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Specialized Mapping
Finding Chromosomal Locations
Using Tetrad Analysis To Study
Genetic Distances
(see Tetrad Analysis Web Module for
Chapter 7 on Text Web Site
http://whfreeman.com/pierce/)
Physical Chromosome Mapping
• Somatic-Cell Hybridization
Using human-rodent somatic cell
hybrids to study the location of
genes on chromosomes
Human-mouse hybrid cells with different
numbers of human chromosomes (blue).
Physical Chromosome Mapping
• How can we determine which
chromosome carries a specific gene?
In human-mouse hybrid cells, a 1:1
correspondence exists between the
presence of the enzymatic activity for
the gene and the presence of the
chromosome carrying the gene.
Problem 4, Page 2-2
Hybrid cells containing human and mouse chromosomes
were analyzed. The grid on the left shows the presence
or absence of each of four human chromosomes in hybrid
cell lines A through D. The grid on the right shows the
presence or absence of human enzyme activity in each of
the cell lines. Assign the gene for each enzyme to the
chromosome that carries the gene.
Human Enzyme
Human Chromosome
ADH PEP HexA GAPDH
A
5
-
7 11
+ +
18
-
A
+
-
+
-
B
+
-
+
+
B
+
+
-
+
C
D
+
+
+
+
-
+
-
C
+
-
+
+
D
-
+
+
-
Physical Chromosome Mapping
• How can we determine which portion of
a chromosome carries a specific gene?
If the enzymatic activity is present in a
cell line with an intact chromosome but
missing from a line with a deletion in that
chromosome, the gene for the enzyme
is in the deleted region.
Gene is located on
the short arm of
Chromosome 4,
in the region missing
from Cell line 3
Enzyme Activity
Present with intact Absent without
Chromosome 4 Chromosome 4
Absent when
short arm of
Chromosome 4
is deleted
Tetrad Analysis
• Analysis of all four products of a
single meiosis
• Two Types of Tetrads
Unordered Tetrad
Ordered Tetrad
Producing an Ordered Tetrad
Genetic Analyses with Tetrads
Cross two haploid cells
a
b
a+
X
a
a+
b+
b
b+
Induce diploid to undergo meiosis
Genetic Analyses with Tetrads
a
b
a
b
Parentals
a+
a
a+
b+
b
X
b+
a+
a
b+
b+
Recombinants
a+
b
MI Segregation Pattern
No crossover
between gene
and
centromere
A first-division
segregation
pattern, MI
MII Segregation Pattern
Crossover
between
gene and
centromere
Types of Tetrads
Parental
Ditype
(PD)
a b
a b
a+ b+
a+ b+
NonTetratype
parental
(T)
Ditype
(NPD)
a b+
a b+
a+ b
a+ b
a b
a b+
a+ b+
a+ b
MI pattern
(both
alleles
adjacent)
a
a
a+
a+
MII
Pattern
(at least
two alleles
separated)
a+
a
a+
a
a
a+
a+
a
Producing MII Segregation Patterns
Producing MII Segregation Patterns
Calculating Genetic Distances
with Tetrad Analysis
Unordered
Example
Yeast
Ordered
(Linear)
Neurospora
Gene-Gene RF= 1/2T + NPD RF= 1/2T + NPD
Distance
total
total
GeneCannot be
Centromere determined
Distance
½ MII/total
Problem 2, Page 2-1
In a Neurospora cross of ab x a+b+, the
following classes and numbers of
tetrads were produced. Neurospora
produces ordered tetrads that undergo
a single mitosis after formation. Pairs of
spores are listed below for simplicity.
Problem 2, Page 2-1
Type
For a
For b
ab
ab
a+b+
a+b+
ab+
ab+
a+b
a+b
ab
ab+
a+b+
a+b
ab
a+b
a+b+
ab+
ab
a+b+
a+b+
ab
ab+
a+b
a+b
ab+
ab+
a+b
a+b+
ab
71
1
18
1
8
0
1
Problem 2, Page 2-1
Distance from a centromere =
Distance from b centromere =
Distance from ab =
Problem 2, Page 2-1
Best solution