Download Incomplete Dominance and Codominance

Solution
The phenotypic ratio of the F1 offspring is two wild-type eye colour to one apricot
eye colour to one honey eye colour. The Punnett square for this cross is shown in
Figure 2.
apricot
E2
E
1
1
E E
E3
2
1
E E
Single-Trait Inheritance
wild
type
3
wild
type
E4
E2E4
E3E4
apricot honey
Figure 2
A cross between a fruit fly with wild-type eye
colour and one with apricot-coloured eyes
1. Since one allele is inherited from each parent, various genotypes are possible.
2. The dominant phenotype is expressed if the offspring is either heterozygous
or homozygous for the dominant allele.
3. The recessive phenotype is expressed only if the offspring is homozygous
for the recessive allele.
4. When there are multiple alleles for a given characteristic, the alleles have a
dominance hierarchy.
Practice
Understanding Concepts
F1 parent
1. Use the information in Table 1 (page 143) to answer the following
questions:
(a) Of the genotypes listed in the table, which would you say represent the homozygous condition under simple single-trait inheritance? Explain.
(b) Of the genotypes listed, which would you say represent the
homozygous recessive condition? Explain.
Applying Inquiry Skills
2. Use the information in Table 1 and the method shown in Figure 2 to
find the F1 phenotypes if a white-eyed fly is crossed with one that has
honey-coloured eyes.
3. Find the F1 phenotypes if a fly with apricot-coloured eyes (E 2E 4) is
crossed with one that has honey-coloured eyes (E 3E 4).
F1 parent
CR
CW
CR
C RC R
C RC W
CW
C RC W
C WC W
C RC R = red
C RC W = pink
C WC W = white
Figure 1
Colour in snapdragons is an example of
incomplete dominance. Red-flowering and
white-flowering snapdragons combine to
produce pink-flowering plants in F1. The F2
generation produces one red to two pink to
one white.
144 Chapter 4
4.5
Incomplete Dominance and
Codominance
Prior to Mendel’s studies, many scientists believed that hybrids would have a
blending of traits. Although Mendel never found any examples of new traits or
blended traits produced by the combinations of different alleles, many do exist
in nature. When two alleles are equally dominant, they interact to produce a new
phenotype. This kind of interaction is known as incomplete dominance.
For example, if red snapdragons are crossed with white snapdragons, all of
the F1 offspring are pink. The pink colour is produced by the interaction of red
and white alleles. This type of incomplete dominance is often called intermediate
inheritance. If the F1 generation is allowed to self-fertilize, the F2 generation produces a ratio of one red to two pink to one white. The Punnett square in Figure 1
helps to explain this result.
Another type of incomplete dominance is referred to as codominance. In this
type of interaction, both alleles are expressed at the same time. Shorthorn cattle
4.5
red bull
white cow
Hr
Hr
Hw
H rH w
H rH w
Hw
H rH w
H rH w
roan cow
roan calf
roan bull
F1 generation
X
F2 generation
red
H rH r
roan
H rH w
roan
H rH w
white
H wH w
Figure 2
In codominance, the expression of one allele
does not mask the expression of the other. In
shorthorn cattle, roan calves have intermingled
red and white hair.
provide an excellent example of codominance (Figure 2). A red bull crossed with a
white cow produces a roan calf. The roan calf has intermingled white and red hair.
A roan calf would also be produced if a white bull were crossed with a red cow.
Practice
Understanding Concepts
1. Explain in your own words the meaning of dominance, codominance,
and incomplete dominance.
Applying Inquiry Skills
2. Determine the F1 phenotypes of a cross between a pink and a white
snapdragon.
3. Find the F1 phenotypes of a cross between a red cow and a roan bull.
4. A geneticist notes that crossing a round radish with a long radish
produces oval radishes. If oval radishes are crossed with oval
radishes, the following phenotypes are noted in the F2 generation:
100 long, 200 oval, and 100 round radishes. Use symbols to explain
the results obtained for the F1 and F2 generations.
Genes and Heredity
145
5. Diabetes is a recessive genetic disorder. A defective gene reduces
insulin production by the pancreas. Insulin is released into the
circulatory system and allows the cells of the body to absorb glucose
from the blood. Individuals who lack insulin have high blood sugar.
In an attempt to trace the inheritance of the defective allele in one
family, the data in Table 1 was gathered.
(a) Construct a pedigree chart showing the passage of the diabetes
allele.
(b) Indicate the probable genotypes of Jennifer and Ryan.
(c) Indicate the probable genotypes of Susan and Walter.
(d) Whose genotype cannot be determined with 100% certainty? Explain.
Table 1
Name
Relationship
Phenotype
Jennifer
mother
normal
Ryan
father
normal
Walter
son of Ryan and Jennifer
diabetic
Susan
wife of Walter
normal
Helen
daughter of Ryan and Jennifer
normal
James
son of Ryan and Jennifer
normal
Colin
son of Susan and Walter
diabetic
Genetic Screening
Figure 3
A genetics counsellor helps a couple understand the
genetic factors involved in diseases and disorders.
Before the development of a process that permitted the extraction of insulin from
animals, many people who had the recessive allele for diabetes in the homozygous
condition died before passing on their genes to offspring. Genetic screening
attempts to identify genetic conditions prior to birth or attempts to predict these
conditions prior to conception (Figure 3). Genetic information is obtained
through a variety of methods including detailed pedigrees and biochemical
testing for known disorders. Methods of prenatal diagnosis can indicate the sex of
the child as well as the presence of many genetic conditions. Amniocentesis and
chorionic villi sampling (CVS) are the most widely used techniques.
Huntington’s chorea is a neurological disorder caused by a dominant allele
that only begins to express itself later in life. The disease is characterized by the
rapid deterioration of nerve control, eventually leading to death. Early detection
of this disease by genetic screening is possible.
Practice
Understanding Concepts
1. Define genetic screening. Describe some technologies used in genetic
screening.
2. What are some advantages of genetic screening? Provide an example.
3. What are some physical dangers associated with genetic screening
methods? Provide an example.
Making Connections
4. What are some social, moral, and ethical objections to genetic
screening? Provide an example.
Follow the links for Nelson Biology 11, 4.5.
GO TO
146 Chapter 4
www.science.nelson.com
4.5
5. What laws, if any, do you think will arise regarding the use of genetic
screening?
DECISION-MAKING SKILLS
Explore an
Issue
Debate: Genetic Screening
Genetic screening techniques are coming of age, and the controversy
that surrounds them is growing by the minute. You have researched
the benefits and problems associated with genetic screening. Do the
benefits outweigh the problems?
Analyze the Issue
Defend a Decision
Evaluate
Define the Issue
Identify
Alternatives
Research
Statement
Genetic screening should be compulsory.
• Write a list of points and counterpoints which you and your group
considered.
• Decide whether your group agrees or disagrees with the statement.
• Prepare to defend your group’s position in a class discussion.
Case Study: A Mystery
There are four different ABO blood types as shown in Table 2. The alleles for
blood types A and B are codominant but are dominant over the allele for type O.
The rhesus factor is a blood factor that is regulated by a gene. The Rh-positive
allele is dominant over the Rh-negative allele. In this activity, you will solve a
murder mystery using genetics.
Evidence
Table 2
Phenotypes
Genotypes
type A
I AI A, I AI O
type B
I BI B, I BI O
type AB
I AI B
type O
I OI O
As a bolt of lightning flashed above Black Mourning Castle, a scream echoed from
the den of Lord Hooke. When the upstairs maid peered through the door, a
freckled arm reached for her neck. Quickly, the maid bolted from the doorway,
locked herself in the library, and telephoned the police. Inspector Holmes arrived
to find a frightened maid and the dead body of Lord Hooke. Apparently, the lord
had been strangled. The inspector quickly gathered evidence. He noted blood on
a letter opener, even though Lord Hooke did not have any cuts or abrasions. The
blood sample proved to be type O, Rh-negative. The quick-thinking inspector
phoned the family doctor for each family member’s medical history. Figure 4
shows the relatives who were in the castle at the time of Lord Hooke’s murder.
Lord Hooke
Tom
Beth
Jane
Tina
Lady Hooke
Ann
Ida
Helen
Roule
Henry
Figure 4
Pedigree chart of Lord and Lady Hooke
Genes and Heredity
147
The inspector gathered the information in Table 3. Some of the family members
were deeply tanned, so the inspector found it difficult to determine whether or not
freckles were present on their arms. Note that having freckles is an inherited trait and
the allele for freckles is dominant over the allele for no freckles.
Table 3
Family member
Blood type
Rh factor
Freckles
Lord Hooke
AB
+
no
Lady Hooke
A
+
no
Helen
A
+
no
Roule
O
+
no
–
?
Henry
Refused blood test
Ida
A
?
Ann
B
+
?
Tom
O
–
no
Jane
A
+
?
Beth
O
–
?
Tina
A
+
yes
The crafty inspector drew the family close together and, while puffing on his
pipe, indicated that he had found the murderer. He explained that one of the
heirs to the fortune was not Lord Hooke’s biological child. The inspector believed
that the child committed the murder.
Analysis
(a) Who was the murderer? State the reasons for your answer.
(b) Describe the procedure you followed to obtain your answer.
(c) How did the inspector eliminate the other family members?
Sections 4.3–4.5 Questions
Understanding Concepts
1. Explain the difference between a dominant and a recessive condition. Provide an example.
2. Guinea pigs with yellow coat colour have the genotype CYCY.
Guinea pigs with cream coat colour (cream-coloured hairs) have
the genotype CYC W, and those with white coat colour have the
genotype C WC W. Is the condition for coat colour one of complete
dominance, incomplete dominance, or codominance? Explain.
Applying Inquiry Skills
3. Phenylketonuria (PKU) is a genetic disorder caused by a dominant
allele. People with PKU are unable to metabolize a naturally occurring amino acid, phenylalanine. If phenylalanine accumulates, it
inhibits the development of the nervous system, leading to mental
retardation. The symptoms of PKU are not usually evident at birth
but can develop quickly if the child is not placed on a special diet.
Figure 5 is a pedigree chart that shows the inheritance of the
defective PKU allele in one family.
148 Chapter 4
4.5
I
1
2
II
1
2
3
4
5
6
7
III
1
2
3
4
5
Figure 5
(a) How many generations are shown in the pedigree chart?
(b) How many children were born to the parents of the first
generation?
(c) What are the genotypes of individuals 1 and 2 in generation I?
(d) How is it possible that in generation II, some of the children
showed symptoms of PKU while others did not? (Hint: Use
a Punnett square to help with your explanation.)
(e) Individuals 6 and 7 in generation II had a child without PKU.
Does this mean that they can never have a child with PKU?
Explain your answer.
4. Multiple alleles control the intensity of pigment in mice. The
gene D 1 designates full colour, D 2 designates dilute colour, and
D 3 is deadly when homozygous. The order of dominance is D 1 >
D 2 > D 3. When a full-coloured male is mated to a dilute-coloured
female, the offspring are produced in the following ratio: two full
colour to one dilute to one dead. Indicate the genotypes of the
parents.
5. Multiple alleles control the coat colour of rabbits. A grey colour
is produced by the dominant allele C. The C ch allele produces a
silver-grey colour, called chinchilla, when present in the homozygous condition, C chC ch. When C ch is present with a recessive
allele, a light silver-grey colour is produced. The allele C h is
recessive to both the full-colour allele and the chinchilla allele.
The C h allele produces a white colour with black extremities.
This coloration is called Himalayan. An allele C a is recessive to
all alleles. The C a allele results in a lack of pigment, called
albino. The dominance hierarchy is C > C ch > C h > C a. Table 4
below provides the possible genotypes and phenotypes for coat
colour in rabbits. Notice that four genotypes are possible for full
colour but only one for albino and chinchilla.
Table 4
Phenotypes
Genotypes
full colour
CC, CC ch, CC h, CC a
chinchilla
C chC ch
light grey
C chC h, C chC a
Himalayan
C hC h, C hC a
albino
C aC a
(continued)
Genes and Heredity
149