Download Partial Linkage

Fig. 15-1
The location of a particular gene can be seen
by tagging isolated chromosomes with a
fluorescent dye that highlights the gene
Fig. 15-4a
EXPERIMENT
P
Generation
F1
Generation

All offspring
had red eyes
Fig. 15-4b
RESULTS
F2
Generation
Fig. 15-4c
CONCLUSION
P
Generation
X
X
w+

w+
X
Y
w
Eggs
F1
Generation
Sperm
w+
w+
w+
w
w+
Eggs
F2
Generation
w
w+
Sperm
w+
w+
w
w
w
w+
Fig. 15-7
The transmission of sex linked recessive genes
XNXN
Sperm Xn

XnY
Sperm XN
Y
Eggs XN
XNXn XNY
XN
XNXn XNY
(a)
XNXn
Eggs
(b)

XNY
XNXn
Sperm Xn
Y
XN
XNXN XNY
Xn
XnXN
Eggs XN
XnY
Xn
(c)

XnY
Y
XNXn XNY
XnXn
XnY
• Sex-linked genes follow specific patterns of
inheritance
• For a recessive sex-linked trait to be expressed
– A female needs two copies of the allele
– A male needs only one copy of the allele
• Sex-linked recessive disorders are much more
common in males than in females
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Partial Linkage
• Linkage is different from sex linkage
• Linked genes tend to be inherited together
because they are located near each other on
the same chromosome. Results from genes
being closely linked on the same chromosome
• Linked genes in genetic experiments deviate
from the results expected from Mendel’s law
of independent assortment.
Dihybrid
Testcross to
Detect
Independent
Assort
YYRR X yyrr
Dihybrid
YrRr
YR yr
Yr
X
yR
yyrr
yr
Sperm
yr
YR
YyRr
yr
yyrr
Eggs
Yr
Yyrr
yR
yyRr
Phenotypic ratio
1:1:1:1
Ratio of parental:Recombinant
1:1
Fig. 15-9-1
EXPERIMENT
P Generation (homozygous)
Wild type
(gray body,
normal wings)
b+ b+ vg+ vg+
Morgan 1912

Double mutant
(black body,
vestigial wings)
b b vg vg
Fig. 15-9-2
EXPERIMENT
P Generation (homozygous)
Wild type
(gray body,
normal wings)

b b vg vg
b+ b+ vg+ vg+
F1 dihybrid
(wild type)
b+ b vg+ vg
Double mutant
(black body,
vestigial wings)
TESTCROSS

Double mutant
b b vg vg
Fig. 15-9-3
EXPERIMENT
P Generation (homozygous)
Wild type
(gray body,
normal wings)
Double mutant
(black body,
vestigial wings)

b b vg vg
b+ b+ vg+ vg+
F1 dihybrid
(wild type)
Double mutant
TESTCROSS

b+ b vg+ vg
Testcross
offspring
b b vg vg
Eggs
b+ vg+
Wild type
(gray-normal)
b vg
b+ vg
b vg+
Blackvestigial
Grayvestigial
Blacknormal
b vg
Sperm
b+ b vg+ vg
b b vg vg b+ b vg vg b b vg+ vg
Fig. 15-9-4
EXPERIMENT
P Generation (homozygous)
Wild type
(gray body,
normal wings)
Double mutant
(black body,
vestigial wings)

b b vg vg
b+ b+ vg+ vg+
F1 dihybrid
(wild type)
Double mutant
TESTCROSS

b+ b vg+ vg
Testcross
offspring
b b vg vg
Eggs
b+ vg+
Wild type
(gray-normal)
b vg
b+ vg
b vg+
Blackvestigial
Grayvestigial
Blacknormal
b vg
Sperm
b b vg vg b+ b vg vg b b vg+ vg
b+ b vg+ vg
PREDICTED RATIOS
If genes are located on different chromosomes:
1
:
1
:
1
:
1
If genes are located on the same chromosome and
parental alleles are always inherited together:
1
:
1
:
0
:
0
965
:
944
:
206
:
185
RESULTS
Fig. 15-10
Testcross
parents
Gray body, normal wings
(F1 dihybrid)
Replication
of chromosomes
Meiosis I
Black body, vestigial wings
(double mutant)
b+ vg+
b vg
b vg
b vg
Replication
of chromosomes
b+ vg+
b vg
b+ vg+
b vg
b vg
b vg
b vg
b vg
b+ vg+
Meiosis I and II
b+ vg
b vg+
b vg
Meiosis II
Recombinant
chromosomes
Eggs
Testcross
offspring
b vg
b+ vg+
b+ vg
b vg+
965
944
206
185
Wild type
(gray-normal)
Blackvestigial
Grayvestigial
Blacknormal
b+ vg+
b vg
b+ vg
b vg+
b vg
b vg
b vg
b vg
Parental-type offspring
Recombination
frequency
=
Recombinant offspring
391 recombinants
2,300 total offspring
 100 = 17%
b vg
Sperm
20% recombination
A B
40%
With crossing over
A B
a b
40%
a b
a B
A b
10%
10%
Testing for Assortment/Linkage
1. Generate a dihybrid
2. Testcross the dihybrid
3. Compare the % of parental to recombinants
A. If 50% parental:50% recombinant – Independent
Assortment
B. If more parental than recombinant – partial linkage
C. If only parental and no recombinant – complete linkage
• The discovery of linked genes and
recombination due to crossing over led Alfred
Strutevant to a method of constructing
genetic maps
• He assumed the farther apart genes are , the
higher the probability that a cross over will
happen between them and therefore the
higher the recombination frequency.
The closer the two genes are on a chromosome the fewer
recombinants
Minimum = 0% recombinants
The further two genes are on a chromosome the more
recombinants
Maximum = 50% recombinants
Linkage therefore can be used as a measure of genetic distance on
chromosome
1 Map Unit = 1 % recombination
Gene Order on Chromosome
B – Vg 17 MU
B – Cn 9 MU
Vg – Cn
9.5 MU
Partial Linkage – two genes are so close on the
same chromosome that
recombination occurs less than 50%
of the time.
Complete Linkage – two genes on the same
chromosome so close that
recombination cannot separate
them.
Independent Assortment – two genes on
different chromosomes or two genes
on the same chromosome but far
enough apart that recombinant
occurs 50% of the time.
Example Problem
In Drosophila long wings is dominant to dumpy wings and round
eyes is dominant to star eyes. A dihybrid fly was generated
by mating a long wing round eye fly with a dumpy wing star
eye fly. This dihybrid fly was testcrossed and the following
progeny were generated.
222 long wing round eye
215 dumpy wing star eye
33 long wing star eye
30 dumpy wing round eye
a. Are these genes completely linked or partially linked?
b. What is the genetic distance between these two genes?
c. How would the results have differed if the genes
independently assorted?
Exception to chromosomal Inheritance
(Organellar Genes)
• The inheritance of traits controlled by genes
present in the chloroplasts or mitochondria
– Depends solely on the maternal parent because
the zygote’s cytoplasm comes from the egg
– Maternal Inheritance
Pedigree Symbols
Nuclear vs Organellar
Human Genetics
Pedigree Analysis
Autosomal vs Sex Linked
Multifactorial Traits
• Heart disease
• Personality
• IQ
Alterations of chromosome number or
structure cause some genetic disorders.
• So far we’ve seen that the phenotype can be
affected by small scale changes involving
individual genes
• Random mutations are the source of all new
alleles, which can lead to a new phenotype.
Abnormal chromosome #: Aneuploidy
Human Aneuploids
• Trisomy 21
• Sex chromosome
– XO – turner syndrome
– XXY – klinefelters
– XYY
Abnormal chromosome numbers
•
•
•
•
•
Polyploidy: Common in plant
~70 % of flowering plants,
Banana are triploid,
Wheat 6n
Strawberries 8n
Alterations in chromosome structure
• Meiosis errors and damaging agents such as
radiation can cause breakage of the
chromosome
• four types of structural damage
Chromosome Structure
reciprocal translocation between 9 and 22
(Philadelphia Chromosome)
Disorders caused by structurally
altered chromosomes
• Cri du chat – deletion in chromosome 5
• Chronic myogenous leukemia
Normal chromosome 9
Normal chromosome 22
Reciprocal
translocation
Translocated chromosome 9
Translocated chromosome 22
Fig. 15-8
X chromosomes
Early embryo:
Two cell
populations
in adult cat:
Active X
Allele for
orange fur
Allele for
black fur
Cell division and
X chromosome
inactivation
Active X
Inactive X
Black fur
Orange fur
Barr Body – inactive X visible in
interphase nucleus
Genomic imprinting
• Def: a parental effect on gene
expression
• Identical alleles may have
different effects on offspring,
depending on whether they
arrive in the zygote via the ovum
or via the sperm.
• Fragile X syndrome: higher
prevalence of disorder and
retardation in males