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Inheritance of Traits
Probability
Carriers
Autosomal
Sex-linked
Pedigrees
Complex patterns of heredity
Incomplete dominance
Codominance
Multiple Alleles
Practice
Inheritance of Traits
• Probability- the likelihood that a
specific event will occur
– When you flip a coin, there is a
50/50 chance it will land tails side
up
– If there are 4 kids in a family,
probability says 2 will be girls and
the other 2 will be boys
Inheritance of Traits
• Carrier- an individual who is heterozygous for an
inherited disorder but does not show symptoms.
– Ex. Linda is a carrier for albinism.
• Genotype-(Aa)
• Phenotype- not albino
• Even though she does not display the symptoms,
she can still pass on albinism to her children.
Inheritance of Traits
• Autosomal- when a gene is autosomal, it
will appear in both sexes
• Sex-linked- when a gene is sex-linked, it is
found only on the X or Y chromosomes so
the effects are only seen in males.
– Remember, males are (Xy) and females are
(XX)
– Examples- colorblindness, hemophilia, Cystic
Fibrosis and Sickle Cell Anemia
Inheritance of Traits
• Sex-linked Punnett Square for colorblindness:
X
Xb
X
Key:
XX- female
Xy- male
Xby- colorblind male
y
XXb- female carrier
XbXb- colorblind female
What are the offspring?
Inheritance of Traits
• Sex-linked Punnett Square for colorblindness:
Xb
X
X
Key:
XX- female
Xy- male
Xby- colorblind male
y
XXb- female carrier
XbXb- colorblind female
What are the offspring?
Inheritance of Traits
• Sex-linked Punnett Square for colorblindness:
Xb
Xb
y
X
Key:
XX- female
Xy- male
Xby- colorblind male
XXb- female carrier
XbXb- colorblind female
What are the offspring?
Pedigrees
• Pedigree- a family history that shows how
a trait is inherited over several generations
– Useful to track the carriers of genetic
disorders in families
– Can be used for both autosomal and sexlinked disorders
Pedigrees
Pedigree for albinism:
Key:
Female
(P1)
Male
(F1)
Female carrier
Male carrier
(F2)
Female with albinism
Male with albinism
*The order of children is from left to right
Married
with children
Pedigrees
• Pedigree for albinism:
A
B
(P1)
C
D
E
F
G
H
(F1)
I
J
K
(F2)
1. Which individuals are heterozygous for albinism?
2. Which are homozygous for albinism?
3. Which individuals have albinism?
4. Which generations have an individual with albinism?
Complex Patterns of Heredity
• Polygenic inheritance- when several genes
influence a characteristic
– Incomplete dominance
– Codominance
– Multiple alleles
Complex Patterns of Heredity
• Incomplete dominance- when an individual
displays an intermediate/middle phenotype of the
parents
– Ex. A red (RR) snapdragon flower is crossed with a
white (rr) one and their offspring are pink (Rr)
Complex Patterns of Heredity
• Codominance- when two dominant
alleles are expressed at the same time
– Ex. Blood types- AB blood
A
A
B
AB
AB
B
AB
AB
Complex Patterns of Heredity
• Codominance
– Ex. Roan horses (have both red and white
hair)
Complex Patterns of Heredity
• Multiple alleles- genes with 3 or more
possible alleles
– Ex. Blood types- A, B, O (AB- codominant)
Practice
Blood Type
• If a man is homozygous for type B blood, and
has a child with a homozygous woman with type
A blood, what are the possible blood types of
the child?
Phenotype
Genotype
Type A
AA or AO
Type B
BB or BO
Type AB
AB
Type O
OO
Practice
Blood type
• A man is heterozygous for type A blood and has
a child with a woman that has AB blood. What
are the possible blood types of the child?
Phenotype
Genotype
Type A
AA or AO
Type B
BB or BO
Type AB
AB
Type O
OO
Practice
Incomplete dominance
• Assume:
– Red= RR
– White = rr or WW or R’R’
– Pink= Rr or RW or RR’
You cross a red flower with a white flower:
What happens?