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LECTURE 6
2.3 Genetic Mapping 2.4 Pedigree
Analysis
OBJECTIVES
At the end of the lesson, students should be able to :• Analyze the example on pedigree
given.• Calculate and map a chromosome’s
genetic loci using recombination data.• Determine the position of genes/loci
along a chromosome based on recombinant frequency data.
• Explain Pedigree analysis.
Genetic Mapping
The proportion of recombinants resulting from a dihybrid test cross is used to calculate the cross over value (COV), a measure of linkage, and if linkage occurs, the distance between genes .
COV is also known as recombination frequency.
Genetic Mapping Formula
COV = total number of recombinant /
total number of offspring x 100
• If the genes are not linked, the expected phenotype ratio of
such a cross is 1:1:1:1 and there is a 50 per cent chance
that alleles on separate chromosomes will be inherited together,
the expected COV is 50 per cent; the lower the value, the closer the
genes.
• Thus the COV can be used to locate the relative positions of genes on chromosomes, a process called chromosome mapping or genetic mapping.
• By convention, one per cent COV is equivalent to one map unit.
• Today the word centimorgan is often used.
Cross over (meiosis)
Homologous chromosome in synapsis
– 4 chromatid (tetrad)
P
Q
p
q
kiasma
r R
Genetic mapping of Drosophila melanogaster genes
Example
P : CCSS x ccss
P : CS/CS x cs/cs
G : CS csF1 : CS/cs x cs/cs
G : CS cs Cs cS cs
x co co
CS/cs cs/cs cS/csCs/cs
480 480 2020
Parent combination
Recombinant
Genotype Individuals
CS/cs 480
cs/cs 480
Cs/cs 20
cS/cs 20
Parent combination
Recombinant (cross over)
Eg : Test cross F1 : CS/cs x cs/cs
Parent combination = (480 + 480)/1000 = 96%
Recombinant = (20 + 20)/1000 = 4% (cov)
If C/c dan S/s genes is far apart, cov > 4%
Jumlah 1000
C S
4 map unit
EXERCISE 1
• P, Q, R dan S genes are linked. • COV for pairs of genes is as follows:
P and Q = 35%P and R = 5%R and Q= 40%Q and S = 10%R and S = 30%
• Map the chromosome.
• Tips: Start with the furthest. (highest cov % )
Solution:R and Q= 40%P and Q = 35%R and S = 30%Q and S = 10%P and R = 5%
R Q
40 map unit
SP5 25 10
Cross over value given for
P and Q = 4%
P and R = 1%
State the cross over value possible for Q and R.
P Q
4
R1 3
Cross over value for Q and R = 3%
P Q41
R
Cross over value for
Q and R = 5%
EXERCISE 2
EXERCISE 3• In tomatoes,
S = smooth s = wrinkledM = red flower m= white flower
• Test cross between smooth, red flower plant with wrinkled, white flower plant resulted in:
Smooth fruit ,red flower = 300Wrinkled fruit , white flower = 300Smooth fruit, white flower = 100Wrinkled fruit, red flower = 100
• Count the map unit for the distance between both S and M genes on its chromosome.
Total progeny = 800
Recombinant = 100 + 100800
= 25%
Map unit for both S and M genes = 25 m.u
S M 25
SM/sm x sm/smSmooth fruit ,red flower = 300 (SM/sm)Wrinkled fruit , white flower = 300 (sm/sm)Smooth fruit, white flower = 100 (Sm/sm)Wrinkled fruit, red flower = 100 (sM/sm)
ConclusionThrough genetic mapping, we can
determine :– Relative distance between two
genes.– Linear sequence of the linked
genes on a chromosome.
Pedigree Analysis
• A pedigree is a family tree that shows genetic interrelationship among parents and offspring across one or more generations.
• Standardized symbols, methods and definitions are used to construct it.
• Eg: Inherited disease in the British Royal family.
I
II
III
1 2
1
1
2 43
Circle = female
Square = male
Roman = generation
Numbers = individuals in generationColored = different phenotype
Eg: “widow’s peak” trait (dominant allele)
W Allele = widow’s peak w allele – no widow’s peak
I
II
III
Color = widow’s peak
Ww ww
ww
WW @ Ww
ww WwWw
Widow’s peak pedigree