intermediate between the two homozygous. Neither allele is dominant/recessive over the other. Neither allele will express itself---therefore it is very important to make a key to determine the differences between the heterozygous and homozygous state.

Practice Incomplete dominance. EX: Labrador Retriever dogs. Yellow labs Black labs Chocolate labs When working these problems it becomes essential to make a key Yellow lab(YY) Black lab (BB) Chocolate lab (BY) The heterozygous offspring is a blend of the parents.

Complete the parent cross If the mother is a yellow lab and dad is a black lab all offspring are chocolate labs Y Y BY B B

Cross two offspring that are chocolate labs Y BB BY YY BB = black lab BY = chocolate lab YY = yellow lab B Y

What did you notice? You must have a key for the homozygous parents (BB = black) (YY = yellow) You must have a key for the heterozygous offspring (BY = chocolate, not black or yellow) You should notice that the heterozygous form is not dominant but a blending or “in between” trait.

Now you try….. Cross a Red Snapdragon flower with a White Snapdragon flower. Set up the Punnett Square and give me both phenotype and genotype Ratios. x

R R W W