Chapter 10
Answers
1
QUICK-CHECK questions
Identify the following as true or false:
a
Mendel’s model assumed that parental characters blended in their offspring.
False: Mendel’s model assumed that parental characters did not blend but
remained distinct in each generation. This assumption was based on Mendel’s
observation that crosses of pure-breeding tall and pure-breeding short plants, for
example, did not produce progeny that were intermediate in height but were
instead all tall.
b
Pea plants normally undergo cross-fertilisation.
False: Pea plants normally undergo self-fertilisation.
c
Mendel’s model applies to inheritance in plants and animals but not to
human inheritance.
False: Mendel’s model applies to all plants and all animals, including humans.
2
List two assumptions of Mendel’s explanatory model for inheritance.
Mendel’s model of inheritance assumes that:
1
Each trait is controlled by a pair of factors.
2
One individual plant carries either two identical or two different factors for a
particular trait.
3
Factors retain their identity across generations.
4
Of each pair of factors, the factor that is expressed in a hybrid (that carries the two
different factors) is said to be dominant and the factor that is not expressed in a
hybrid is said to be recessive.
5
During gamete formation, the members of each pair of factors separate to different
gametes.
6
During gamete formation, members of different pairs of factors behave
independently of each other.
7
The results from specific crosses are identical, regardless of which parent is pure
breeding for a given trait.
(Note: Some of Mendel’s assumptions have been refined by later research. In working
with peas, Mendel dealt only with autosomal traits, so his model does not account for
the inheritance of sex-linked traits; for example, assumption 7 above does not hold in
the case of sex-linked traits. We also now know that one gene may have multiple
alleles and this means that assumption 1 above does not always hold. In regard to
assumption 6, see the answer to question 5.)
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Chapter 10: QUICK-CHECK answers
3
What impact did Mendel’s model make on the scientific community at the time it
was first reported?
Mendel’s model made no impact on the scientific community when his results were
first reported and his model of inheritance was ignored.
4
Name the three biologists who rediscovered Mendel’s work.
In 1900, three scientists, all of whom were working on crosses of plants, independently
rediscovered Mendel’s results. They were de Vries who was carrying out experiments
in Holland, Correns who was working in Germany and Tschermak who was working in
both Belgium and Austria.
5
Whose experiments were the first that suggested that Mendel’s factors did not
always behave independently?
It was the English biologists Bateson and Punnett who carried out experiments in the
early part of the twentieth century that first suggested that different pairs of Mendel’s
factors did not always behave independently. Bateson and Punnett demonstrated, for
example, that the factors controlling flower colour in peas did not assort independently
of the factors controlling petal shape.
6
How did the work of Morgan change Mendel’s model?
TH Morgan was the first person to provide experimental evidence that Mendel’s factors
were physically located on chromosomes.
7
What is the transforming factor?
The transforming factor was the name given to the chemical substance that was
shown by Griffith to change (transform) harmless, rough-type bacteria into virulent,
smooth-type bacteria.
8
Briefly explain how the work of the following scientists contributed to an
understanding of the chemical nature of Mendel’s factors.
a
Griffith
In 1928, Frederick Griffith was the first to demonstrate experimentally that
Mendel’s factors were a chemical substance.
b
Avery
In 1943, Oswald Avery carried out experiments that demonstrated that Mendel’s
factors were made of the chemical substance deoxyribonucleic acid (DNA).
9
What are the sub-units of DNA?
The sub-units of DNA are complex molecules called nucleotides.
10
If the sub-units of DNA were analysed, what parts would be identified?
Analysis of nucleotides shows that they are composed of three parts:
 a sugar, known as deoxyribose
 a phosphate group
 a N-containing base.
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Chapter 10: QUICK-CHECK answers
The sugar and the phosphate are identical in all nucleotides found in DNA, but the
base present in a nucleotide can vary and may be adenine (A), thymine (T), cytosine
(C) or guanine (G).
11
The relative proportion of the G nucleotide in DNA from human gut cells was
found to be 1.4 and that of T was 0.9. What other conclusions are possible?
Because DNA forms a double helix in which nucleotides with A pair with T and those
with C pair with G, other valid conclusions are that:
 the proportion of C nucleotide is 1.4
 the proportion of A nucleotide is 0.9.
12
What is meant by a ‘double helix’?
‘Double helix’ refers to the structure of a DNA molecule. Each DNA molecule consists
of two nucleotide chains that are coiled around each other and run in opposite
directions. Each chain consists of a sugar–phosphate backbone with the bases on the
inside of the double helix. The structure is stabilised by weak hydrogen bonds that
form between specific nucleotide pairs, with A always pairing with T, and C always
pairing with G.
13
What would be expected to happen if a solution of DNA was
a
gently heated
When a DNA solution is gently heated, the weak hydrogen bonds between the
nucleotide pairs in the DNA double helix are disrupted. As a result, the two
nucleotide chains of the double helix separate so that the DNA exists in the form
of single nucleotide chains. This process is called dissociation.
b
then allowed to cool?
When the solution is then allowed to cool, the single nucleotide chains form again
into a double helix with its A–T pairs and its C–G pairs stabilised by hydrogen
bonds.
14
Consider part of a DNA chain with the nucleotides:
...TTAGGAC...
Which of the following is part of the complementary strand:
a ...CCGAAGT...?
b ...TTAGGAC...?
c ...AATCCTG...?
The sequence of bases in one strand of a DNA double helix is complementary to the
sequence in the second strand of the double helix. So, the complementary strand is
(c).
15
A piece of DNA contains 20 000 bp. Is this more likely to be a whole
chromosome or a whole gene? Explain.
This amount of DNA (20 000 base pairs) is more likely to be a gene. The DNA
molecule in a whole chromosome, even a small chromosome, contains millions of
base pairs.
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Chapter 10: QUICK-CHECK answers
16
If genes were isolated from a cat, a cyanobacterium and a cauliflower, what
similarity would be seen?
The genes from each kind of organism would all be found to be made of DNA.
17
If the two human genes for making blood-clotting factor and salivary amylase
were compared:
a
in what way would they be similar?
The two different genes would be similar in that they would both be made of DNA.
b
in what way different?
The two genes would differ in the base sequence or order of the four different
bases along their length.
18
Give an example of a code.
A code is a set of symbols of some kind that contains information; to understand the
code, the symbols must be deciphered. Many examples of codes exist, including
morse code, chemical symbols, semaphore, musical scores, traffic signs and sign
language.
19
In what form is information held in a DNA molecule?
The information held in a DNA molecule is present in the order of the nucleotides A, T,
C and G along its template strand.
20
Which of the following statements is most accurate? Explain your choice.
a
DNA is converted to the amino acid sub-units of protein.
b
DNA contains the coded information for joining amino acids to form protein.
c
DNA turns into protein.
The accurate statement is (b).
Statements (a) and (c) are incorrect because DNA is not a molecule that
undergoes a chemical reaction to form a new product.
Statement (b) is accurate because it identifies DNA as an information-containing
molecule. DNA remains unchanged and it is only the coded information that it
contains that is used.
21
How many instructions (for adding amino acids) are present in the base
sequence: TTAGGG?
The information in DNA is present in the form of base triplets (three-base sequences)
called codons, so the 6-base sequence TTAGGG theoretically contains the information
for adding two amino acids to a protein chain. (See tables A1 and A2 on page 656, to
check that TTA codes for the addition of the amino acid asparagine, and that GGG
codes for the addition of the amino acid proline.)
22
What code translates as ‘START joining amino acids to form a protein’?
The START code in the template strand of a DNA molecule is TAC. When translated
to an mRNA molecule, the START code is AUG, the complement of the DNA code.
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Chapter 10: QUICK-CHECK answers
23
What are the meanings of the following codes in DNA: CAA and ACT?
In the template strand of a DNA molecule, the code CAA means ‘Add the amino acid
valine to a protein (polypeptide) chain'.
The code ACT means ‘STOP adding amino acids to a protein chain’.
24
List one of the benefits of the Human Genome Project.
Benefits of the Human Genome Project include applications in:
 medicine such as:
 more precise characterisation of diseases due to single gene mutation
 improved understanding of the action of disease-causing mutant alleles that will
potentially lead to improved treatment of these diseases
 identification of genes that predispose people to certain abnormal conditions that
will potentially result in prevention of these conditions.
 evolutionary biology, including better insights into human evolution and human
prehistory
 human biology, including advances in understanding of the genetic control of
human development.
25
Identify the first eukaryotic organism to have its genome sequenced.
The first eukaryotic organism to have its genome sequenced was Saccharomyces
cerevisiae, a unicellular organism better known as brewer’s yeast.
26
Is the following statement true or false?
Most of the genomes of two unrelated persons would be different.
If the genomes of two unrelated persons are compared, they would be found to be
very similar. The estimated difference is just 0.1 per cent.
27
What is meant by the term ‘comparative genomics’?
Comparative genetics is the study of the genomes or complete DNA sequences of
various species, including the identification of similarities and differences.
28
How do a substitution mutation and a deletion mutation differ?
In a substitution mutation, one base in a DNA sequence is replaced by another base,
such as G being replaced by A. This kind of mutation affects just one codon and is
also known as a point mutation.
In a deletion mutation, one base in a DNA sequence is removed. This kind of mutation
affects the codon in which the deletion occurs as well as all codons downstream from
that position and is sometimes called a frameshift mutation.
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Chapter 10: QUICK-CHECK answers
29
The DNA in the gametes of an industrial worker is altered because of exposure
to a chemical.
a
Is this a spontaneous or an induced mutation?
The mutation in the worker’s gametes is an induced mutation since the cause is
known. The chemical to which the worker was exposed and that caused the
mutation is an example of a mutagenic agent.
b
Is this a somatic or a germline mutation?
This is a germline mutation since it has affected the gametes or germline of the
industrial worker.
c
Can this mutation be transmitted to the worker’s children?
Because this is a germline mutation, it may be passed to the next generation and
so can be transmitted to the worker’s children.
30
Using words or diagrams, distinguish between the members of each of the
following pairs:
a
intron and exon
An intron is the part of the DNA sequence of a gene that is translated into premRNA in the nucleus but is then excised so that it is not transcribed.
An exon is the part of the DNA sequence of a gene that is transcribed into mRNA
and translated into a protein (polypeptide) chain.
b
coding region and flanking region.
The coding region of a gene is the region of a DNA template strand that contains
the base sequence that will be translated into a protein (polypeptide) chain. This
region begins with a start codon and ends with a stop codon.
The flanking regions of a gene are both the region of DNA upstream of the gene
(towards the 5 end) and regions of DNA downstream of the gene (towards the 3
end). The upstream region does not contain codons but has specific base
sequences known as promoters that affect the activity of the gene. The
downstream region includes a base sequence that terminates the gene
transcription.
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Chapter 10: QUICK-CHECK answers
31
True or false? All genes contain the same number of exons.
False: The number of exons in different genes varies.
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