4.8 Phenotypes Are Often Affected by More Than One Gene

 Multiple Choice
Which statement best describes a phenotype controlled by several genes?
a) It is caused by one gene with two alleles.
b) It is always due to interaction between linked genes.
c) It may be influenced by different genes and their products, even without direct interaction.
d) It always shows continuous variation.

c

 Short Answer
Give an example of a phenotype controlled by multiple genes but not necessarily through gene interaction.

Eye development (epigenesis) or hereditary deafness.

Multiple Choice
Epistasis occurs when:
a) Two genes assort independently.
b) The expression of one gene masks or modifies the expression of another gene.
c) Both genes contribute equally to the phenotype.
d) One gene suppresses itself through feedback inhibition.

b

True or False
The Bombay phenotype is an example of epistasis because a recessive mutation at one locus prevents expression of the A and B blood group alleles.

true

Multiple Choice
When analyzing epistasis, it is generally assumed that:
a) Genes are linked and show incomplete dominance.
b) Genes assort independently and show complete dominance.
c) Genes are always codominant.
d) The F₁ generation shows complete linkage.

b

Short Answer
What type of variation do epistatic interactions typically produce — continuous or discontinuous?

Discontinuous variation.

Multiple Choice
In mice, agouti (A_) and black (aa) colors occur only when the B allele is present. Mice with genotype bb are albino. This is an example of:
a) Dominant epistasis
b) Recessive epistasis
c) Complementary gene interaction
d) Codominance

b

Short Answer
In recessive epistasis, which genotype at one locus masks the expression of alleles at another locus?

The homozygous recessive genotype (e.g., bb).

Multiple Choice
In squash, a dominant A allele prevents color expression, making the squash white regardless of B or b alleles. This is an example of:
a) Dominant epistasis
b) Recessive epistasis
c) Complementary interaction
d) Incomplete dominance

a

Short Answer
In dominant epistasis, the masking effect is caused by which type of allele?

A dominant allele.

Multiple Choice
In sweet peas, both dominant alleles (A_B_) are needed for purple flowers; any homozygous recessive combination (aa or bb) produces white flowers. This is an example of:
a) Dominant epistasis
b) Recessive epistasis
c) Complementary gene interaction
d) Novel phenotypes

c

True or False
In complementary gene interaction, only one dominant allele is needed to produce the phenotype.

False — both dominant alleles (A and B) are required.

Multiple Choice
In summer squash:

• A_B_ = disc shape
  
  

• A_ or B_ = spherical
  
  

aabb = long shape
This is an example of:
a) Dominant epistasis
b) Recessive epistasis
c) Novel phenotype interaction
d) Complementary gene interaction

c

Short Answer
In a novel phenotype interaction, how does having both dominant alleles (A and B) affect the trait compared to having only one or none?

Having both dominants (A_B_) produces a new or unique phenotype distinct from having only one dominant or none.

Multiple Choice
Even when epistasis modifies the 9:3:3:1 dihybrid ratio, which two genetic principles still apply?
a) Linkage and codominance
b) Segregation and independent assortment
c) Epigenesis and incomplete dominance
d) Mutation and recombination

b