X-Linked Inheritance
X-Linked Traits: ________________________________________________________________
______________________________________________________________________________
Examples: _____________________________________________________________________
______________________________________________________________________________
X-linked inheritance results in a _________ rate of incidence of these traits in _______. This is
because a gene on the ___ chromosome in the male has no matching allele on the ______
chromosome. Therefore, any gene on the X chromosome, whether ___________ or
___________, is expressed in males. X-linked traits tend to ______ generations.
Sample Genotypes and Phenotypes for Colourblindness
Males
Females
Genotypes
Phenotypes
Genotypes
Phenotypes
XY
XX
XC Y
XC X
XC XC
Example 1
A male with hemophilia and a female have children. What are the genotypic and phenotypic
ratios of their offspring?
Parent Genotypes:
Possible Gametes:
Genotypic Ratio:
Phenotypic Ratio:
Example 2
The gene for PTC tasting (T) is dominant to non-tasting (t). A non-tasting woman who is a
carrier for colourblindness is married to a heterozygous tasting, normal man. What is the
chance that they will produce each of the following children?
a) a son
b) a non-tasting son
c) a non-tasting colourblind son
d) a tasting colourblind daughter
e) two tasting colourblind sons
Parent Genotypes:
x
Possible Gametes:
a)
b)
c)
d)
e)
There is a ______ or ___________chance of producing a son.
There is a ______ or ___________chance of producing a non-tasting son.
There is a ______ or ___________chance of producing a non-tasting, colourblind son.
There is a ______ or ___________chance of producing a tasting colourblind daughter.
There is a ______ or ___________chance of producing two tasting colourblind sons.
** Both parents would have to have the Xc allele to be able to have a daughter with
colourblindness.**
Hmwk. p. 200 # 3, 4, 10
X-Linked Inheritance
X-Linked Traits: Any traits controlled by genes on the X
chromosome are called X-linked traits.
Examples: Red/Green Colourblindness, Hemophilia (limited
blood clotting ability), and Duchenne Muscular Dystrophy.
X-linked inheritance results in a higher rate of incidence of
these traits in males. This is because a gene on the X
chromosome in the male has no matching allele on the Y
chromosome. Therefore, any gene on the X chromosome,
whether dominant or recessive, is expressed in males.
X-linked traits tend to skip generations.
Sample Genotypes and Phenotypes for Colourblindness
Males
Females
Genotypes
XY
XCY
Phenotypes
Normal male
Colourblind male
Genotypes
XX
XCX
XCXC
Phenotypes
Normal female
Normal female (carrier)
Colourblind female (very rare)
Example 1
A male with hemophilia and a female have children. What are the
genotypic and phenotypic ratios of their offspring?
Parent Genotypes:
Possible Gametes:
X
XhY x XX
Xh Y X
Xh Y
XhX XY
Genotypic Ratio: 1 XhX: 1 XY
Phenotypic Ratio: 1 normal female: 1 normal male
Example 2
The gene for PTC tasting (T) is dominant to non-tasting (t). A nontasting woman who is a carrier for colourblindness is married to a
heterozygous tasting, normal man. What is the chance that they will
produce each of the following children?
f) a son
g) a non-tasting son
h) a non-tasting colourblind son
i) a tasting colourblind daughter
j) two tasting colourblind sons
Parent Genotypes: ttXcX
Possible Gametes: tXc tX
TX
TY
tX
tY
tXc
TtXcX
TtXcY
ttXcX
ttXcY
x
TtXY
TX TY tX tY
tX
TtXX
TtXY
ttXX
ttXY
f) There is a 4/8 or 50% chance of producing a son.
g) There is a 2/8 or 25% chance of producing a non-tasting son.
h) There is a 1/8 or 12.5% chance of producing a non-tasting,
colourblind son.
i) There is a 0/8 or 0% chance of producing a tasting colourblind
daughter.
j) 1/8 x 1/8 = 1/64 or 1.6% chance of producing two tasting
colourblind sons.
** Both parents would have to have the Xc allele to be able to have a
daughter with colourblindness.**
Hmwk. p. 200 # 3, 4, 10