LESSON #1.8:
SEX-LINKED TRAITS,
PEDIGREE CHARTS,
MULTIPLE ALLELES
PART A: SEX-LINKED TRAITS
• Sex-linked traits are controlled by genes located on the
sex chromosomes.
• A recessive trait located on the X chromosome is more
likely to express itself in males
A = Dominant
a = recessive
• male genotypes:
XAY, XaY
• female genotypes:
XAXA, XAXa, XaXa
PART A: SEX-LINKED TRAITS
• A “Recessive lethal X-linked disorder” is a trait that
when both recessive alleles are present it results in
death/malformation of offspring (occurs more often in
males)
SAMPLE PROBLEM 1:
In humans, the recessive allele that causes a form of
red-green colour-blindness ( c ) is found on the X
chromosome.
• Identify the phenotypes and genotypes of the F1
generation from a colour-blind father and a mother
who is homozygous for perfect vision.
• Trait: Colour-blindness in humans
• C = NOT colour blind
• c = Colour blind
SAMPLE PROBLEM 1:
• Parent phenotypes: Colour-blind x not colour-blind
• Parent genotypes:
• Parent gametes:
XcY
Xc
Y
x
x
XCXC
XC
XC
SAMPLE PROBLEM 1:
F1
XC
XC
Xc
Y
SAMPLE PROBLEM 1:
F1
Xc
Y
XC
XCXc
XCY
XC
XCXc
XCY
• F1 Phenotypes: 100% of daughters are NOT colour-blind
100% of sons are NOT colour-blind
• F1 Genotypes: 100% of daughters are XCXc
100% of sons are XCY
SAMPLE PROBLEM 2:
• Identify the phenotypes and genotypes of the F1
generation from a father who has perfect vision and a
mother who is heterozygous for colour-blindness.
• Parent phenotypes: not colour-blind x not colour-blind
• Parent genotypes:
• Parent gametes:
XCY
XC
Y
x
x
XCXc
XC
Xc
SAMPLE PROBLEM 1:
F1
XC
Xc
XC
Y
SAMPLE PROBLEM 1:
F1
XC
Y
XC
XCXC
XCY
Xc
XCXc
XcY
• F1 Phenotypes: 100% of females have normal vision
50% of sons are colour-blind, 50% have
normal vision
• F1 Genotypes: 50% of females are X CXC, 50% females XCXc
50% of males are XCY, 50% males XcY
• WORKSHEET
PART B: PEDIGREE CHARTS
• Pedigree charts are constructed to show the
inheritance of genetic conditions within generations of
a family
Fraternal twins 
Identical twins 
2
1
1
I
2
II
3
4
1
5
2
3
III
• Pedigree charts help in determining whether a trait is
controlled by an autosomal dominant, autosomal
recessive, or sex-linked allele
• Autosomal dominant - if both parents have the trait
and the offspring do not (“carriers” are not possible)
AA-affected; Aa-affected; aa-normal
• Autosomal recessive -if neither parent have the trait
but some of their offspring do
AA-normal; Aa-carrier; aa-affected
• Pedigree charts help in determining whether a trait is
controlled by an autosomal dominant, autosomal
recessive, or sex-linked allele
• Sex-linked- if only females are “carriers”; males and
females show the trait unevenly
XAXA- normal; XAXa-carrier; XaXa-affected
XAY-normal; XaY-affected
PEDIGREE CHART FOR THE SICKLE-CELL
ANEMIA DISORDER:
PEDIGREE CHART FOR THE SICKLE-CELL
ANEMIA DISORDER:
Conclusion: What can you determine about a family,
using a pedigree chart?
• # of generations/individuals
• # of carriers
• phenotypes/genotypes
• # of individuas
• whether it is autosomal dominant or recessive
• WORKSHEET
PART C: MULTI-ALLELISM
• Some traits are controlled by two or more different
alleles. This gives more possible phenotypes.
**not to be confused with dihybrid crosses which
involves two different traits; each controlled by their
own alleles!
ABO BLOOD TYPING
• Your blood type is established before you are born, by
genes inherited from your parents; you receive one
blood type allele from each parent.
• Your blood type is determined by alleles coding for the
presence or absence of the Type A and Type B antigen
molecules on the red blood cells.
ABO BLOOD TYPING
• the gene coding for blood type has three different alleles:
IA = A antigen on the red blood cells
IB = B antigen on the red blood cells,
i
= has neither antigen
• If everyone has two copies of these genes, there
are six possible combinations (genotypes) :
ABO BLOOD TYPING
IA IA
IA i
both resulting in Type A blood
• IB IB
• IB i
both resulting in Type B blood
IA IB  resulting in Type AB blood
(Type A & B are codominant)
• i i  resulting in Type O blood
(recessive)
ABO BLOOD TYPING
** A & B are codominant with each other, but
dominant over i
SAMPLE PROBLEM 1:
Suppose that a mother has blood Type A ( IA i ) and the
father has blood Type B ( IB i ). Determine the possible
genotypes and blood types (phenotypes) for their
children
TRAIT: blood types
• Parent phenotypes:
(mother)
Type A
• Parent genotypes:
• Parent gametes:
IAi
IA
i
x
x
x IB
(father)
Type B
IBi
i
SAMPLE PROBLEM 1:
IA
IB
i
i
SAMPLE PROBLEM 1:
IA
i
IB
IAIB
IBi
i
IAi
ii
F1 Phenotypes: 25% Type AB; 25% Type A; 25% Type B
25% Type O
F1 Genotypes: 25% IAIB; 25% IAi (heterozygous);
25% IBi (heterozygous); 25% i i
(homozygous
recessive)
THE RHESUS FACTOR
The rhesus factor is another antigen discovered on
red blood cells; two possible alleles:
• Rh-positive (Rh+) is dominant
(~ 85% of Canadians have this antigen)
• Rh-negative (Rh-) is recessive
(15%)
SAMPLE PROBLEM 2:
For human blood type, the alleles for types A and B are
codominant, but both are dominant over the type O
allele. The Rh factor is separate from the ABO blood
group and is located on a separate chromosome. The
Rh+ allele is dominant to Rh-.
• Indicate the possible phenotypes from the mating of a
woman, type,O, Rh-, with a man, type A, Rh+ (both
homozygous)
SAMPLE PROBLEM 2:
TRAIT: blood types and rhesus factor
(father)
• Parent phenotypes: Type A +
• Parent genotypes:
IA IA++
x
x
• Parent gametes: IA+ IA+ IA+ IA+ x
(mother)
Type Oi i- i- i- i- i-
SAMPLE PROBLEM 1:
IA+
iiii-
IA+
IA+
IA+
SAMPLE PROBLEM 1:
F1 Phenotypes:
100% Type A +
IA+
IA+
IA+
IA+
i-
IAi +-
IAi +-
IAi +-
IAi +-
i-
IAi +-
IAi +-
IAi +-
IAi +-
i-
IAi +-
IAi +-
IAi +-
IAi +-
i-
IAi +-
IAi +-
IAi +-
IAi +-
F1 Genotypes:
100% IAi +**These will be DIHYBRID Crosses!!!
Video and…
PRACTICE TIME!