Gene Linkage
Inheritance Patterns for
---Two Genes on the Same Chromosome
---Genes on the Sex Chromosomes
Sex Determination
Sex Chromosomes: homologous
chromosomes that differ in size and
genetic composition between males and
females
Human
Chromosome
Size
Genetic
Composition
X
Y
Larger
Smaller
Multiple
genes
unrelated
to gender
Very few genes
SRY region determines
maleness
Sex Determination
Autosome: any chromosome that is
not a sex chromosome
Humans have
22 autosome pairs
and 1 pair of sex
chromosomes
Sex Determination
Female
XX x
X
Male
XY
X
X
XX
XX
Y
XY
XY
Phenotypic Ratio of Offspring
½ Female + ½ Male
Dad
XhY
Sex Linked Genes
Hemophilic Male Non-hemophilic
Female
(father is hemophilic)
XhY
Males are
hemizygous for
traits on the
X chromosome
XHXh
x
XH
Xh
Xh
Xh
XHXh
XhXh
Y
XHY
XhY
Females can be
homozygous or
heterozygous for
traits on the
X chromosome
Phenotypic Ratio of Offspring
¼ hemophilic males + ¼ non-hemophilic males
¼ hemophilic females + ¼ non-hemophilic females
Patterns of Inheritance
•
•
•
•
Autosomal Dominant
Autosomal Recessive
Sex-Linked Dominant (X-linked)
Sex-Linked Recessive (X-linked)
Pedigree for An Autosomal Dominant Trait
Pedigree for Albinism:
An Autosomal Recessive Trait
Pedigree for Hemophilia,
An X-linked Recessive Trait
Pedigree for Colorblindness,
An X-linked Recessive Trait
Patterns of Inheritance
•
•
•
•
•
Autosomal Dominant
Autosomal Recessive
Sex-Linked Dominant (X-linked)
Sex-Linked Recessive (X-linked)
Mitochondrial Inheritance
Linked Genes
Gene
location
Gamete
types
Mendel’s
experiments
Linked Genes
Genes on separate
chromosomes or far
apart on the same
chromosome
Equal numbers of all
possible allele
combinations
Genes on the same
chromosome
More parental
combinations than
recombinant
combinations
How Do We Study
Gene Linkage?
Example from the Fruit Fly
Red eyes, x Pink eyes
Beige body
Ebony body
RRBB
rrbb
F1:
Red eyes, Beige body
RrBb
How Do We Study
Gene Linkage?
Testcross: cross to individual of
known genotype
F1: Red eyes X Pink eyes
Beige body
Ebony body
RrBb
rrbb
How Do We Study
Gene Linkage?
F2 phenotype
Red eyes
Beige body
Pink eyes
Ebony body
Red eyes
Ebony body
Pink eyes
Beige body
Number of
Offspring
398
Expected for
Unlinked Genes
250
382
250
108
250
112
250
How Do We Study
Gene Linkage?
If genes are linked:
Red eyes, x Pink eyes
Beige body
Ebony body
F1:
R
B
r
b
R
B
r
b
Red eyes, Beige body
R
B
r
b
How Do We Study
Gene Linkage?
F1: Red eyes, Beige body
R
B
Alleles are in
Cis Configuration
X
r
b
Four types of gametes are produced
Parental
Recombinant
R
B
R
b
r
b
r
B
How Do We Study
Gene Linkage?
F1:Red eyes X Pink eyes
Beige body
Ebony body
R B
r b
r b
r b
r b
R B
r b
R b
r B
R B
r b
R b
r B
r b
r b
r b
r b
How Do We Study
Gene Linkage?
F2 phenotype
Number of
Offspring
398
Chromosome
arrangement
RB//rb
Parental
Pink eyes
Ebony body
382
rb//rb
Parental
Red eyes
Ebony body
108
Rb//rb
Recombinant
Pink eyes
Beige body
112
rB//rb
Recombinant
Red eyes
Beige body
Genetic Map Units
1% recombination = 1 map unit
108 + 112 x 100 = 22%
1000
These genes are located 22 map units
apart on the same chromosome.
Map is drawn as:
R
I
22 map units
B
I
How Do We Study
Gene Linkage?
Knowing the gene distance allows us to
predict the expected percentages of each
type of gamete.
r
R
X
Parental
r
R
B
Alleles are in
Trans Configuration
b
Recombinant
B
39%
r
b
39%
78%
R
b
11%
B
11%
22%
Gene Linkage Summary
A. Linked genes tend to be inherited together in
their original, parental arrangement.
B. Alleles of linked genes can be separated by
crossing over (recombination) during meiosis.
C. Distance between genes is calculated as 1%
recombination = 1 map unit.
D. Gene distances can be calculated for genes that
are less than 50 map units apart. At 50 map units
and beyond, we cannot distinguish between
genes far apart on the same chromosome and
genes on separate chromosomes.