Bio- Genetics: Incomplete Dominance
Name:_______________________Hr:____
So far, we’ve discussed only Complete Dominance, meaning that only two phenotypes are possible for
three different genotypes.
H=hairy toes
Genotypes
HH
Hh
hh
h=non-hairy toes
Phenotypes
hairy toes
hairy toes
non-hairy toes
Something new! Incomplete Dominance occurs when one allele is not completely dominant over the other
allele for a trait. This means that the heterozygous phenotype is somewhere in between the two
homozygous phenotypes.
For instance: Snapdragons (flowers)
R=red
Genotypes
RR
Rr
rr
r=white
Phenotypes
red
pink**
white
Follow the same steps in order to solve problems:
1. Assign letters to represent the genes
2. Determine the parental phenotypes and genotypes.
3. Predict the outcome of a cross between the possible gametes from the two parents.
4. State phenotypic and genotypic ratios present in the offspring.
5. Don’t forget to answer the question within the problem.
Example:
Cross a homozygous dominant snapdragon with a homozygous recessive snapdragon. Assume incomplete
dominance.
Step 1: RR=red
rr=white
Rr=pink
Step 2: Parents’ phenotypes: red (homozygous dominant) x white (homozygous recessive)
Parent’s genotypes: RR x rr
Step 3:
Step 4:
R
R
r
Rr
Rr
r
Rr
Rr
Genotypic Phenotypic
Ratio
Ratio
100% Rr
100% pink
Example:
Shape of a radish is controlled by gene that shows incomplete dominance. There are two alleles for the gene,
round shape and long shape. The phenotypes possible in the radishes are round, oval, or long. Suppose you
have seeds for round and oval plants, and that you cross these plants. What would be the phenotypes present
in the next generation of plants produced from this cross?
Step 1: RR=round rr=long
Rr=oval
Step 2: Parents’ phenotypes: round x oval
Parent’s genotypes: RR x Rr
Step 3:
Step 4:
R
R
R
RR
RR
r
Rr
Rr
Genotypic
Ratio
Phenotypic
Ratio
50% RR
50% Rr
50% round radishes
50% oval radishes
Example:
Using the same information from the problem above regarding radishes, cross two oval radishes. What is
the probability of getting round radishes from this cross?
Step 1: RR=round rr=long
Rr=oval
Step 2: Parents’ phenotypes: oval x oval
Parent’s genotypes: Rr x Rr
Step 3:
Step 4:
R
r
R
RR
Rr
r
Rr
rr
Genotypic
Ratio
Phenotypic
Ratio
25% RR
50% Rr
25% rr
25% round radishes
50% oval radishes
25% long radishes
Step 5:
The probability of getting round radishes from this cross is 25%.
Bio- Genetics: Codominance
Name:_______________________Hr:____
Codominance is when BOTH alleles contribute to the phenotype of the organism. Therefore, in this case a
heterozygous phenotype will express both alleles separately and simultaneously. (You will see both alleles
in the phenotype.)
For example: In shorthorn cattle, red coat color is codominant with white coat color. The heterozygous
condition results in a color known as ROAN, which is intermingled red and white hairs. (Roan is not pink)
Remember the letters we assign are arbitrary (we make them up; you can choose anything).
Let’s say R=red and r=white (white is not recessive; it is still dominant)
Genotypes
RR
Rr
rr
Phenotypes
red
roan**
white
Follow the same steps in order to solve problems:
1. Assign letters to represent the genes
2. Determine the parental phenotypes and genotypes.
3. Predict the outcome of a cross between the possible gametes from the two parents.
4. State phenotypic and genotypic ratios present in the offspring.
5. Don’t forget to answer the question within the problem.
Example:
A roan bull is mated with a red cow. Show the genotypes and phenotypes of the offspring.
Step 1: RR=red
rr=white
Rr=roan
Step 2: Parents’ phenotypes: roan x red
Parent’s genotypes: Rr x RR
Step 3:
Step 4:
R
r
R
RR
Rr
R
RR
rr
Genotypic Phenotypic
Ratio
Ratio
25% RR
50% Rr
25% rr
25% red offspring
50% roan offspring
25% white offspring
Example:
In rabbits of black and white coloration, F is the allele for fur color. If F= black fur allele f= white fur
allele then…
FF= Black rabbit
ff=White rabbit
Ff=Black AND white rabbit
What is the probability of getting black bunnies from mating a white rabbit with a black and white rabbit?
Step 1: FF=black
ff=white
Ff=black and white
Step 2: Parents’ phenotypes: black x black and white
Parent’s genotypes: FF x Ff
Step 3:
Step 4:
F
F
F
FF
FF
f
Ff
Ff
Genotypic Phenotypic
Ratio
Ratio
50% FF
50% Ff
50% black bunnies
50% black and white bunnies
Step 5:
The probability of getting black bunnies is 50%.
Practice Problems:
The interactions of genes that cause sickle-cell anemia show another example of C0-DOMINANCE.
Sickled cells (abnormally shaped red blood cells) are unable to carry oxygen properly and often clump up in
the blood vessels. The physical symptoms of anemia are the result of all sickled cells. Those symptoms
include fatigue, shortness of breath, tingling of extremities, and blueness of the skin.
However, individuals who are heterozygous (Ss) produce both normal red blood cells (RBCs) and sickled
red blood cells. Neither allele is dominant—both are expressed in the phenotype. This is called sickle cell
trait.
SS— Normal RBCs
ss— Sickled RBCs
Ss--- Sickled RBCs and Normal RBCs
1. Suppose a man with sickle cell anemia marries a woman with the sickle cell trait. What are the predicted
genotypes and phenotypes of their children?
2. Suppose a woman with normal RBCs marries a man with the sickle trait. Will any of their children have
sickle cell anemia? Will any of their children carry the sickle cell trait?
3. You are a genetic counselor and a couple comes in to visit you. They are very concerned that they will
have a child with sickle cell anemia, because they both have the sickle cell trait. What will you say to
them? What is the chance that their child will have sickle cell anemia? Sickle cell trait? Have normal
RBCs?
4. In pigs, having a curly tail is dominant over having a straight tail. If a piglet is heterozygous for the tail
gene, his or her tail will be crooked. Suppose a heterozygous pig mates with a straight-tailed pig. What
is the probability of having curly tail piglets?
5. In an island bird population, short, crushing beaks are incompletely dominant over long, delicate beaks.
The heterozygotes have medium length beaks. Show the genotypic and phenotypic ratios of a cross
between two birds with short, crushing beaks.