Transcript AB AB ab AB

Gene linkage
seminar
No 405 Heredity
Key words:
complete and incomplete gene linkage, linkage
group, Morgan´s laws, crossing-over,
recombination, cis and trans linkage phases,
strenght of linkage, Morgan´s number - p,
centimorgan, gene map unit, Bateson´s number - c,
gene mapping.
3. Mendel´s principle of combination
(independent assortment)
members of two and more allelic pairs segregate
independently - there are as many types of gamets as
possible combinations among maternal and paternal
chromosomes (alleles)
Genes on the same chromosome show linkage
- they have tendency to be transmitted together through
meiosis
Genes on 1 chromosome = linkage group
Morgan´s laws
1. The genes on chromosomes are in linear order
2. Number of linkage groups is equal to the
number of pairs of homologous chromosomes
Only the recombination of parental alleles results into the
formation of gametes with the configuration of linked alleles
different from the configuration of parent alleles
cis phase
P
trans phase
A
a
AB
A
a
B
b
ab
b
B
AB
ab
Ab
ab
Ab
x
AB
F1
AB
Ab
ab
aB
x
Ab
aB
aB
aB
Complete gene linkage
– dihybrid form only two types of gametes with ratio 1:1
• F2
cis
trans
AB
ab
AB
AB
AB
ab
AB
ab
AB
ab
ab
ab
GP
Ab
aB
Ab
Ab
Ab
aB
Ab
aB
Ab
aB
aB
aB
1:2:1
1:2:1
FP
3:1
B1
AB
ab
AB
ab
ab
ab
ab
1
1:2:1
:
1
ab
Ab
aB
Ab
ab
aB
ab
1
:
Recombination – incomplete linkage
cis A
A a
a
A
A a
a
B
b B
b
b
B b
B
A
B
A
b
a
a
B
b
Non-recombinant
AB ab
Recombinant
Ab aB
A
b
A
a
B
b
trans
a
B
Ab
AB
aB
ab
Proof of recombination: from B1 (testcross)
cis
trans
B1 AB>Ab aB < ab
AB < Ab aB > ab
Mechanism of recombination – crossing over
C.O. = reciprocal exchange of segments between
nonsister chromatids of homologous chromosomes
Probability of c.o. – dependent on the alleles distance
0<P<1
P = 0 complete linkage, P = 1 free (independent)
combination
Crossing over - linked genes A/a, B/b
Strength of linkage p =
fr
from B 1 (testcross)
fr + fnr
fr – frequency of recombinant gametes, fnr = frequency of
nonrecombinant gametes (phenotypes in B1)
0 < p < 0,5 p = Morgan´s number
Unit: cM (centimorgan) = % of recombinants =
expression of relative distance of alleles = map unit 1cM =
rekombinant frequency 1% (0,01)
p = 0 complete linkage
p = 0,5 free (independent) combination
• Bateson´s number c: expresses the ratio
between recombined and non-recombined
alleles (fromB1)
Phase cis =
Phase trans: c =
a1 + a4
a2 + a3
a2 + a3
a1 + a4
1< c< ∞
for independent combination c = 1
a1- frequency of gametes AB (zygotes AaBb from B1)
a2 - frequency of gametes Ab (zygot es Aabb from B1
a3 - frequency of gametes aB (zygot es aaBb from B1)
a4 - frequency of gametes ab (zygotes aabb from B1)
Morgan´s number – from B1
Phase cis: p =
trans: p =
a2 + a3
a1 + a2 + a3 + a4
a1 + a4
a1 + a2 + a3 + a4
1. In B1 (testcross) generation of parental cross
AB/AB x ab/ab
902 individuals of phenotype AB, 898 of ab, 98 of Ab
and 102 of aB phenotypes was detected.
Calculate strength of linkage between genes A and B
(number p).
2. How would you prove that two allelic pairs are linked
or not?
3. Hybrid of parental cross Ab/Ab x aB/aB is crossed
with individual ab/ab in testcross. What is the ratio of
genotypes in progeny comprising 1850 individuals?
The strength of the gene linkage between A and B
genes is 40% of recombinations.
4. Rabbits bearing the allele En are spotted, whereas en codes the
absence of colour spots. Allele l codes for the long (Angora) hair, and allele
L codes for the normal short rabbit hair. Both allele pairs are in linkage.
a)You should cross homozygous spotted rabbits with the normal length of
the hair with Angora non-spotted rabbits. Describe genotypes of the P and
F1 generations. Determine the gene linkage phase
b)During the testcross (F1 x recessive homozygote) the following
offsprings were obtained:
83 spotted rabbits with normal lenght of hair
12 spotted rabbits with long hair
14 rabbits without spots and with normal length of hair
89 rabbits without spots and with long hair
Write the course of the backcross.
c) Determine the strength of the gene linkage between both genes, and
find out what is the ratio between the amount of parental and recombined
gametes produced by the F1 dihybrid.
d)Calculate the phenotype and genotype ratio in the F2 generation
5. Determine the order of genes on chromosome if you know
that p=5% for genes A and B, p=3% for genes B and C and
p=2% for genes A and C.
6. Dominant allele D is coding for Rh+ factor, recessive
genotype dd is coding for Rh- phenotype (absence of Rh
factor on the surface of erythrocytes). Elliptic -oval shape
of erythrocytes (eliptocytosis) is coded by the dominant
allele E, whereas the homozygously recessive genotype
ee codes for normal round shape of erythrocytes. Both
genes are in genetic distance of 20 cM.
There is a man with the eliptocytosis, whose
mother had a normal shape of erythrocytes and was
homozygous for Rh+ and whose father was Rh- and
heterozygous in allelic pair for eliptocytosis. This man
got married to a healthy woman bearing Rh- factor.
a) What is the probability that the first child will be Rhwith eliptocytosis?
b) What is the probability that the first child will be
eliptocytic and Rh+?
c)If the first child is Rh+, what is the probability that it
will suffer from the eliptocytosis too?
7. Determine the cross ratios in B1 of the hybrid AB/ab,
if c=2 and the number of offsprings in the B1 generation
is 1585.
8. What is the cross ratio of the hybrid Ab/aB in the F2
generation, if c=5?
9.The woman (X DH/X dh) is a carrier (heterozygote) of
recessive alleles for a daltonism (colour-blindness) and a
hemophilia. Relative distance of genes is 10 cM.
a)In which percentual ratio the recombined gametes in the
woman arise? Write their genotypes.
b)What is the probability that woman will have a hemophilic
and daltonic son? What is the probability that she will have
a hemophilic but not daltonic son or a daltonic but not
hemophilic son or a healthy son? You should assume that
the father is X DH/Y.
c) The same you should calculate for the case that the mother
has got all the allelic pairs in the trans phase.
10.The colour-blind man had married a phenotypically
normal woman. Their first son is healthy, two sons suffer
from the hemophilia one daughter is colour-blind, and
the second daughter is healthy.
a) Draw a pedigree and determine the genotypes of each
family member.
b) What is the probability that sons of both daughters
will suffer from hemophilia if they will marry healthy
partners?
Thompson and Thompson: Clinical genetics, chapter 10 : Human gene mapping ,
pg. 207-222, 7th edition