MUTATIONS - TSFX€¦ · MUTATIONS A mutation is a permanent change in the genetic material (or DNA sequence). Mutations ... Let’s determine the effects of three types of point

� The School For Excellence 2015 The Essentials – Unit 4 Biology – Book 1 Page 70

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.


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 � CCA � 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 CGA GCG CGA AGC TGA - 3' DNA: 3' - TAC TGA GTG GCT CGC GCT TCG ACT - 5' mRNA: 5' - AUG ACU CAC CGA GCG CGA AGC UGA - 3' Protein: Met Thr His Arg Ala Arg Ser Stop 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


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)


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 A A G C A T G G T A G G

mRNA: 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.

UU

CC

AA

GG

GGU GGU -- GlyGly

GGC GGC -- GlyGly

GGA GGA -- GlyGly

GGG GGG -- GlyGly

GAU GAU -- AspAsp

GAC GAC -- AspAsp

GAA GAA -- GluGlu

GAG GAG -- GluGlu

GCU GCU -- AlaAla

GCC GCC -- AlaAla

GCA GCA -- AlaAla

GCG GCG -- AlaAla

GUU GUU -- ValVal

GUC GUC -- ValVal

GUA GUA -- ValVal

GUG GUG -- ValVal

GG

UU

CC

AA

GG

AGU AGU -- SerSer

AGC AGC -- SerSer

AGA AGA -- ArgArg

AGG AGG -- ArgArg

AAU AAU -- AsnAsn

AAC AAC -- AsnAsn

AAA AAA -- LysLys

AAG AAG -- LysLys

ACU ACU -- ThrThr

ACC ACC -- ThrThr

ACA ACA -- ThrThr

ACG ACG -- ThrThr

AUU AUU -- IleIle

AUC AUC -- IleIle

AUA AUA -- IleIle

AUG AUG -- Met/startMet/start

AA

UU

CC

AA

GG

CGU CGU -- ArgArg

CGC CGC -- ArgArg

CGA CGA -- ArgArg

CGG CGG -- ArgArg

CAU CAU -- HisHis

CAC CAC -- HisHis

CAA CAA -- GlnGln

CAG CAG -- GlnGln

CCU CCU -- ProPro

CCC CCC -- ProPro

CCA CCA -- ProPro

CCG CCG -- ProPro

CUU CUU -- LeuLeu

CUC CUC -- LeuLeu

CUA CUA -- LeuLeu

CUG CUG -- LeuLeu

CC

UU

CC

AA

GG

UGU UGU -- CysCys

UGC UGC -- CysCys

UGA UGA -- stopstop

UGG UGG -- TrpTrp

UAU UAU -- TyrTyr

UAC UAC -- TyrTyr

UAA UAA -- stopstop

UAG UAG -- stopstop

UCU UCU -- SerSer

UCC UCC -- SerSer

UCA UCA -- SerSer

UCG UCG -- SerSer

UUU UUU -- PhePhe

UUC UUC -- PhePhe

UUA UUA -- LeuLeu

UUG UUG -- LeuLeu

UU

GGAACCUUThird Third

BaseBase

Second BaseSecond BaseFirstFirst

BaseBase

mRNAmRNA Code DictionaryCode Dictionary


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:

UU

CC

AA

GG

GGU GGU -- GlyGly

GGC GGC -- GlyGly

GGA GGA -- GlyGly

GGG GGG -- GlyGly

GAU GAU -- AspAsp

GAC GAC -- AspAsp

GAA GAA -- GluGlu

GAG GAG -- GluGlu

GCU GCU -- AlaAla

GCC GCC -- AlaAla

GCA GCA -- AlaAla

GCG GCG -- AlaAla

GUU GUU -- ValVal

GUC GUC -- ValVal

GUA GUA -- ValVal

GUG GUG -- ValVal

GG

UU

CC

AA

GG

AGU AGU -- SerSer

AGC AGC -- SerSer

AGA AGA -- ArgArg

AGG AGG -- ArgArg

AAU AAU -- AsnAsn

AAC AAC -- AsnAsn

AAA AAA -- LysLys

AAG AAG -- LysLys

ACU ACU -- ThrThr

ACC ACC -- ThrThr

ACA ACA -- ThrThr

ACG ACG -- ThrThr

AUU AUU -- IleIle

AUC AUC -- IleIle

AUA AUA -- IleIle


AA

UU

CC

AA

GG

CGU CGU -- ArgArg

CGC CGC -- ArgArg

CGA CGA -- ArgArg

CGG CGG -- ArgArg

CAU CAU -- HisHis

CAC CAC -- HisHis

CAA CAA -- GlnGln

CAG CAG -- GlnGln

CCU CCU -- ProPro

CCC CCC -- ProPro

CCA CCA -- ProPro

CCG CCG -- ProPro

CUU CUU -- LeuLeu

CUC CUC -- LeuLeu

CUA CUA -- LeuLeu

CUG CUG -- LeuLeu

CC

UU

CC

AA

GG

UGU UGU -- CysCys

UGC UGC -- CysCys

UGA UGA -- stopstop

UGG UGG -- TrpTrp

UAU UAU -- TyrTyr

UAC UAC -- TyrTyr

UAA UAA -- stopstop

UAG UAG -- stopstop

UCU UCU -- SerSer

UCC UCC -- SerSer

UCA UCA -- SerSer

UCG UCG -- SerSer

UUU UUU -- PhePhe

UUC UUC -- PhePhe

UUA UUA -- LeuLeu

UUG UUG -- LeuLeu

UU

GGAACCUUThird Third

BaseBase


BaseBase


Insertions lead to new colour and petal forms of Ipomoea purpurea


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:

UU

CC

AA

GG

GGU GGU -- GlyGly

GGC GGC -- GlyGly

GGA GGA -- GlyGly

GGG GGG -- GlyGly

GAU GAU -- AspAsp

GAC GAC -- AspAsp

GAA GAA -- GluGlu

GAG GAG -- GluGlu

GCU GCU -- AlaAla

GCC GCC -- AlaAla

GCA GCA -- AlaAla

GCG GCG -- AlaAla

GUU GUU -- ValVal

GUC GUC -- ValVal

GUA GUA -- ValVal

GUG GUG -- ValVal

GG

UU

CC

AA

GG

AGU AGU -- SerSer

AGC AGC -- SerSer

AGA AGA -- ArgArg

AGG AGG -- ArgArg

AAU AAU -- AsnAsn

AAC AAC -- AsnAsn

AAA AAA -- LysLys

AAG AAG -- LysLys

ACU ACU -- ThrThr

ACC ACC -- ThrThr

ACA ACA -- ThrThr

ACG ACG -- ThrThr

AUU AUU -- IleIle

AUC AUC -- IleIle

AUA AUA -- IleIle


AA

UU

CC

AA

GG

CGU CGU -- ArgArg

CGC CGC -- ArgArg

CGA CGA -- ArgArg

CGG CGG -- ArgArg

CAU CAU -- HisHis

CAC CAC -- HisHis

CAA CAA -- GlnGln

CAG CAG -- GlnGln

CCU CCU -- ProPro

CCC CCC -- ProPro

CCA CCA -- ProPro

CCG CCG -- ProPro

CUU CUU -- LeuLeu

CUC CUC -- LeuLeu

CUA CUA -- LeuLeu

CUG CUG -- LeuLeu

CC

UU

CC

AA

GG

UGU UGU -- CysCys

UGC UGC -- CysCys

UGA UGA -- stopstop

UGG UGG -- TrpTrp

UAU UAU -- TyrTyr

UAC UAC -- TyrTyr

UAA UAA -- stopstop

UAG UAG -- stopstop

UCU UCU -- SerSer

UCC UCC -- SerSer

UCA UCA -- SerSer

UCG UCG -- SerSer

UUU UUU -- PhePhe

UUC UUC -- PhePhe

UUA UUA -- LeuLeu

UUG UUG -- LeuLeu

UU

GGAACCUUThird Third

BaseBase


BaseBase


Deletions are just one cause of cancers


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 trisomiescan arise in the gametes formed, or normal or balanced gametes can be produced.

Normalchromosome

pair

Fusion of two non-

homologous chromosomes

Possible gamete formations


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: c. Determine the mRNA sequence: d. Translate the mRNA:

3. 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.


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 (non-disjunction) at anaphase I or II. The gametes produced can have one extra or one missing chromosome.

For Example: Patau syndrome (Trisomy 13)

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.

Clinical Tools Inc.


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.

Turner’s syndrome

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.

Constriction near the end of the long arm of an X chromosome.

Second only to Down Syndrome as a cause of intellectual disability. Males have high foreheads, unbalanced faces, large jaws, large testicles, prone 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.


(d) Zenkeys are unable to produce offspring. Using your knowledge of gamete formation,

suggest why the Zenkey is sterile.

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1 mark

Total 6 marks

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?

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1 mark (b) By what process are gametes formed?

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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.


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:

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_________________________________________________________________________ Homozygous (pure breeding) genotype:

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_________________________________________________________________________ Heterozygous (hybrid) genotype:

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_________________________________________________________________________ Hemizygous genotype:

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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’.


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.


Define the following: Dominant phenotype:

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Recessive phenotype:

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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.


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: 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.

3. 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.


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


EXAMPLE 1 A cross between two different pure breeding pea plants is made using a punnet square:

Parents: TT x tt Gametes: all T all t

T T

t Tt Tt

t Tt Tt F1 Genotypes: F1 Phenotypes:


EXAMPLE 2 A cross between two individuals from the F1 generation is made. This is known as a monohybrid cross.

Parents: Tt x Tt Gametes: T or t T or t

F2 Genotypes: F2 Phenotypes:

Note: The expected phenotypic ratio from a cross between two heterozygous organisms (for one gene only) is known as the monohybrid ratio.

monohybrid ratio


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.

EXAMPLEIn 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:


Explanation:

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stockphotos.it


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 NOTresult in a continuous range of phenotypes.


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:

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_________________________________________________________________________ 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:


*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: R1R1 x R2 R2 P. Gametes: R1 R2 F1 Genotypes: R1R2 F1 Phenotype: All pink F1 Genotypes: R1R2 x R1R2 F1 Gametes: R1 or R2 R1 or R2

R1 R1

R2 R1R2 R1R2

R2 R1R2 R1R2

Genotypes: RrRr : RrRw : RwRw

1 : 2 : 1 Phenotypes: Red : Pink : White 1 : 2 : 1 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.

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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:

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Possible female genotypes and phenotypes:

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• 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 X = no allele YH = hairy ear allele


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:


EXAMPLE 2 Now predict the offspring of a haemophiliac male and a female carrier: P Genotypes: Gamete Genotypes:



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.

EXAMPLEDetermine 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: MM : Mm : mm 1 : 2 : 1 Expected Offspring Phenotypic Ratio: Manx : Tailed 3 : 1 Actual Offspring Phenotypic Ratio: __________________________ 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.

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MUTATIONS - TSFX€¦ · MUTATIONS A mutation is a permanent change in the genetic material (or DNA sequence). Mutations ... Let’s determine the effects of three types of point