Chromosome Theory
of Inheritance
Chromosome Theory

Today’s understanding:
Genes, which determine genetic traits, are
located on chromosomes

Historical Evidence:
Sutton
 Morgan
 Bridges

X – Linked Traits
HUMAN EXAMPLE
Autosomal Inheritance


Genes for these traits on #1 22
Both ♂ & ♀ have two genes for every trait
X-linked Inheritance

X- Linked trait – traits whose gene is located
on the “X” chromosome, therefore
Females (XX) have two genes
 Males (XY) only have one……

Human Sex Chromosomes
Some X-linked Traits
H = normal blood clotting
h = hemophilia
D = normal muscular development
d = muscular dystrophy
C = normal color vision
c = color blindedness
Possible Genotypes of BIO
SCI230 ♀
C
C
C
c
c
c
†† †† ††
genotype
phenotype
Possible Genotypes of SCI 230♀
C
C
C
c
c
c
†† †† ††
genotype
Homozygous
Dominant
phenotype
Normal Color
Vision
Possible Genotypes of SCI 230 ♀
C
C
C
c
c
c
†† †† ††
genotype
Homozygous
Dominant
phenotype
Normal Color
Vision
heterozygous
Possible Genotypes of SCI 230 ♀
C
C
C
c
c
c
†† †† ††
genotype
Homozygous
Dominant
heterozygous
phenotype
Normal Color
Vision
Normal but
“carrier”
Possible Genotypes of SCI 230 ♀
C
C
C
c
c
c
†† †† ††
genotype
Homozygous
Dominant
heterozygous
phenotype
Normal Color
Vision
Normal but
“carrier”
Homozyg.
Recessive
Possible Genotypes of SCI 230 ♀
C
C
C
c
c
c
†† †† ††
genotype
Homozygous
Dominant
heterozygous
Homozyg.
Recessive
phenotype
Normal Color
Vision
Normal but Color
“carrier”
blind!
Possible Genotypes of SCI 230 ♂
c
C
†
Genotype
phenotype
γ
†
γ
Possible Genotypes of SCI 230 ♂
c
C
†
γ
Genotype
hemizygous
dominant
phenotype
†
γ
Possible Genotypes of SCI 230 ♂
c
C
†
γ
Genotype
hemizygous
dominant
phenotype
normal
color vision
†
γ
Possible Genotypes of SCI 230 ♂
c
C
†
γ
†
γ
Genotype
hemizygous
dominant
phenotype
normal
color vision
hemizygous
recessive
Possible Genotypes of SCI 230 ♂
c
C
†
Genotype
phenotype
γ
†
γ
hemizygous
dominant
hemizygous
recessive
normal
color vision
Color blind!
HINT
(for working problems w X-linkage)
RECORD THE SEX CHROMSOMES
of BOTH PARENTS !!
Sample Problem
A carrier for hemophilia marries a normal
male.
1.
2.
3.
4.
What is the probability that their daughter is a
hemophiliac?
What is the probability that their son is a
hemophiliac?
What is the probability that their daughter is a
carrier?
What is the probability that their son is a
carrier?