Transcript Lac A

GENETIC CODE
FILE 8:
POINT MUTATIONS
1.
In a mRNA sequence (wt) there is a triplet UUU. After a mutation, the triplet changes
in UUA. What kind of mutation happened and which effects have the mutation on
the protein encoded by the gene?
To switch from UUU to UUA is necessary a mutation in the DNA
From
TTT
to
TTA
AAA
AAT
This is a Transversion: it refers to the substitution of a purine for a pyrimidine or
vice versa, in deoxyribonucleic acid (DNA).
The aminoacid Phenylalanine encodes for UUU will be
substitute with Leucine (UUA).
This is a Missense mutation. The effect on the protein
function is not predictable.
2.
This is a sequence of wt mRNA
5’- AUG AGA CCC ACC….
What kind of effects has a mutation the substitute the fifth base from G to U?
5’– AUG AUA CCC ACC…
In the second triplet the mutation changes the
codon and the aminoacid encoded: Arg
(Encoded by AGA) to Ile (encoded by AUA).
What kind of effects has a deletion of the sixth base?
5’– AUG AUA CCC ACC…
The deletion causes a frameshift of the codon code. All the aminoacids after will
be different.
Compare the two previous mutations:
First case: Probably the protein will be active (Missense mutation)
Second case: the protein is totally different from the original protein and probably
will not be active. (FrameShift Mutation).
3.
A nucleotide sequence below
5
5'
3'
10
15
20
25
30
35
ATTCGATGGGATGGCAGTGCCAAAGTGGTGATGGC
TAAGCTACCCTACCGTCACGGTTTCACCACTACCG
3'
5'
a) Knowing that the transcription of this sequence is from left to right (5’->3’),
write the resulting mRNA sequence:
5’ AUUCGAUGGGAUGGCAGUGCCAAAGUGGUGAUGGC
b) Knowing that this mRNA sequence contains the translation start codon, identify
the starting codon and indicate the amino acid sequence of the resulting peptide.
Met- Gly- Trp-Gln-Cys- Gln- Ser-Gly- Asp- Gly
5'
3'
ATTCGATGGGATGGCAGTGCCAAAGTGGTGATGGC
TAAGCTACCCTACCGTCACG GTTTCACCACTACCG
3'
5'
C) Find which consequences will have on the amino acid sequence:
- a transition of the T base pair in position 18;
In the DNA sequence: switch from TA to CG;
In the mRNA sequence there is a switch between U and
C thus between the triplet UGC (Cys) to CGC (Arg) that
causes a MIS-SENSE mutation.
5'
3'
ATTCGATGGGATGGCAGTGCCAAAGTGGTGATGGC
TAAGCTACCCTACCGTCACG GTTTCACCACTACCG
3'
5'
- a transversion of the CG base pair in position 20;
Case 1: from CG to GC
mRNA: switch between C to G
From UGC (Cys) to UGG (Trp)
MIS-SENSE MUTATION: protein with one
different aminoacid
Case 2: from CG to AT
mRNA: switch between C to A
From UGC (Cys) to UGA (STOP)
NON SENSE MUTATION thus a truncated
protein
5'
3'
ATTCGATGGGATGGCAGTGCCAAAGTGGTGATGGC
TAAGCTACCCTACCGTCACGGTTTCACCACTACCG
3'
5'
- an insertion of a base pair after pair 11 (AT)
+1
AUG GGA NUG GCA GUG CCA AAG UGG UGA UGG C
Met-Gly-aaX-Ala-Val-Pro-Lys-Trp-Stop
After the insertion all the aminoacids will be different: frame-shift.
The frame-shift creates a stop codon.
d) How can we abolish the insertion mutation in position 11?
If in a position near the first insertion a deletion happens, we can restore the
correct frame of aminoacids. For example in position 15 (intragenic suppressionsuppressor mutation)
+1
AUG-GGA-NUG-GAG-UGC-CAA-AGU-GGU-GAU-GGC
Met-Gly-aaX-Glu-Cys-Gln-Ser-Gly-Asp-Gly
The second mutation restore the correct reading frame.
Only the aminoacids located between the two mutations will be different.
DNA with insertion and subsequent suppressor mutation:
5’ ATTCGATGGGANTGGAGTGCCAAAGTGGTGATGGC 3’
3’ TAAGCTACCCTNACCTCACGGTTTCACCACTACCG 5’
This is a polypeptidic sequence of a protein. Wild type and mutant sequences are
compared. What type of mutations did happen?
WT
: Met-Arg-Phe-Thr…
Mutant 1: Met-Ile-Phe-Thr…
Mutant 2: Met-Ser-Ile-Tyr
We compare mutant 1 with the wt: the second amino acid is different
Arg could be encoded by CGU, CGC, CGA, CGG, AGA, AGG
Ile could be encoded by AUU, AUC, AUA
It is probable that the codon encoding Arg could be AGA, and a mutation
occoured orginating AUA
Phe could be encoded by UUU/UUC and Thr by ACU/ACC/ACA/ACG,
The possible DNA sequence will be:
Sequence of wt: AUG – AGA – UUPy – ACN
Py=C o T
Sequence of mutant 1: AUG – AUA – UUPy – ACN -
WT
: Met-Arg-Phe-Thr…
Mutant 1: Met-Ile-Phe-Thr…
Mutant 2: Met-Ser-Ile-Tyr
We compare mutant 2 with wt: all the amicoacids after the Methionine are
different.
A frameshift mutation caused by an insertion.
AUG – AGA – UUPy – ACN –
Ser is encoded by UCN or AGU-AGC ,
We can hypotesize the sequence:
AUG –AGN–AUU-PyACWith N=U/C
The third codon AUU = Ile
The fourth codon will be UAC = Tyr
5An Escherichia coli mutant auxotroph for tryptophan (Trp-) has an amino acid
substitution in tryptophan-synthetase: Glycine at position 210 is replaced by an
Arginine.
On the basis of the genetic code, find which kind of mutation (on the DNA) you think
has caused the amino acid replacement
Gly Codons
:
GGU
GGC
GGA
GGG
Arg Codons
We can hypothesize a single base substitution
First base G could switch to C or A
GGU->CGU; GGC->CGC; GGA->CGA; GGG->CGG;
GGA->AGA; GGG->AGG
:
CGU
CGC
CGA
CGG
AGA
AGG
In the Escherichia coli metA gene a base substitution occurred. Because of this
mutation, in the mRNA a UAA codon is present inside the gene.
- Which consequence will this mutation have on protein synthesis?
The triplet UAA is a stop codon,
Thus the protein synthesis will be
interrupted.
The primary transcript of chicken ovalbumin RNA is composed by 7 introns (white)
and 8 exons (black):
If the Ovoalbumin DNA is isolated, denaturated and hybridized to its cytoplasmic
mRNA, which kind of structure do we expect?
Exon
INTRON
mRNA
Structure with loops corresponding to introns.
if a deletion of a base pair occurs in the middle of the second intron, which will be
the likely effects on the resulting polypeptide?
We don’t have any effect if the mutation is not in the splicing site.
if a deletion of a base pair occurs in the middle of the first exon, which will be the
likely effects on the resulting polypeptide?
We have a frame-shift effect or a truncated protein.
FILE 9
A man has the chromosome 21 translocated on the 14. Draw the karyotype (only
the chromosomes involved in the mutation).
What kind of gametes will be produced by this person?
Which will be the consequences on the progeny, if this man has a child together
with a normal woman?
14 21
14 21
14-21
Karyotype
21
Pairing of
homologous
chromosomes
during meiosis
14-21 21
14
GAMETES
14 14-21
14-21
Parent with translocation
Normal Parent
Normal Gamete
Gametes (First parent)
Gametes (First parent)
14-21 21
14 14-21
14
21 Trisomy : Down’s Syndrome
2 chromosomes 14, DOWN
3 chromosomes 21 SYNDROME
21 Monosomy : not compatible with life
2 chromosomes 14,
1 chromosomes 21
NOT VITAL
14 Trisomy : not compatible with life
3 chromosomes 14,
2 chromosomes 21
NOT VITAL
Monosomy 14 n:ot compatible with life
1 chromosome 14,
2 chromosomes 21
21
Zigote healthy carrier: normal phenotype
14 21
14-21
Normal Zigote: normal phenotype
2 chromosomes 14,
2 chromosomes 21
2 chromosomes 14,
2 chromosomes 21
NOT VITAL
CARRIER
TRASLOCATION
NORMAL
Which will be the consequences on the progeny, if this man has a child together
with a normal woman?
1/6+1/6+1/6: ZIGOTEs are not
compatible with life
1/6
1/6
1/6
2In humans trisomy of chromosome 21 is responsible of the Down syndrome.
- which gametes originated an affected person?
- draw a scheme of the meiotic stages that can give rise to the mutated gamete
and indicate the name of this process.
- Chromosomes are distributed to gametes incorrectly
- The gametes either are missing or have an extra chromosome 21
- It is caused by the NONDISJUNCTION of chromosome 21 during meiosis
(homologous chromosomes or sister chromatids).
3.
A man carries a heterozygous paracentric inversion.
A B C D
E
F
G
H
a b c d
g
f
e
h
Draw a scheme of homologous chromosomal pairing during meiosis
F
E
ABCD
Draw only one cromatide for each
chromosome.
f
e
g
G
H
h
abcd
Which gametes are produced (in particular which gametes are missing)? Explain
why.
Parental gametes
A B C D
E
F
G
H
a b c d
g
f
e
h
F
RECOMBINATION
(CROSSING-OVER)
E
ABCD
f
e
g
G
H
h
abcd
A B C D
RECOMBINANT GAMETES
h
E
f
g
e
F
G
d c b a
H
We will have PARENTAL GAMETES but we don’t have vital recombinant gametes
produced by a crossing over happens in the inverted region
Effect: a DICENTRIC CHROMOSOME (TWO CENTROMERES) and a ACENTRIC
FRAGMENT CHROMOSOME (lost).
Does the presence of the mutation change the fertility of this man?
No, if inversion is not extended and is limited to a small part of chromosome.
4.
Deletion of a small region on Y chromosome in humans can prevent the individual
development as a male. How can you explain this result?
The deletion is on a locus of Y
chromosome where the SRY gene (Sex
determining Region Y) is located. It
encodes for the TDF, Testis Determining
Factor.
Missing of this gene prevent the
development as a male, thus the
individual develops as a FEMALE
6 - how can a triploid organism originate?
Draw a scheme of meiosis process in a triploid cell with n = 3.
From the cross between gamete n + gamete 2n.
Chromosomes A, B and C
gamete 2n= AABBCC
gamete n = ABC
individual 3n=AAABBBCCC
In a triploid cell which gametes are produced?
BB (1/2)
CC (1/2)
C (1/2)
AABBCC (1/8)
AABBC (1/8)
B (1/2)
CC (1/2)
C (1/2)
AABCC (1/8)
AABC (1/8)
BB (1/2)
CC (1/2)
C (1/2)
ABBCC (1/8)
ABBC (1/8)
B (1/2)
CC(1/2)
C (1/2)
ABCC (1/8)
ABC (1/8)
AA (1/2)
A (1/2)
2/8 are gametes that could generate an individual 6/8 are not compatible with life
7Asiatic cotton and American wild cotton have both 26 chromosomes. The
American cultivated cotton, that is derived from the previous species by
alloploidy, has 52 chromosomes. Explain, with a scheme, how it originates.
Alloploidy=A hybrid individual having two or more sets of chromosomes derived from two different species.
We hypotize that both species 2n = 26
ASIATIC COTTON (A) = 13 chromosomes
AMERICAN COTTON (B) = 13 chromosomes
If in the hybrid we have 26 chromosomes from two different species the plant is
vital but it is sterile.
If in the hybrid a doubling of chromosomes occurs, we have an allopolyploid that is
fertile because each chromosome has its homologous.
2 A + 2 B = 26 + 26 = 52 Chromosomes
FILE 10
1.
In Escherichia coli, the lac (lactose) operon, is made of the following genes and
sites. Specify what is the function of the ones indicated below:
- promoting site
- operator site
- repressor gene
- structural genes.
PROMOTER
DNA region where RNA
polymerase sits to start
transcription.
REPRESSOR
gene that encodes for a protein that
negatively regulates transcription.
OPERATOR
DNA locus where the repressor could bind
to stop transcription.
STRUCTURAL GENES
Genes that are usefull for a cellular
function; for example metabolism of
lactose.
2. What would be the result of a base substitution that inactivates the following genes:
LacZ-
Since the gene encodes for b-galattosidase enzyme, a mutation in this gene
probably inactivates the function of the gene.
LacIThis gene encodes for the repressor of lactose operon. The mutation will have
different effects depending on the protein domain where it occours:
a) If the mutation inactivates the protein (frame-shift, stop codon, missense), we have the absence of the repressor and constitutive transcription
of the structural genes (recessive mutation LacI-)
b) If the mutation alters the allosteric domain of the protein where the
inducer binds, we have constitutive repression of the structural genes
because the repressor is bound to its site and is not influenced by the
presence of lactose. (dominant mutation, LacIs)
3.
What would happen if a base deletion occurs in the operator region?
OPERATOR is the DNA locus bound by the repressor to
stop transcription.
After the mutation in the operator, the repressor could be unable to
recognize the locus. In fact the repressor (protein) is able to recognize the
Operator (sequence).
Thus, we have the constitutive expression of the genes. The mutant in
operator constitutive lacOc (cis DOMINANT)
Cis indicates that the mutation acts on the nearest genes.
4.
Which of the following genotypes will be able to produce β-galactosidase and/or
permease in the presence of lactose?
bgal perm.
Genotype 1 Lac I+
Lac P+
Lac O+
Lac Z+
Lac Y+
Lac A+
+ …..
+
……
Genotype 2 Lac I+
Lac P+
Lac O+
Lac Z+
Lac Y-
Lac A+
+ …..
……
Genotype 3 Lac I-
Lac P+
Lac O+
Lac Z+
Lac Y+
Lac A+
+ ..….
+
……
Genotype 4 Lac I+
Lac P+
Lac Oc
Lac Z+
Lac Y+
Lac A+
+ ……
+
……
Genotypes 3 and 4 -> constitutive transcription of Lac operon (no repression)
4.
If the lactose is not present, in which mutants the expression of genes change?
bgal perm.
Genotype 1 Lac I+
Lac P+
Lac O+
Lac Z+
Lac Y+
Lac A+
- -…..
……
Genotype 2 Lac I+
Lac P+
Lac O+
Lac Z+
Lac Y-
Lac A+
- …..
……
Genotype 3 Lac I-
Lac P+
Lac O+
Lac Z+
Lac Y+
Lac A+
+ ..….
+
……
Genotype 4 Lac I+
Lac P+
Lac Oc
Lac Z+
Lac Y+
Lac A+
+ ……
+
……
Genotypes 1 and 2 will not express the genes (REPRESSION)
Genotypes 3 and 4 will express constitutively the enzymes
5. What does it mean that the Oc mutation is dominant in cis? How can I
demonstrate it?
CIS dominant mutation: it expresses the dominant phenotype but it affects only
the expression of genes on the same DNA molecule where the mutation occurs.
LacOc, affects only neighbouring genes (for exemple a plasmid)
To demonstrate it we construct an heterozygote (diploid) with:
The mutation lacZ- (Bgal enzyme) located in cis to lacOc no production of bgalattosidase
The gene lacZ+ in trans (on plasmid) with wt lacO
lacOc
lacZPhenotype in absence of induction:
mutation is cis-dominant -> no b-gal activity
lacO+
lacZ+
mutation is trans-dominant -> b-gal activity
6.
What kind of phenotype will has a bacteria LacI+ O+ Z+ Y+ A+ carrying on a
plasmid lacIs O+ Z+ Y+ A+ ? Which conditions can we use to analyse its
phenotype?
The bacteria will be: LacI+ O+ Z+ Y+ A+ / LacIs O+ Z+ Y+ A+ heterozygote for gene
LacI+ /LacIS
LacI+ encodes for the repressor that is able to bind lactose (induction and
derepression) and is able to recognise sequence of Operator (repression)
LacIS encodes for a repressor with a mutation that unable the protein to interact
with lactose (constitutive repression).
LacIS repressor is always bound on the Operator repressing the transcription of
Operon Lac : the operon Lac genes are not transcribed and expressed
In order to analyse the phenotype I’ll grow the mutant in a condition where I
can detect the expression of enzyme…with the presence of lactose (induction).
We expect that the bacteria with the plasmid does not express the structural
gene  it does not grow if the unique source of carbon is Lactose. (dominant
mutation)
The wt bacteria (control) express the structural genes and grow on Lactose.
7.
Two bacteria have a Trp- phenotype, then?
The Trp- bacteria are unable to synthesize Tryptophan
How can I verify whether the two mutations are in the same gene?
I complement them: I produce bacteria carrying both mutations one on a
plasmid, the other on the chromosome.
To analyse the phenotype I plate them on a medium without Tryptophan
CASE 1: If bacteria grow, the two mutations complement each other, because
they affect two different genes.
CASE 2: If bacteria don’t grow, the two mutations do not complement each
other, because they affect the same gene.
POPULATION GENETICS
The study of the allele frequencies in a population
The Hardy Weinberg equilibrium
“Allele and genotype frequencies in a population tend to remain constant in the
absence of disturbing influences”
-non-random mating
-mutations
-selection
-limited population size
-random genetic drift
Conditions for HW equilibrium are never met in nature.
The equations
p+q=1
p=frequency of allele A
q=frequency of allele a
p2 + 2pq + q2 =1
p2=frequency of AA
q2=frequency of aa
2pq=frequency of Aa
An Example
Assume a population in which 36% of the population are homozygous for a
certain recessive allele a. Assume the population is at equilibrium.
What is the frequency of the recessive allele in this population?
36%aa
q2=0.36
q=f(a)= 0.6
What is the frequency of the dominant allele in this population?
q =0.6
p = f(A) =1-0.6=0.4
What percentage of the population are homozygous for the dominant allele A?
p2=f(AA)=0.4 x 0.4=0.16 16%
What percentage of the population are heterozygous for the trait?
2pq=f(Aa)=2 x 0.6 x 0.4 =0.48  48%
Why do we have to start the problem with the percentage or numeber of the
homozygous recessive in the population?
Because only the recessive phenotype indicates only the homozygous recessive
genotype…the dominant phenotype indicates heterozygous and homozygous
1 - Determine the gene and genotypic frequencies of the following populations:
I
n° of individuals with
genotype:
AA
Aa
aa
100
400
500
II
300
200
500
III
700
200
100
Genotypic frequency of
AA
0,1
Aa
aa
A
a
0,4
0,5
0,3
0,7
In this case we know the number of heterozygous
We think about the number of alleles
p =f (A) = (2 x 100) + 400 = 0,3
2 x 1000
q = f(a)=1 – 0,3 = 0,7
Gene Frequency of
1 - Determine the gene and genotypic frequencies of the following populations:
Genotypic frequency of
AA
Aa
I
n° of individuals with
genotype:
AA
Aa
aa
100
400
500
II
300
200
500
0,3
0,2
III
700
200
100
p = (2 x 300) + 200 = 0,4
2 x 1000
q = 1 – 0,4 = 0,6
aa
0,5
Gene Frequency of
A
a
0,4
0,6
1 - Determine the gene and genotypic frequencies of the following populations:
I
n° of individuals with
genotype:
AA
Aa
aa
100
400
500
II
300
200
500
III
700
200
100
p = (2 x 700) + 200 = 0,8
2 x 1000
q = 1 – 0,8 = 0,2
Genotypic frequency of
AA
0,7
Aa
0,2
aa
0,1
Gene Frequency of
A
0,8
(A(AA)+A(AA))+A(Aa)/(total alleles)
a
0,2
2 - Are the populations of exercise 1 at equilibrium? Where necessary, use statistical
methods to verify it.
Population 1
p = 0,3 q = 0,7
f(AA) = p2 = 0,09
f(Aa) = 2pq = 0,42
f(aa) = q2 = 0,49
Genotipi
Xo
Xa
(Xo – Xa)2 : Xa
AA
100
90
1,11
Aa
400
420
0,95
aa
500
490
0,20
c2 = 2,26
Degree of Freedom = 2(p,q)-1= 1
P = 0,10-0,25
Population is in equilibrium
2 - Are the populations of exercise 1 at equilibrium? Where necessary, use statistical
methods to verify it.
Population II
p = 0,4 q = 0,6
f(AA) = p2 = 0,16
f(Aa) = 2pq = 0,48
f(aa) = q2 = 0,36
Genotipi
Xo
Xa
AA
300
160
Aa
200
480
aa
500
360
Expected and observed values are too different.
The population is not in equilibrium
(Xo – Xa)2 : Xa
2 - Are the populations of exercise 1 at equilibrium? Where necessary, use statistical
methods to verify it.
Population III
p = 0,8 q = 0,2
f(AA) = p2 = 0,64
f(Aa) = 2pq = 0,32
f(aa) = q2 = 0,04
Genotipi
Xo
Xa
(Xo – Xa)2 : Xa
AA
700
640
5,6
Aa
200
320
45
aa
100
40
90
c2 = 140,6
GL = 2– 1 = 1
P <0,01
The population is not at equilibrium
3 - The frequencies of LM and LN alleles in a group of 200 black
Americans were 0,8 e 0,2, respectively. Calculate the expected
frequencies for individuals with M, N and MN blood group.
If the population is at equilibrium:
Group M = LM LM = p2 = 0,8 x 0,8 = 0,64 x 200 = 128
Group MN = LM LN = 2pq = 2 x 0,8 x 0,2 = 0,32 x 200 = 64
Group N = LN LN = q2 = 0,2 x 0,2 = 0,04 x 200 = 8
Population I
f(aa) = 810/1000 = 0,81
Since f(aa) = q2 f(a) = √0,81 = 0,9
f(A) 1 – 0,9 = 0,1
Population II
f(aa) = 360/1000 = 0,36
f(a) = √0,36 = 0,6
f(A) 1 – 0,6 = 0,4
Population III
f(aa) = 490/1000 = 0,49
f(a) = √0,49 = 0,7
f(A) 1 – 0,7 = 0,3
The individuals with
phenotype A can be AA or
Aa.
We are sure that
individuals with
phenotype a are: aa
We start from this
information (aa) to figure
out f(A)
5 - What is the expected frequency for dominant homozygous and heterozygous genotypes
in a population at equilibrium in which the homozygous recessive genotype frequency is
0,09?
f(aa) = 0,09
f(a) = √0,09 = 0,3
f(A) = 1 – 0,3 = 0,7
f(AA) = 0,7 x 0,7 = 0,49
f(Aa) = 2 x 0,7 x 0,3 = 0,42
6 - if in a population at equilibrium the frequency of Rh- phenotype is 0,0025,
which is the expected frequency of heterozygous individuals?
f(Rh-) = 0,0025 = f(dd)
f(d) = √0,0025 = 0,05
f(D) = 1 – 0,05 = 0,95
f(Dd) = 2 x 0,95 x 0,05 = 0,095
7 In Drosophila melanogaster the w recessive sex-linked allele, is responsible of
the white colour of the eyes. In a population the frequency of this allele is 0,3.
Which frequencies of male and female with white-eyes is expected if the
population is at equilibrium?
Male white eyes = w Y
Female white eyes = w w
Frequency:
Males white eyes = f(w) = 0,3
Females white eyes = f(ww) = q2 = 0,3 x 0,3 = 0,09
If alleles are X-linked, females may be homozygous or heterozygous, but males carry
only a single allele for each X-linked locus. For X-linked alleles in females, the H-W
frequencies are the same as those for autosomal loci.
In males, however, the frequencies of the genotypes will be p and q, the same as the
frequencies of the alleles in the population.
8 - Daltonism is due to a sex-linked recessive allele. In a population the daltonic
male frequency is 0,1; which is the one of daltonic females?
Daltonic MALE = d Y (XY) emyzygote freq dY = freq allele d
Daltonic FEMALE = d d (XX)
Frequency Daltonic Males = 0,1
Frequency allele d in MALES = 0,1
Daltonic FEMALES = f(d d) = 0,1 x 0,1 = 0,01
Healty carriers= 2*0.1*0.9=0.18
Fitness involves the ability of organisms— or, more rarely, populations or species— to survive
and reproduce in the environment in which they find themselves . The consequence of this
survival and reproduction is that organisms contribute genes to the next generation.
Fitness= Its lifetime reproductive success
Fitness absolute
200/100 = 2
Fitness relative (W)
2/2 = 1
400/200 = 2
100/100 = 1
2/2 = 1
The selection is against the phenotype homozygous recessive
1/2= 0,5
10 - A population is composed of: AA 250, Aa 500, aa 250 individuals, when a
selection factor acts against the homozygote recessive and its fitness decreases to 0.
The variation of genic frequency (selection effect) could be figured out taking into
consideration the contribution of the gametes of different genotypes in the next
generation.
Consider the previous population, try to figure out the variation of genetic frequency
of a after a generation of selection.
1
1
250/1000 = 0,25
1 x 0,25 = 0,25
0
500/1000 = 0,5
250/1000 = 0,25
1 x 0,5 = 0,5
0,25 x 0 = 0
1000x2 total of alleles
2x250 aa=250a+250a
2(0.25+0.5) total of freq
[2(0) + 0,5] /2(0,25 + 0,5 + 0) = 0,33 2x0 aa=0a+0a
[2(250) + 500]/2(1000) = 0,5
q1 – q0 = 0,33 – 0,5 = -0,17
As expected the frequency of a allele decreases
11 – In a population there is a selection against the homozygous recessive with
fitness = 0 but the genotypic frequencies are:
AA 0,64; Aa 0,32; aa 0,04,
Try to figure out the variation of genetic frequency of a after a generation of
selection
1
1
0
0,64
0,32
0,04
0,64 x 1 = 0,64 0,32 x 1 = 0,32 0,04 x 0 = 0
[0,32 + 2(0,04)]/2(0,64 + 0,32 + 0,04) = 0,20
[0,32 + 2(0)]/2(0,64 + 0,32 + 0) = 0,16
q1 – q0 = 0,16 – 0,20 = -0,04
Previous population Dq = -0,17 (> of q0 (0,5))
This population q0 = 0,2  With the same selection coefficient, the effect of
selection against homozygous recessive change on varying of genic
frequency
12 – In a population the gene frequency of A is 0,3 and the frequency of a is 0,7, and
there is a selection against the heterozygous
(s =coefficient of selection=1-w= 0,4).
Try to figure out the variation of genetic frequency of a after a generation of selection
genotypes
fitness
AA
1
Aa
aa
1 – 0,4 = 0,6
Genotypic frequency
p2 = 0,32 = 0,09
Gametic contribution
1 x 0,09 = 0,09
2pq = 0,42
1
q2 = 0,72 = 0,49
0,6 x 0,42 = 0,252 1 x 0,49 = 0,49
Frequency of q0
[0,42 + 2(0.49)]/2(0.09+0,42+0,49)=0,7
Frequency of q1
[0,252 + 2(0,49)]/2(0,09 + 0,252 + 0,49) = 0,74
Dq
q1 – q0 = 0,74 – 0,7 = 0,04
- What is the final result of this selection?
The recessive allele will be fixed and the dominant allele will be delete from
the population
13 – The population is composed of: 640 AA, 320 Aa and 40 aa,
The fitness of homozygous dominant is 0,8 and the fitness of homozygous
recessive is 0,1.
Try to figure out the variation of genetic frequency of after a generation of selection
genotypes
fitness
Genotypic frequency
Gametic contribution
AA
Aa
0,8
1
640/1000=0,64
320/1000=0,32
0,64 x 0,8=0,51
0,32 x 1 = 0,32
aa
0,1
40/1000=0,04
0,04 x 0,1=0,004
Frequency of q0
[320+2(40)]/2(1000) = 0,2
Frequency of q1
[0,32+2(0,004)]/2(0,51+0,32+0,004) = 0,19
Dq
0,19 – 0,20 = -0,01
Considering people that become part of the family
through marriage don’t have the mutant allele,
calculate the probability that an affected (sick) child is
born from the indicated cross
?
Daltonism is a recessive x-linked disease.
A girl has a daltonic father and a normal mother but
the mum’s father is daltonic.
Which is the probability that the girl is daltonic?
Draw the tree
Genes A and B are concatenated at 20
mu. Which gametes are produced by an
individual: AaBb?
If this individual is cross with a
homozygous recessive, which
phenotypic classes and which
frequencies will we obtain?
For each of the following crosses:
- determine the genotype of the individuals used for the cross;
- determine if there is concatenation of the analyzed genes ;
- if genes are concatenated, determine their map distance;
- draw a schematic picture of the map of crossed individuals.
Parental Phenotypes
AB x ab
Progeny Phenotypes:
AB
40
Ab
63
aB
58
Ab
37
Gene A, B and C are located in this
order on the same chromosome. The
distances are A-B 20 mapunits and B-C
10 mapunits. If an individual triple
heterozygous is crossed with a
recessive homozygous, which
phenotypical classes are expected in
the progeny and with which
frequencies?
If the interference is 1?
Analysing a sample of individuals we have these phenotypic results
MM 60
MN 92
NN 20
Determine allelic frequencies of M and N
Determine if the population is at equilibrium (use
statistical analysis)
7 Gene A, B and C are located in this order on the same chromosome. The distances are M-N
2 map units and N-O 8 map units (genes are in cis). If an individual triple heterozygous is
crossed with a recessive homozygous, which phenotypical classes are expected in the
progeny and with which frequencies?
Draw the map
M
M
2
N
n
8
O
o
x
m
m
Parental
MNO
mno
Recombinants1
Mno mNO
Recombinants2
Mno mnO
DR
MnO mNo
Freq DR= 0.02 x 0.08 =0.0016/2=0.0008
Freq Ric 1=0.02-0.0016=0.0184/2= 0.0092
Freq Ric 2 =0.08 – 0.0016= 0.0784/2 =0.0392
Parental = 1- 0.0016-0.0184-0.0784=0.9016/2= 0.450
n
n
o
o