Download MUTATIONS

MUTATIONS
A mutation is a permanent change in the genetic material (or DNA sequence). Mutations
can affect a single point in a gene, or larger sections of DNA (e.g. chromosomal
mutations).
Mutations can result in new characteristics that are inherited if the mutation appears in the
germ-line cells where gametes are formed. Germ-line cell mutations influence the next
generation whereas a somatic cell mutation only affects the individual concerned.
•
Mutations are spontaneous events and are therefore random in action.
•
A mutant is the individual who inherits the change.
•
Mutations can also be induced by mutagens (e.g. ionising radiation such as UV and
X-rays, or chemicals such as DDT).
•
Not all mutations are harmful. A mutation may
produce a new characteristic that provides a
survival advantage. Some mutations are obviously
harmful but most are neutral in their effect.
•
Point mutations are the most common type of
mutation. They involve minor changes to the DNA
and are often called single gene mutations.
Examples include substitutions, deletions etc.
Example of a Mutagen – UV radiation:
At risk:
•
•
•
Fair skinned individuals
Children
Individuals exposed to considerable solar radiation
Clare Oliver became the human face of the
dangers of gaining tans from the solarium industry.
Her case of terminal skin cancer gained nationwide
publicity, particularly in the months leading to her
death in 2007.
A multi-million dollar graphic campaign had a
devastating effect on the numbers of teenagers
using solariums for tanning purposes. The
solarium industry has since collapsed in Australia,
after Victoria became the first state to strictly
regulate what is now a defunct industry.
The School For Excellence 2015
The Essentials – Unit 4 Biology – Book 1
Page 70
GENE MUTATIONS
Let’s determine the effects of three types of point mutations on a small section of DNA:
Original section of DNA: … GGT ATA CCC TGA TCC …
Substitution: … GGT TTA CCC TGA TCC …
Addition: … GGT ATA ACC CTG ATC C..
Deletion: …
GGT AAC CCT GAT CC. …
^
•
As can be seen above, in general, additions and deletions have a more devastating
effect on the base sequence of a gene than substitutions, as all triplets beyond the
mutation are potentially changed (frame shift mutations). Hence, the potential effect
on phenotype is typically more devastating.
•
A single amino acid change in the protein that a gene codes for can still have
significant consequences. When valine is substituted for glutamic acid at a particular
point in the beta chain of haemoglobin, the effect on the shape and function of red
blood cells is dramatic, resulting in the inherited disease sickle cell anaemia.
•
Occasionally a substitution results in a codon that still codes for the same amino acid
(neutral point mutation – e.g. CCC Æ CCA Æ proline). This is due to degeneracy
in the genetic code.
•
Sickle cell anaemia is an example of a missense mutation, i.e. a point mutation where
the change in a single nucleotide causes the substitution of a different amino acid.
•
A nonsense mutation is a point mutation that results in a premature stop codon in
the transcribed mRNA, and a nonfunctional protein product. They can be caused by
substitutions, additions or deletions, as well as other changes in DNA.
In the example below, the second scenario involves a substitution that has resulted in a
faulty polypeptide chain which is missing its final four amino acids:
1.
5' - ATG ACT CAC
DNA: 3' - TAC TGA GTG
mRNA: 5' - AUG ACU CAC
Protein:
Met Thr
His
2.
5' - ATG ACT CAC TGA GCG CGA AGC TGA - 3'
DNA: 3' - TAC TGA GTG ACT CGC GCT TCG ACT - 5'
mRNA: 5' - AUG ACU CAC UGA GCG CGA AGC UGA - 3'
Protein:
Met Thr
His Stop
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CGA GCG CGA
GCT CGC GCT
CGA GCG CGA
Arg
Ala Arg
AGC
TCG
AGC
Ser
TGA - 3'
ACT - 5'
UGA - 3'
Stop
The Essentials – Unit 4 Biology – Book 1
Page 71
HOW THE ENVIRONMENT CAN CHANGE YOUR DNA
Chemical pollutants can alter the way in which DNA replicates.
Normal DNA replication is represented below:
In an example of a mutation caused by the environment, a chemical pollutant (a mutagen)
oxidises a guanine base (see below). The oxidised guanine now binds with adenine instead
of cytosine, and during subsequent interphase events, DNA polymerase will mistakenly
create an A-T pairing in place of the original G-C pair at this point in the DNA molecule.
(New Scientist)
The School For Excellence 2015
The Essentials – Unit 4 Biology – Book 1
Page 72
POINT MUTATIONS
Mutations can be caused by:
1.
Substitutions
Substitutions are the most common form of mutation. They involve the replacement of
one base by another. One codon may be altered so that it now codes for one different
amino acid in the protein sequence.
The most detrimental substitution mutations seem to be when the 1st or 2nd base of a
codon is altered.
E.g. GGU, GGC, GGA and GGG all code for Glycine. Altering either the 1st or 2nd base
will definitely result in a new amino acid being coded for. Complete the missing DNA,
mRNA and amino acid molecules in the following example:
Example: In the normal DNA sequence
mRNA:
A A G C A T G G T A G G
U U C
Amino acids:
If the first A base is substituted for a T:
The new DNA sequence is:
mRNA:
Amino acids:
Substitution mutations may result in no change of amino acid because the genetic code
is degenerate. This means that many codons can code for the one amino acid, so the
mutation may have a neutral affect.
mRNA Code Dictionary
Second Base
First
Base
U
C
A
G
Third
Base
U
UUU
UUC
UUA
UUG
-
Phe
Phe
Leu
Leu
UCU
UCC
UCA
UCG
-
Ser
Ser
Ser
Ser
UAU
UAC
UAA
UAG
-
Tyr
Tyr
stop
stop
UGU
UGC
UGA
UGG
-
Cys
Cys
stop
Trp
U
C
A
G
C
CUU
CUC
CUA
CUG
-
Leu
Leu
Leu
Leu
CCU
CCC
CCA
CCG
-
Pro
Pro
Pro
Pro
CAU
CAC
CAA
CAG
-
His
His
Gln
Gln
CGU
CGC
CGA
CGG
-
Arg
Arg
Arg
Arg
U
C
A
G
A
AUU
AUC
AUA
AUG
-
Ile
Ile
Ile
Met/start
ACU
ACC
ACA
ACG
-
Thr
Thr
Thr
Thr
AAU
AAC
AAA
AAG
-
Asn
Asn
Lys
Lys
AGU
AGC
AGA
AGG
-
Ser
Ser
Arg
Arg
U
C
A
G
G
GUU
GUC
GUA
GUG
-
Val
Val
Val
Val
GCU
GCC
GCA
GCG
-
Ala
Ala
Ala
Ala
GAU
GAC
GAA
GAG
-
Asp
Asp
Glu
Glu
GGU
GGC
GGA
GGG
-
Gly
Gly
Gly
Gly
U
C
A
G
The School For Excellence 2015
The Essentials – Unit 4 Biology – Book 1
Page 73
2.
Insertions (Additions)
Insertions occur when an extra base(s) is inserted into the DNA sequence. A frame
shift is caused as all bases located after the point of insertion in the gene are moved or
displaced one position. This may affect many codons.
Complete the missing DNA, mRNA and amino acid molecules in following example:
Example: In the normal DNA sequence
G T A C T A A A C G T C
mRNA:
Amino acids:
If the base G is inserted after the 1st T:
The new DNA sequence is:
mRNA:
Amino acids:
Insertions lead to new colour and petal forms of Ipomoea
purpurea
mRNA Code Dictionary
Second Base
First
Base
U
C
A
G
Third
Base
U
UUU
UUC
UUA
UUG
-
Phe
Phe
Leu
Leu
UCU
UCC
UCA
UCG
-
Ser
Ser
Ser
Ser
UAU
UAC
UAA
UAG
-
Tyr
Tyr
stop
stop
UGU
UGC
UGA
UGG
-
Cys
Cys
stop
Trp
U
C
A
G
C
CUU
CUC
CUA
CUG
-
Leu
Leu
Leu
Leu
CCU
CCC
CCA
CCG
-
Pro
Pro
Pro
Pro
CAU
CAC
CAA
CAG
-
His
His
Gln
Gln
CGU
CGC
CGA
CGG
-
Arg
Arg
Arg
Arg
U
C
A
G
A
AUU
AUC
AUA
AUG
-
Ile
Ile
Ile
Met/start
ACU
ACC
ACA
ACG
-
Thr
Thr
Thr
Thr
AAU
AAC
AAA
AAG
-
Asn
Asn
Lys
Lys
AGU
AGC
AGA
AGG
-
Ser
Ser
Arg
Arg
U
C
A
G
G
GUU
GUC
GUA
GUG
-
Val
Val
Val
Val
GCU
GCC
GCA
GCG
-
Ala
Ala
Ala
Ala
GAU
GAC
GAA
GAG
-
Asp
Asp
Glu
Glu
GGU
GGC
GGA
GGG
-
Gly
Gly
Gly
Gly
U
C
A
G
The School For Excellence 2015
The Essentials – Unit 4 Biology – Book 1
Page 74
3.
Deletions
Deletions occur when a nucleotide is deleted from the sequence. A frame shift is again
caused, as all bases move back one position. Most codons following the deletion will
be affected.
Complete the missing DNA, mRNA and amino acid molecules in following example:
Example: In the normal DNA sequence
A A A G G T A T A C C C
mRNA:
Amino acids:
If the 1st base G is deleted the new DNA sequence is:
mRNA:
Amino acids:
Deletions are just one cause of cancers
mRNA Code Dictionary
Second Base
First
Base
U
C
A
G
Third
Base
U
UUU
UUC
UUA
UUG
-
Phe
Phe
Leu
Leu
UCU
UCC
UCA
UCG
-
Ser
Ser
Ser
Ser
UAU
UAC
UAA
UAG
-
Tyr
Tyr
stop
stop
UGU
UGC
UGA
UGG
-
Cys
Cys
stop
Trp
U
C
A
G
C
CUU
CUC
CUA
CUG
-
Leu
Leu
Leu
Leu
CCU
CCC
CCA
CCG
-
Pro
Pro
Pro
Pro
CAU
CAC
CAA
CAG
-
His
His
Gln
Gln
CGU
CGC
CGA
CGG
-
Arg
Arg
Arg
Arg
U
C
A
G
A
AUU
AUC
AUA
AUG
-
Ile
Ile
Ile
Met/start
ACU
ACC
ACA
ACG
-
Thr
Thr
Thr
Thr
AAU
AAC
AAA
AAG
-
Asn
Asn
Lys
Lys
AGU
AGC
AGA
AGG
-
Ser
Ser
Arg
Arg
U
C
A
G
G
GUU
GUC
GUA
GUG
-
Val
Val
Val
Val
GCU
GCC
GCA
GCG
-
Ala
Ala
Ala
Ala
GAU
GAC
GAA
GAG
-
Asp
Asp
Glu
Glu
GGU
GGC
GGA
GGG
-
Gly
Gly
Gly
Gly
U
C
A
G
The School For Excellence 2015
The Essentials – Unit 4 Biology – Book 1
Page 75
CHROMOSOMAL MUTATIONS (MACROMUTATIONS)
Chromosome mutations can involve:
i.
Gross structural alterations of chromosomes (e.g. translocations, inversions) or
ii.
Changes in numbers of whole chromosomes within a nucleus (e.g. polyploidy in apple
varieties, trisomy-21 with Down’s Syndrome).
1.
Translocations
A section of one chromosome attaches to the end of another chromosome. A possible
consequence of translocation is that chromosomes may not perform disjunction
(separation) during meiosis.
Example:
In the following translocation gamete formation is disturbed. Monosomies or trisomies
can arise in the gametes formed, or normal or balanced gametes can be produced.
Normal
chromosome
pair
Fusion of two
nonhomologous
chromosomes
Possible gamete formations
The School For Excellence 2015
The Essentials – Unit 4 Biology – Book 1
Page 76
2.
Inversions
Inverting a section of bases in the DNA molecule changes the order of the bases, e.g.
Numerous codons can be affected, and significant changes in the protein’s amino acid
sequence can result. A dysfunctional protein is quite likely to be synthesised.
Example: In the sequence
A T C G T T A C C T C G:
a.
Determine the mRNA sequence:
b.
Translate the mRNA:
If the TGC is inverted to CGT:
3.
c.
Determine the mRNA sequence:
d.
Translate the mRNA:
Duplications
A section of chromosome is repeated.
Example: In the sequence A T G A A A, the ATG is duplicated.
Consequences:
Either duplications or deletions of bases in the dystrophin gene can lead to Duchenne
muscular dystrophy, an X-linked condition resulting in muscle wasting and eventual
death in young men.
The School For Excellence 2015
The Essentials – Unit 4 Biology – Book 1
Page 77
4.
Non-Disjunction
Aneuploidy occurs when an organism possesses an abnormal number of
chromosomes that is not a whole multiple of the haploid number.
During meiosis homologous chromosomes occasionally do not separate (nondisjunction) at anaphase I or II.
The gametes produced can have one extra or one missing chromosome.
For Example:
Patau syndrome (Trisomy 13)
Clinical Tools Inc.
In some cases whole sets of chromosomes do not separate during meiosis, resulting in
diploid gametes. When a diploid and haploid gamete fuse, a triploid cell forms. Triploid
individuals tend to be sterile. Triploidy is mainly observed in plants (e.g. triploid apple
varieties).
The situation where an organism has multiple copies of chromosomes is called
polyploidy. Animals rarely survive polyploidies, and triploidy in human embryos is the
most common cause of miscarriage.
The School For Excellence 2015
The Essentials – Unit 4 Biology – Book 1
Page 78
A SELECTION OF CHROMOSOME MUTATIONS
Try to complete the following table of chromosomal mutations:
Condition
Cause
Symptoms
Down syndrome
(Trisomy 21)
Flat facial appearance, slanting eyes,
broad hands with short fingers, single
crease across palm, malformed ears,
short stature, heart defects in ~40%,
typically reduced IQ etc.
Patau syndrome
(Trisomy 13)
Intellectual disability, defects in heart,
kidneys and scalp. Affected individuals
rarely survive.
XXY or XXXY
Male, tall and thin with small testes,
failure of sperm production,
enlargement of breasts, absence of
facial and body hair.
Female, infertile, possess female
external genitalia but no ovaries,
hence, no menstrual periods. Typically
short with various developmental
defects, e.g. webbing of the neck.
Turner’s syndrome
Second only to Down Syndrome as a
cause of intellectual disability. Males
Constriction near the
have high foreheads, unbalanced
end of the long arm of
faces, large jaws, large testicles, prone
an X chromosome.
to violent outbursts. 1/3 of females
intellectually disabled.
Chris Burke is an actor with Down syndrome. At birth, his parents were
told to institutionalise him. Instead they decided to raise him at home and
nurture his talents, with the help of his two older sisters and brother.
Burke got his first professional acting job in 1987 in the American
Broadcasting Corporations’ TV movie Desperate. Network executives at
ABC were impressed by his performance in Desperate and created Life
Goes On with Burke's character, Charles "Corky" Thacher, as the main
role. Corky was the first character in a network television series with
Down syndrome.
Burke's revolutionary role conveyed a realistic portrayal of people with Down syndrome and
changed the way audiences viewed people with disabilities. Life Goes On propelled Burke
into fame and widespread recognition. The series ran from 1989–1993. His most recent TV
appearance was on ER in 2002.
Burke was a Golden Globe Award Nominee, Best Actor in a Supporting Role in a Series,
Mini-Series or Motion Picture Made for TV, in 1990.
The School For Excellence 2015
The Essentials – Unit 4 Biology – Book 1
Page 79
QUESTION 21
The Zenkey is a hybrid animal produced from a cross between a species of zebra with a diploid
number of 44 and a donkey with a diploid number of 62.
(a)
What is the diploid number of the Zenkey?
_______________________________________________________________________
1 mark
(b)
By what process are gametes formed?
_______________________________________________________________________
1 mark
(c)
Starting with the cell shown below with a pair of homologous chromosomes, draw what
happens to the cell and its chromosomes during the process by which gametes are
produced.
(d)
Zenkeys are unable to produce offspring. Using your knowledge of gamete formation,
suggest why the Zenkey is sterile.
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
1 mark
Total 6 marks
The School For Excellence 2015
The Essentials – Unit 4 Biology – Book 1
Page 80
INHERITANCE TERMINOLOGY
An individual’s genetic instructions (genes) are inherited from the parents.
For any homologous pair of chromosomes, one chromosome is inherited from the mother
and the other from the father. For example, one chromosome no. 7 is inherited from the
mother (maternal chromosome of the pair) and the second no. 7 is inherited from the father
(paternal chromosome of the pair).
Define the following:
Genotype:
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
Homozygous (pure breeding) genotype:
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
Heterozygous (hybrid) genotype:
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
Hemizygous genotype:
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
Remember:
•
One gene can have several alleles (forms/variations). The alleles are normally
assigned letters. An allele that codes for a dominant trait is given the capital letter
(e.g. B, XH etc.) and the allele that codes for the recessive trait is given the lower case
letter (e.g. b, Xh etc.).
•
Traits are dominant or recessive. In the state of Victoria, genes or alleles cannot be
referred to as dominant or recessive. Instead, you can refer to these units of DNA as
‘alleles coding for the dominant or recessive traits’.
The School For Excellence 2015
The Essentials – Unit 4 Biology – Book 1
Page 81
EFFECT OF GENOTYPE AND
ENVIRONMENT ON PHENOTYPE
The allele combination of an organism is called its genotype. The interaction of an
organism’s genotype with the environment results in the organism’s phenotype. The
phenotype is the observable characteristics of an organism (e.g. attached ear lobes or
unattached ear lobes). It is an expression of an organism’s genotype in its structural,
biochemical, physiological and behavioural characteristics.
i.e. the genotype of an organism will not be expressed in an inappropriate environment.
Phenotype = Genotype x Environment
•
An example of the interaction of genotype and environment occurs with the fur colour of
the Himalayan rabbit. This predominantly white rabbit has black feet, ears, tail and
nose, and its colouration was originally thought to have been entirely under genetic
control, with no environmental influence.
However, if a cold pad is fixed to the rabbit’s back for a few weeks, black hair starts to
develop beneath the pad. In contrast, if the rabbit lives in warm tropical conditions, the
entire animal remains white.
Hence, a lack of heat is the stimulus required to turn on the allele coding for black
pigment. A temperature-sensitive allele produces black fur on the ears, nose, feet and
tail of the otherwise white rabbit under cold conditions. The environment obviously
affects the phenotype.
•
In plants, chlorophyll will only develop if light is available. The alleles responsible for
chlorophyll production will not be expressed in the absence of light. In addition, the
period of uninterrupted darkness on subsequent nights is the stimulus for flowering in
plants.
•
In identical twins (each twin has an identical genotype) the influence of the environment
on phenotype is easily recognised. In the USA, identical twins fostered to different
families at birth were reunited for the first time at an age of 17 years. The twin from a
sporting family (the boy regularly played grid iron) on a consistently high energy diet
was both taller and much more muscular than his brother, who was an adept classical
pianist, and did not involve himself in intense physical activity.
•
When traits are compared, we find that family members have more in common than
classmates or friends.
The School For Excellence 2015
The Essentials – Unit 4 Biology – Book 1
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Define the following:
Dominant phenotype:
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
Recessive phenotype:
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
Carrier:
An individual who is heterozygous at a given gene locus, possessing one allele coding for
the normal trait as well as another potentially harmful allele. The carrier is thus
phenotypically normal, but can pass on the faulty allele to offspring.
The School For Excellence 2015
The Essentials – Unit 4 Biology – Book 1
Page 83
MENDELIAN INHERITANCE
Our first understanding of the inheritance of genetic factors came from the work of the
Austrian monk Gregor Mendel (1822 – 1884). The assumption made is that the different
“factors”, i.e. genes, have their loci on different chromosomes, so that alleles of different
genes can be inherited independently.
Mendel completed extensive inheritance studies using garden peas. He observed the
following characteristics:
1.
Pea shape : Round or wrinkled.
2.
Pea colour : Yellow or green.
3.
Plant height : Tall or short.
•
Mendel determined that some traits are dominant. The allele coding for the dominant
trait is assigned a capital letter. The trait is dominant, not the allele.
•
Some traits are recessive. The allele coding for the recessive trait is assigned a lower
case letter. The trait is recessive, not the allele.
Mendel’s theory:
3.
1.
Characteristics are controlled by two inherited
factors (alleles). If the individual contains two
factors (alleles) coding for the dominant trait or
two factors (alleles) coding for the recessive
trait, then the individual is said to be pure
breeding (homozygous). Individuals with one
factor (allele) coding for the dominant trait and
one factor (allele) coding for the recessive trait
are called hybrids (heterozygous).
2.
During gamete formation, the two inherited
factors (genes) separate randomly. This is
called the principle of segregation (Mendel’s
First Law). It states that during gamete
formation (meiosis) each pair of
genes/chromosomes is separated into different
gametes.
The law of independent assortment (Mendel’s Second Law) states that the
behaviour of each pair of genes (chromosomes) is not influenced by the
behaviour of other pairs of genes (chromosomes).
Remember, Mendel’s Second Law only holds true if the genes’ loci are on different
chromosome pairs.
The School For Excellence 2015
The Essentials – Unit 4 Biology – Book 1
Page 84
DEGREES OF DOMINANCE
Complete dominance occurs because enough protein for a particular phenotype is
produced due to the action of either a single allele coding for the dominant trait (e.g. in the
genotype Tt) or two alleles coding for the dominant trait (e.g. with the genotype TT).
•
Mendel’s experiments show that traits are either dominant or recessive. In all of his
investigations, phenotypes never appeared which were intermediate between the
dominant and recessive characteristics.
Today, however, numerous instances are recognised that are not examples of
complete dominance, where intermediate phenotypes are recognised in many cases.
COMPLETE DOMINANCE
Let’s looks at Mendel’s pea pod colour as an example of Complete Dominance.
•
Dominant trait: A trait that is expressed with a heterozygous genotype. Only a single
copy of the allele is required for the trait’s complete expression.
•
Recessive trait: Refers to a trait that is only expressed in a homozygous genotype.
•
Pure breeding: An organism which, when crossed with itself or others like it, always
produces offspring like itself, i.e. it is homozygous.
•
F1: First filial generation, the offspring of two pure breeding parents with contrasting
characters.
•
F2: Second filial generation, the offspring resulting from the mating of two F1
individuals.
Example:
Pea plants can have one of two traits for height; tall or dwarf.
The tall plant trait is dominant:
T = tall
t = dwarf
Note:
1.
Try and avoid using genotypes such as Ss, Cc, Pp and Ff, as in these
instances the upper and lower case forms of the letters are very similar and
are often confused. Obviously, if you are instructed in the exam to use such
an example (e.g. S and s), you should use those letters.
2.
Whenever you perform a monogenic cross, take it for granted that you
must write out the genotypes, phenotypes and the expected ratios, of all
offspring after completing the cross. If a question asks for the chance of
any particular phenotype appearing in the offspring, you must state the
chance in a sentence. Don’t just simply complete the cross!
In the crosses made on the following pages, these abbreviations may be employed:
P = parent
F1 = 1st filial generation
F2 = 2nd filial generation
The School For Excellence 2015
The Essentials – Unit 4 Biology – Book 1
Page 85
EXAMPLE 1
A cross between two different pure breeding pea plants is made using a punnet square:
Parents:
TT
Gametes:
all T
x
tt
all t
T
T
t
Tt
Tt
t
Tt
Tt
F1 Genotypes:
F1 Phenotypes:
The School For Excellence 2015
The Essentials – Unit 4 Biology – Book 1
Page 86
EXAMPLE 2
A cross between two individuals from the F1 generation is made. This is known as a
monohybrid cross.
Parents:
Gametes:
Tt
T or t
x
Tt
T or t
F2 Genotypes:
F2 Phenotypes:
monohybrid ratio
Note: The expected phenotypic ratio from a cross between two heterozygous organisms (for
one gene only) is known as the monohybrid ratio.
The School For Excellence 2015
The Essentials – Unit 4 Biology – Book 1
Page 87
TEST CROSS
Both a homozygous dominant and a heterozygous dominant individual will exhibit the same
phenotype, despite the obvious differences in genotype.
A test cross can be used to determine the genotype of an individual who exhibits a
dominant trait. In this procedure the individual exhibiting the dominant trait is mated with a
homozygous recessive individual.
Offspring ratios are then used to deduce the unknown genotype of the individual.
EXAMPLE
In mice, B = black fur and b = white fur. A black mouse was found by a student who wished
to determine if the mouse was homozygous or heterozygous. Show how a test cross could
be used to determine the genotype of the mouse.
Solution
Offspring Genotypes:
Offspring Phenotypes:
Offspring Genotypes:
Offspring Phenotypes:
Explanation:
stockphotos.it
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
The School For Excellence 2015
The Essentials – Unit 4 Biology – Book 1
Page 88
CODOMINANCE
Codominance occurs in situations where both alleles in the genotype are expressed equally
in the phenotype.
•
Heterozygous offspring have characteristics of both parents, i.e. both alleles are fully
expressed resulting in a phenotype in which both forms of the trait are exhibited. This
differs from incomplete dominance where there is an intermediate characteristic.
•
A good example is the ABO blood group gene.
Two alleles of this gene code for Type A or
Type B antigens, each of which may be present
on the surface of red blood cells in humans.
The alleles for Type A antigens and Type B
antigens are codominant. A Type O individual
only possesses Type O antigens, and the Type
O blood type is the recessive condition.
MULTIPLE ALLELES
Most genes have two versions or alleles, e.g. two alleles in pea plants code for height,
(T for tall and t for dwarf).
Some genes have more the two alleles, e.g. The ABO blood group gene has three alleles. In
such cases multiple allelism exists.
•
Every person has two copies of the ABO blood group gene. The gene is found on
chromosome 9.
•
There are three alleles (versions) of the gene. However, because we only have two
copies of chromosome 9 we can only have a maximum of two alleles of the gene.
IA - codes for Type A blood (A antigens on the red blood cell).
IB - codes for Type B blood (B antigens on the red blood cell).
i - codes for Type O blood (neither A nor B antigens on the red blood cell).
•
Individuals of Type A blood can therefore have the genotype IA IA or IA i.
•
Individuals of Type B blood can have the genotype IB IB or IB i.
•
Individuals of Type O blood can only have the genotype ii.
•
If the two alleles that code for dominant traits are both present in the cell (i.e. IA IB
genotype), the individual shows the phenotype
Type AB blood. As both alleles are completely
expressed, this is an example of codominance.
•
Normally a gene that has many alleles can have
more than two phenotypes. However, it does NOT
result in a continuous range of phenotypes.
The School For Excellence 2015
The Essentials – Unit 4 Biology – Book 1
Page 89
EXAMPLE 1
Fred (blood Type A) and Francene (blood Type B) have just had their first child, Frank
(blood Type O). Upon hearing of Frank’s blood type, Fred concluded that Francene had
been less than faithful, and was filing for a divorce.
Showing all working, explain whether or not Fred was justified in
his decision to divorce Francene.
P. Phenotypes:
Type A
x
Type B
P. Genotypes:
Gamete genotypes:
Offspring genotypes:
Offspring Phenotypes:
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
EXAMPLE 2
Determine the expected offspring from a Type AB and
heterozygous Type A individual.
P. Phenotypes:
Type AB
x
Type A
P. Genotypes:
Gamete genotypes:
Offspring genotypes:
Offspring Phenotypes:
The School For Excellence 2015
The Essentials – Unit 4 Biology – Book 1
Page 90
*INCOMPLETE
DOMINANCE
With incomplete dominance the offspring exhibit an intermediate characteristic between
the two pure breeding parental phenotypes. (In some texts it is stated that the dominant
phenotype is not fully expressed). The heterozygote phenotype is a blend of the two
respective pure breeding phenotypes.
EXAMPLE 1
A red primrose plant is crossed with a white primrose plant.
R1 = red allele
R2 = white allele
P. Genotypes:
P. Gametes:
R1R1
x
R2 R2
R1
R2
F1 Genotypes:
R1R2
F1 Phenotype:
All pink
F1 Genotypes:
F1 Gametes:
R1R2
x
R1R2
R1 or R2
R1 or R2
R1
R1
R2
R1R2
R1R2
R2
R1R2
R1R2
Genotypes:
RrRr
1
:
:
RrRw
2
:
:
RwRw
1
Phenotypes:
Red
1
:
:
Pink
2
:
:
White
1
telefora.com
An intermediate trait now exists; in this case it is pink. If the plant was showing
codominance, red and white sections could be expected on each flower.
BIOCHEMICAL BASIS OF DOMINANCE
A heterozygous person will produce enough functional protein from the one allele coding for
the dominant trait that the person shows the dominant phenotype.
If both alleles coding for different variations produce functional proteins, then the traits are
codominant.
If the allele coding for the dominant trait produces insufficient amounts of protein to show the
full dominant phenotype, incomplete dominance results.
Note: Many texts simply lump codominance and incomplete dominance under the one
banner, and notation may involve superscripts, subscripts or neither of these.
Unfortunately, there are no strict universal rules of notation.
The School For Excellence 2015
The Essentials – Unit 4 Biology – Book 1
Page 91
SEX LINKED INHERITANCE
Sex linkage occurs when the locus of the gene in question is on a sex chromosome.
The sex chromosomes are considered to be the 23rd pair in humans. They determine the sex
of the individual. In humans, females possess two X chromosomes/somatic cell and males
possess an X and a Y chromosome/somatic cell.
•
The X chromosome is much larger than the Y chromosome, hence, it contains more
DNA and more genes. The characteristics coded for on the X chromosome, which do
not necessarily have anything to do with the sex of the individual, are said to be
X linked, because the gene loci are on the X chromosome.
•
As human females carry two homologous X chromosomes their genotypes, they can be
either homozygous or heterozygous for any X-linked condition. As males carry an X
and Y chromosome, only hemizygous genotypes are possible for sex linked
conditions. The Y chromosome is not only shorter than the X chromosome, it also
doesn’t carry any of the genes found on the X chromosome.
•
Daughters receive an X chromosome from each parent. Sons receive an X
chromosome from their mother and a Y chromosome from their father. Hence, boys
inherit their X-linked traits from their mothers.
Example: Haemophilia – a blood disorder caused by a faulty Factor 8 protein:
•
The F8C (Factor 8 protein) gene occurs on the X chromosome, but not on the Y
chromosome.
The gene has one allele coding for factor 8 protein (XH) and one coding for an
abnormal protein (Xh) which causes haemophilia. The production of factor 8 protein is
dominant over the production of abnormal protein.
The gene is X linked, hence, the recessive disease appears more often in males. As a
consequence, there is no superscript letter denoting the presence of an allele on the Y
chromosome in a male genotype, eg. XHY.
Possible male genotypes and phenotypes:
_____________________________________________________________________
_____________________________________________________________________
Possible female genotypes and phenotypes:
_____________________________________________________________________
_____________________________________________________________________
•
Genes located on the Y chromosome are said to be Y-linked, and they
obviously only appear in males. Y-linked traits are only passed from
father to son.
The Y chromosome is small and does not contain many genes,
hence, few traits are Y-linked. One example is the hairy ear gene.
Y+ = normal allele
YH = hairy ear allele
The School For Excellence 2015
X = no allele
The Essentials – Unit 4 Biology – Book 1
Page 92
Note: The sex of the individual is part of the phenotype in a sex-linked cross.
e.g. a normal male is a different phenotype to a normal female. The sex of the
individual must be stated when phenotypes are given.
EXAMPLE 1
Illustrate how only male children from normal parents can inherit haemophilia:
P phenotypes:
carrier female
x
normal male
P genotypes:
Gamete genotypes:
Offspring Genotypes:
Offspring Phenotypes:
EXAMPLE 2
Now predict the offspring of a haemophiliac male and a female carrier:
P Genotypes:
Gamete Genotypes:
Offspring Genotypes:
Offspring Phenotypes:
The School For Excellence 2015
The Essentials – Unit 4 Biology – Book 1
Page 93
LETHAL GENOTYPES
In some cases a particular genotype can be lethal (e.g. MM in Manx cats). Such genetic
combinations are said to result from lethal genotypes. All living Manx (tailless) cats are
heterozygous.
EXAMPLE
Determine the chances of producing a tailed kitten from the living offspring of a cross
between two Manx cats (M – Manx; m – tail-less).
P. Phenotypes:
Manx cat
x
Manx cat
P. Genotypes:
Mm
x
Mm
P. Gametes:
M;m
M;m
M
m
M
MM
Mm
m
Mm
mm
Offspring Genotypic Ratio:
Expected Offspring Phenotypic Ratio:
Actual Offspring Phenotypic Ratio:
MM
:
Mm
:
mm
1
:
2
:
1
Manx
:
3
:
Tailed
1
__________________________
Hence, the chance of a tailed kitten arising in the living offspring is ____________.
If you ever encounter an offspring phenotypic ratio of 2:1, instead of the expected 3:1, after
numerous offspring have been produced from a particular cross between two heterozygous
individuals, it is most probable that the homozygous dominant genotype is lethal.
The School For Excellence 2015
The Essentials – Unit 4 Biology – Book 1
Page 94