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MORE GENETIC INTERACTIONS
1. Read the essay Incomplete Dominance p. 514 and then attempt the problems below:
INCOMPLETE DOMINANCE
In incomplete dominance, the dominant allele does not totally "cover up" the effect of the recessive allele, thus
an intermediate phenotype or "blending" occurs.
Key: In Carnations: RR = red rr = white Rr = pink
Problem: A cross was made between a pink carnation and a white carnation.
a. What was the Genotype for both parents?
b. What were the gamete possibilities from each parent?
c. Draw and fill in a Punnett square.
d. What is the probability of getting red carnations from this cross?
e. What is the probability of getting pink carnations from this cross?
f. What is the probability of getting white carnations from this cross?
CODOMINANCE
In codominance, two different alleles for the same gene are EQUALLY active, thus BOTH showing in the
phenotype.
Key: In cattle: HRHR = red hair
HWHW = white hair
R W
H H = roan (white and red hair mixed)
Problem: Two cattle having roan hair are bred.
a. What was the genotype of both parents?
b. What gametes can be produced from each parent?
c. Draw and fill in a Punnett square.
d. What is the probability of getting a roan calf?
e. What is the probability of getting a red calf?
f. What is the probability of getting a white calf?
2. Many traits include the interaction of more than two genes, and may also include interactions with the
environment. Read the following essay and answer the questions. Genetic Complexity, p. 511
1. List three types of complex human traits:
a.
b.
c.
2. List two factors that influence complex traits
a.
b.
3. What substance produces skin pigments?
4. What non-genetic factor influences skin pigments?
POLYGENIC INHERITANCE
In polygenic inheritance, a single trait may be controlled by two or more different genes, giving a wide range of
possible genotype expressions.
Key: In humans, skin color is believed to be influenced by at least FOUR different genes. Dominant
alleles promote more melanin pigment, and recessive alleles reduce melanin pigment.
AABBCCDD = Dark Black Skin
aabbccdd = Pale White Skin (Darker than albino, which is NO pigment at all)
Problem: A man and a woman with the following skin color genotypes married:
AaBBCcdd X aabbCcDd
a) Show all the gamete possibilities from each parent.
b) Draw and fill in a Punnett square (make this AFTER you figure out your gametes.)
c) List the genotypes from the square that is of the DARKEST skin color.
d) Is the skin color darker than either of the parents?
e) Could a child be produced with LIGHTER skin color than either of the parents?
f) List the genotype with the LIGHTEST skin color.
g)
What is the probability (make it a fraction out of 16) that a child could be of a lighter skin
color than either parent?
5. Explain why light skin would be advantageous in northern latitudes.
6. Explain why dark skin would be advantageous in equatorial latitudes.
MULTIPLE ALLELES
When there are multiple alleles, there may still be just one gene per trait (2 alleles per gene, right?) The
difference is there are multiple possibilities for alleles.
In humans, eye color brown (EB) is dominant to green (EG), which is dominant to blue (eb)
Problem: Billy’s mom has green eyes and his dad’s eyes are brown. Holy smokes! Billy’s eyes
a) What are the genotypes? Mom
Dad
Billy
b) What gametes can his parents produce?
c) Make a punnett square:
d) Billy’s brother Bobby was just born. What is the probability Bobby’s eyes will be:
Brown
Green
Blue
are blue!