Download Beyond Mendel`s Laws

S E C T I O N
4.4
Beyond Mendel’s Laws
E X P E C TAT I O N S
Explain how incomplete
dominance and co-dominance
can account for inheritance of
intermediate characteristics.
Solve genetics problem
involving multiple alleles.
Figure 4.21 The inheritance
pattern of multicoloured corn
would have been unknown to
Mendel.
In his studies of pea plants, Mendel found that
inherited traits were either dominant or recessive.
The dominant allele in an individual was always
expressed, even if the recessive allele was present.
However, some organisms show different patterns
of inheritance. For example, the variety of colours
of corn shown in Figure 4.21 does not follow the
inheritance pattern outlined by Mendel — many
colours of kernel are expressed rather than only
one. How can we explain the inheritance of traits
that do not follow simple Mendelian genetics?
Incomplete Dominance
Not all traits are purely dominant or purely
recessive. In some instances neither of the alleles
controlling the trait are dominant. When this
happens, a blending of the two traits can occur,
called incomplete dominance. Apparent blending
of traits to give an intermediate expression can
occur in individuals that are heterozygous.
Examples of incomplete dominance can be found
in many species of plants, including the
snapdragon (see Figure 4.22).
White or red snapdragon flowers are
homozygous, while pink flowers are heterozygous.
In this example, the letters R and R′ are used
(rather than R and r) to indicate alleles that show
incomplete dominance (see Figure 4.23). Two red
alleles (RR) are necessary to produce a red flower.
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Figure 4.22 Snapdragon flowers can be white, red, or pink.
Pink flowers result from a cross of individuals with white and
red flowers.
Individuals with only one R allele are unable to
make enough red pigment to produce red flowers,
and they appear pink. Individuals that are white
(R′R′) produce no red pigment.
In traits that show incomplete dominance, the
alleles segregate the same way as in crosses
mentioned earlier in this chapter. However, because
neither of the alleles are dominant, only pink
flowers (RR′) are produced in the F1 generation.
What happens when two individuals with pink
pink
(RR′)
red
(RR)
R
red
R
R
R′
white
R
R′
RR′
RR′
RR
RR′
RR′
R′R′
all
pink
white
(R′R′)
R′
RR′
RR′
pink
(RR′)
All pink flowers
R′
1 red : 2 pink : 1 white
Figure 4.23 Flower colour in the snapdragon is an example of incomplete
dominance. Pink flowers are heterozygous (RR′), where neither allele is dominant.
flowers are crossed? The phenotypes of the
F2 generation are: 25% red, 50% pink, and 25%
white, or a ratio of 1 : 2 : 1. Notice, that the
F2 generation does not show a Mendelian ratio for
phenotype of 3 : 1. The ratios for genotype and
phenotype are both 1 : 2 : 1 because there is no
dominant allele. This supports Mendel’s law of
independent assortment. Another example of
incomplete dominance occurs in the pitch of the
human male voice. The lowest and highest pitches
occur in men who are homozygous for these
alleles. Intermediate pitches occur in men
heterozygous for this condition. Fortunately for
Mendel, the pea plant traits that he studied were
not controlled by incomplete dominance.
Otherwise he may not have been able to develop
the basic principles of genetics.
Figure 4.24 In some varieties of chicken, two alleles for a
trait may be expressed equally, such as in this bird with
barred plumage.
Co-dominance
Multiple Alleles
In some cases, both alleles for a trait may be
dominant. Such alleles are said to be co-dominant
because both alleles are expressed in the
heterozygous individual. For example, feather
colour in chickens is governed by two dominant
alleles. Black birds are homozygous for the B allele
and white birds are homozygous for the W allele.
What happens when a black rooster is crossed
with a white hen? If the colours blended, you
would expect offsping with grey plumage. If only
the B allele was dominant, then only blackfeathered young would result. However, the result
of the cross is offspring with checkered black-andwhite plumage, as shown in Figure 4.24. Some of
the feathers are white and some are black.
Many genes have more than two alleles, or
multiple alleles. An example of multiple alleles
occurs in human blood types. In this case, three
alleles are involved: A, B, and O. Table 4.2 on
page 144 shows the blood types in humans and the
possible genotypes for each. Each person has two
of the three alleles and each allele determines if the
red blood cells do or do not possess certain
glycoproteins. People with type A blood have A
glycoproteins on their blood cells. People with type
B blood have B glycoproteins. People with type AB
blood have both glycoprotiens. People with blood
type O do not have either A or B glycoproteins.
The three alleles for human blood types are IA, IB,
and i. Alleles IA and IB are dominant over i.
However, IA and IB are co-dominant and are
expressed equally.
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Table 4.2
Human blood types
Phenotype (blood type)
Genotypes
A
IAIA or IAi
B
IBIB or IBi
AB
IAIB
O
ii
There are two possible genotypes for blood types
A and B, one homozygous, and one heterozygous.
In order to have type AB blood, a person would
have to inherit one IA and one IB allele. People
with type O blood must inherit two ii alleles. You
will learn about the inheritance of human bloodrelated illnesses in Chapter 7.
Human blood type is only one example of a trait
that is governed by more than one pair of alleles.
Some traits are controlled by far greater numbers of
alleles. These types of traits are the result of
Sample Problem
Human Blood Types
If a woman has blood type AB, and a man has
blood type A, what possible blood types will
their children have?
What is required?
You are asked to determine the possible
genotypes of offspring from a cross.
What is given?
You know the genotype of the mother (AB) and
father (A). The possible gametes from the mother
are IAIB. The possible gametes from the father are
IAIA if he is homozygous or IAi if he is
heterozygous.
Plan your strategy
You must make the following crosses:
IAIB × IAIA
IAIB × IAi
Act on your strategy
Place the alleles of each parent along the
columns and rows of the Punnett square and
complete the possible crosses. If the father is
blood type A homozygous, then:
Mother
I
IA
A
B
I
IAIA
IAIB
IAIA
IAIB
Father
IA
If father is blood type A heterozygous, then:
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MHR • Genetic Continuity
Mother
IA
IB
IA
IAIA
IAIB
IAi
IBi
Father
i
There are four possible genotypes: IAIA , IAIB, IAi,
IBi. (Note: the genotypes IAIA and IAi will both
produce the same phenotype — type A blood).
Thus, the possible blood types of the children
are: A, AB, and B.
Check your solution
If the father is homozygous, the possible blood
types of the children are A and AB. If the father
is heterozygous, the possible blood types of the
children are A, AB, and B.
Practice Problems
1. A man has blood type A and his wife has
blood type B. A child has blood type O.
Could these individuals be the parents of this
child? Explain.
2. Suppose a man with blood type B marries a
woman with type AB blood. What blood
types would you expect to find among their
children? What could tell you whether the
man was homozygous or heterozygous for the
B blood type?
3. In foxes, a pair of alleles, P and p, interact as
follows: PP is lethal, usually during the
embryonic stage; Pp produces platinumcoloured fur, and pp produces silver foxes.
Could a fox breeder establish a true-breeding
variety of platinum foxes? Explain.
multiple gene inheritance and are very complex.
Skin colour is a good example of this type of
inheritance. There are many more than just the two
or three phenotypes attributed to skin colour. In
fact there may be several phenotypes, or even a
continuous variation that cannot be split up into
convenient, easily defined categories. The more
genes that contribute to a single trait, the greater
the number of categories of the trait, with
increasingly fine differences between the
categories. These complex patterns of inheritance
will be addressed in the next chapter.
Since Mendel’s time, knowledge of the
mechanisms that control the inheritance of traits
has developed considerably. It is now understood
that the inheritance of one allele can, at times,
affect the inheritance of a second allele, or can
affect how and when a trait is expressed in an
individual. As you have learned not all inheritance
follows the simple dominant-recessive pattern. In
THINKING
incomplete dominance, heterozygotes exhibit an
intermediate phenotype between the two
contrasting phenotypes. Co-dominant alleles, such
as those determining blood type, are both present
in the phenotype of heterozygous individuals.
Many traits are determined by several alleles.
COURSE CHALLENGE
Blood types are often used to help establish the
identity of people. How could blood types be used to help
determine the presence of disease or as evidence in criminal
investigations?
LAB
Inheritance of Coat Colour
in Rabbits
3. Would it be possible to obtain white rabbits if one
parent is white and the other is Chinchilla? Explain.
Background
4. Would it be possible to obtain Chinchilla rabbits if one
parent is Himalayan and the other is white? Explain.
Coat colour in rabbits is governed by four different alleles.
Each allele is responsible for producing a different coat
colour: dark grey, Chinchilla, Himalayan, and white. Each
rabbit has only two alleles. Study the relationship among
the alleles in the table and then complete the lab.
Phenotype
(coat colour)
Allele
5. A Chinchilla rabbit is mated with a Himalayan. Some
of the offspring are white. What are the parents’
genotypes?
Pattern of inheritance
Dark grey
C
dominant to all other alleles
Chinchilla
cch
dominant to Himalayan and to white
Himalayan
h
c
dominant to white
White
c
recessive
You Try It
1. List all the possible genotypes for a
(a) dark grey rabbit
(b) Chinchilla rabbit
(c) Himalayan rabbit
(d) white rabbit
2. Predict the phenotype of a rabbit with the following
genotypes. Explain your answers.
(a) chcch
(b) Cch
Heredity • MHR
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SECTION
REVIEW
1.
K/U Define the following terms: carrier, incomplete
dominance, co-dominance, multiple allele
inheritance, multiple gene inheritance.
2.
K/U Explain the law of probability as it applies to the
inheritance of different traits.
3.
I Tay Sachs disease is the result of two recessive
alleles. Frank’s brother died from Tay Sachs disease,
but Frank is healthy. Stephanie’s brother died from
Tay Sachs disease but Stephanie is healthy.
9.
10.
MC Mrs. Doe and Mrs. Roe had babies at the same
time. Mrs. Doe took home a girl and named her
Nancy. Mrs. Roe received a boy and named him
Richard. However, Mrs. Roe was sure she had a girl,
and sued the hospital. Blood tests showed that
Mr. Roe was type O and Mrs. Roe was type AB.
Mr. and Mrs. Doe were both type B. Nancy was
type A and Richard was type O. Had an exchange
occurred? Explain your answer.
11.
I A rose-combed rooster is mated with two rosecombed hens. Hen A produces 14 chicks, all rosecombed. Hen B produces 9 chicks, 7 of which are
rose-combed and 2 single-combed. What are the
likely genotypes of the parent birds? Explain.
12.
I In four o’clock plants, red flowers are incompletely
dominant over white flowers. The heterozygous
flowers are pink. If a red-flowered four o’clock plant
is crossed with a white-flowered four o’clock plant,
what will be the flower colour of
(a) If Frank and Stephanie have a child, what is the
probability that the child will inherit Tay Sachs
disease?
(b) If Frank and Stephanie have a child, what is the
probability that the child will not inherit the
disorder?
4.
C Explain how an individual can be a carrier for a
particular disease yet not have the disease. Is this
explanation true for all people who are carriers?
5.
K/U Differentiate between co-dominance and
multiple allele inheritance.
6.
C How many different genotypes of the human
blood type are possible? Explain. How many different
phenotypes are possible? Why are these numbers
different? Explain.
7.
MC A woman sues a man for the support of her
child. She has blood type A, her child has type O,
and the man has type B. Could the man be the
father? Explain your answer.
8.
I The Punnett square shows a dihybrid cross of
F1 pea plants. Purple flowers (P) are dominant and
white flowers are recessive. Tall plants (T) are
dominant and short plants are recessive. Determine
the phenotype ratios of the F2 generation. What
Mendelian law does this ratio demonstrate?
Female gametes
PT
Pt
pT
pt
PPTT
PPTt
PpTT
PpTt
I A mother with blood type AB has a child with the
same blood type. What are the possible genotypes of
the father?
(a) the F1 generation?
PT
(b) the F1 generation crossed with its red parent?
Male gametes
(c) the F1 generation crossed with its white parent?
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Pt
PPTt
PPtt
PPTt
Pptt
UNIT ISSUE PREP
pT
PpTT
PpTt
ppTT
ppTt
pt
PpTt
Pptt
ppTt
pptt
MHR • Genetic Continuity
If you have decided to investigate research into
inheritance of complex traits in your Unit 2 Issue
Analysis, make sure you are familiar with the different
models of inheritance of traits.