Chapter 4

Question 1

Explain incomplete or partial dominance.
(semi-dominance)

Answer 1

The intensity of pigmentation in this species depends on the amount of a product specified by the color gene

ex. WW homozygotes will have twice as much of the product as Ww heterozygotes do, therefore show a deeper color.

When the Ww phenotype is midway between the phenotypes of the two homozygotes, the partially dominant allele is sometimes said to be semidominant.

Question 2

How is an antigen detected?

Answer 2

An antigen is detected by its ability to react with factors obtained from the serum portion of the blood.
ex. Serum anti-M, recognizes only the M antigen on human blood cells; anti-N, recognizes only the N antigen on these cells.

Question 3

What is codominance? Give an example?

Answer 3

Codominance implies that there is an independence of allele function. Neither allele is dominant, or even partially dominant, over the other.

ex. Heterozygotes for the two alleles in antigen detection, M N, produce both kinds of antigens. Since the two alleles contribute independently to the phenotype of the heterozygotes, they are said to be codominant

Question 4

How are alleles represented in a codominant scenario?

Answer 4

Codominant alleles are represented by superscripts on the symbol for the gene.

Ex. In bloodtyping, the M allele is L^M & L^M while the N allele is L^N & L^N, and the heterozygote is L^M & L^N

Question 5

Give an example of a scenario with multiple alleles?

Answer 5

The coat color in rabbits

ex. The color-determining gene, denoted by the lowercase letter c, has four alleles:
c (albino)
c^h (himalayan)
c^ch (chinchilla)
c^+ (wild-type)

Question 6

What symbol is used for wildtypes?

Answer 6

A superscript plus sign after the letter for
the gene or sometimes the gene is omitted and only the plus sign is used.
ex.
c^+ or +

Question 7

What are mutants in rabbits?

Answer 7

Altered forms of the wildtype allele that must have arisen sometime during the evolution of the rabbit.

Question 8

Explain an allelic series in rabbits.

Answer 8

The four alleles of the c gene in rabbits can be combined with each other to make six different kinds of heterozygotes:
c^h & c
c^ch & c
c+ & c
c^ch & c^h
c+ & c^h
c+ & c^ch.

The wild-type allele is completely dominant over all the other alleles in the series.
Ex. c+ > c^ch > c^h > c.

Question 9

What is the use of allelic series used for?

Answer 9

To study the functional relationships among the members of a series of multiple alleles by making heterozygous combinations through crosses between homozygotes.

Question 10

What are nonfunctional alleles called?
What are Partially functional alleles?
Give examples in rabbits.

Answer 10

Nonfunctional alleles:
Are null or amorphic (always completely recessive)

Partially functional:
Alleles are hypomorphic (recessive to alleles that are more functional, including the wild-type allele.)

ex. c+ > c^ch > c^h > c.

Question 11

How is a mutant allele created?

Answer 11

A mutant allele is created when an existing allele changes to a new genetic state—a
process called mutation

Question 12

How can you determine the allelic identity of a new mutation? (recessive mutation ONLY)

Answer 12

Crosses are done to combine the new recessive mutation with recessive mutations of known genes.

1. If the hybrid progeny show a mutant phenotype, then the new mutation and the tester mutation are alleles of the same gene.
2. If the hybrid progeny show a wild type phenotype, then the new mutation and the tester mutation are not alleles of the same gene.

Question 13

Give an example of how to test recessive
mutations for allelism in Drosophila.

Answer 13

In wild-type flies, the eyes are dark red.
Cross the homozygous mutant strains with each other to produce hybrid progeny.

a. If the hybrids have bright red eyes, we will conclude that cinnabar and scarlet
are alleles of the same gene.

b. If they have dark red eyes, we will conclude that they are mutations in different genes.

Question 14

What are sterile mutations?

Answer 14

Mutations that prevent reproduction
Some sterile mutations affect both sexes, but most affect either males or females.

Question 15

What are lethal mutations?
What is their phenotypic effect?

Answer 15

Mutations that interfere with necessary vital functions.
Their phenotypic effect is death.

Question 16

Describe the difference between early and late dominant lethal mutations as well as recessive lethal mutations.

Answer 16

Dominant lethals that act early in life are lost one generation after they occur because the individuals that carry them die.

Dominant lethals that act later in life, after reproduction, can be passed on to the next generation.

Recessive lethals may linger a long time in a population because they can be hidden in heterozygous conditions by a wild-type allele.

Question 17

Give an example of a lethal mutation in mice.

Answer 17

Ex.The yellow-lethal mutation, AY, in the mouse.

The mutation is a dominant visible, causing the fur to be yellow instead of its wildtype gray-brown.

The AY mutation is a recessive lethal, killing AY homozygotes early in their development.

The AY AY homozygotes die during embryonic development

Question 18

What are polypeptides?

Answer 18

Macromolecules that are built of a linear chain of amino acids

Question 19

What makes up a protein?

Answer 19

Two or more polypeptides may combine to form a protein.

Question 20

What are enzymes and what do they do?

Answer 20

Enzymes are proteins that function as:

1. Catalysts in biochemical reactions (speed up the reactions)
2. Form the structural components of cells
3. Are responsible for transporting substances within and between cells

Question 21

Explain the difference in phenotypic effects between dominant mutations and recessive mutations.

Answer 21

Dominant mutations have phenotypic effects in heterozygotes and homozygotes.

Recessive mutations have phenotypic effects only in homozygotes.

Question 22

Explain how a recessive mutation works.
(Loss of function alleles)

How is it affected when paired with a wild-type allele in a heterozygous condition?

Answer 22

Recessive mutations often involve a loss of gene function. (When the gene no longer specifies a polypeptide or when it specifies a nonfunctional or underfunctional polypeptide.)

Such alleles have no effect in heterozygous conditions with a wild-type allele because the wild-type allele specifies a functional polypeptide that will carry out its normal role in the organism.

The phenotype of a mutant/wild heterozygote will therefore be the same, or essentially the same, as that of a wild-type homozygote.

Question 23

Give an example of a recessive mutation that results in the partial loss of gene function. (hint: rabbits)

Answer 23

The himalayan allele of the coat color gene in mammals such as rabbits and cats specifies a polypeptide that functions only in the parts of the body where the temperature is reduced.

This partial loss of function explains why animals homozygous for the himalayan allele have pigmented hair on their extremities—tail, legs, ears, and tip of the nose—but not on the rest of their bodies.

In the extremities, the polypeptide specified by this allele is functional, whereas in the rest of the body, it is not. The expression of the himalayan allele is therefore temperature-sensitive.

Question 24

What are dominant-negative mutations?

Answer 24

Dominant mutations that interfere with the function of the wild-type allele by specifying polypeptides that inhibit, antagonize, or limit the activity of the wild-type polypeptide.

Question 25

Give two examples of dominant-negative mutations in heterozygotes and homozygotes of the T gene in mice.

Answer 25

In heterozygous conditions, these mutations cause a shortening of the tail.

In homozygous conditions, they are lethal.

Question 26

What are gain-of-function mutations?

Answer 26

A mutation that endows a gene product with a new function.

They are a dominant mutation that causes a mutant phenotype in heterozygotes.

Question 27

What types of mutations can occur with recessive mutations? Dominant mutations?

Answer 27

Recessive mutations are more likely to be loss-of-function mutations

Dominant mutations can be gain-of-function, dominant-negative, or loss-of-function mutations.

Question 28

What is incomplete penetrance?
Give an example.

Answer 28

When individuals do not show a trait even though they have the appropriate genotype.
Ex. Polydactyly

Question 29

What causes polydactyly?

Answer 29

This condition is due to a dominant mutation, P, that is manifested in some of its carriers.

Question 30

What is expressivity used to describe? Give an example?

Answer 30

If a trait is not manifested uniformly among the individuals that show it.

ex. The dominant Lobe eye mutation
in Drosophila.

The phenotype associated with this mutation is extremely variable. Some heterozygous flies have tiny compound eyes, whereas others have large, lobulated eyes; between these extremes, there is a full range of phenotypes.

The Lobe mutation is therefore said to have variable expressivity

Question 31

What does incomplete penetrance and variable expressivity indicate?
What causes this modulation?

Answer 31

They indicate that the pathway
between a genotype and its phenotype is subject to considerable modifications.

Geneticists know that some of this modulation is due to:
1. Environmental factors
2. Factors in the genetic background.

Evidence for such factors comes from breeding experiments showing that two or more genes can affect a particular trait.

Question 32

What is epistasis?

Answer 32

When two or more genes influence a trait, an allele of one of them may have an overriding effect on the phenotype.

When an allele has such an overriding effect, it is said to be epistatic to the other genes that are involved.

Question 33

Explain the epistatic effect of shepherd’s purse on triangular seed capsules.

Answer 33

Crossing: AaBb x AaBb

Result:
Ovoid capsules are produced only if
a plant is homozygous recessive for both genes. (aa x bb)

The dominant allele of one gene is
epistatic over the recessive allele of
the other.

Question 34

Explain the epistatic control of squash colour by multiple recessive allele
combinations.

Answer 34

Crossing: CcGg x CcGg

Result:
-Plants that carry the dominant allele C
produce white fruit (hetero or homo)

-cc plants that carry the dominant G allele are yellow (hetero or homo)

-cc and gg plants are green

Question 35

Explain the unlinked recessive allele controls that affect the flower colour in sweet pea plants.

Answer 35

Crossing: CcPp x CcPp

Result:
Any recessive pairing cc, pp, ccpp causes white flowers

Question 36

Explain epistasis in Labrador retrievers

Answer 36

Crossing: BbEe x BbEe

Result:
1. black coat colour is dominant to brown
(brown fur must be bb)

2. a second gene (E) determines whether or not pigment will be deposited in the hair
   ee = yellow, regardless of BB, Bb, or bb

Black: 9
Brown: 3
Yellow: 4

Question 37

What is heterosis (hybrid vigour) used for?

Answer 37

-Successively crossing inbred lines
leads to genetic purity

• Increase in desired traits
• Increase in undesirable traits
  – Inbreeding depression