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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 equally
Inheritance of Traits
• Sex-linked- when a gene is sex-linked, it is
found only on the X chromosome so the effects
are seen in males more often than females
– 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:
X
X
Xb
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:
X
Xb
y
Xb
Key:
XX- female
Xy- male
Xby- colorblind male
XXb- female carrier
XbXb- colorblind female
What are the offspring?
Sex linked:
• Sometimes you can write sex linked
genotypes like this:
(N = normal; n= abnormal)
• XNXN
• XNXn
• XnXn
XNy
Xny
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
Pedigree Key
•
•
•
•
Circle- female
Square- male
Clear circle/square- does not show trait/disorder
Shaded circle/square- has trait disorder
Carries can be displayed several different ways:
– half will be shaded
– it will be clear with a line through the middle
– it will be clear
• Horizontal line between circle and square indicates
two married people
• Vertical line extending from a parent line leads to
offspring
– The order of children is left to right
Pedigrees
• Pedigree for albinism
• (autosomal):
(P1)
Key:
Female
Male
Female carrier
(F1)
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 (autosomal):
(P1)
B
A
(F1)
D
C
E
F
G
H
(F2)
I
J
K
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?
Pedigree for colorblindness
(sex-linked- X-linked Recessive Trait)
A
F
X
1.
2.
3.
4.
5.
6.
How many males are colorblind?
How many females are colorblind?
What is the genotype for square A?
What is the genotype for circle F?
Is circle F colorblind?
What is the genotype for circle X?
Complex Patterns of Heredity
• We have been talking about “Mendelian
Genetics” where the phenotype of one
allele dominants over another
• Now were are going to talk about “Non
Mendelian Genetics”
– Intermediate inheritance
• Polygenic inheritance
• Incomplete dominance
• Codominance
– Multiple alleles
Intermediate Inheritance
• Intermediate inheritance- when the
heterozygous offspring (Bb, Rr, etc)
display a trait that is not exactly like either
parent
– One trait DOES NOT simply dominate over
the other
Polygenic Inheritance
• Polygenic
inheritance
– Many genes
contribute to the
phenotype
– Provides genetic
variation
– Examples:
• Skin color, hair color,
height
Incomplete domiance
• Incomplete dominance- when an individual
displays an intermediate/middle phenotype of the
parents (neither is dominate or recessive)
– Ex. A red (RR) snapdragon flower is crossed with a
white (rr) one and their offspring are pink (Rr)
R
R
r
Rr
Rr
r
Rr
Rr
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?
Incomplete Domiance
• Cross a red flower with a pink flower
R
R
R
RR
RR
r
Rr
Rr
• What happens?
Codomiance
• Codominance- when two dominant alleles
are expressed at the same time
– Ex. Roan horses or cattle
• have both red AND white hair
Codominance Practice
• Assume:
– Red= RR
– White = rr or WW or R’R’
– Red and White= Rr or RW or RR’
You cross a red horse with a white horse:
What happens?
Codominance
• Another example of codominance is
blood type
• If someone has AB blood that means
they have both A blood AND B blood
A
A
B
AB
AB
B
AB
AB
Multiple Alleles
• Multiple alleles- more than 2 forms of a
gene are expressed
• Example- rabbit coat color
– It isn’t just black OR brown; brown OR white
– Rabbit fur could be brown, black, white, or a
combination of black and white
Multiple Alleles
• Another common example of multiple
alleles is blood type
– IAIA = homozygous dominant A blood
– IAIO = heterozygous dominant A blood
– IBIB = homozygous dominant B blood
– IBIO = heterozygous dominant B blood
– IAIB = heterozygous but has both A and B
blood (is also codominant)
– iOiO = homozygous recessive
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 (IAIA or IAiO)
Type B
BB or BO (IBIB or IBiO)
Type AB
AB (IAIB)
Type O
OO (iOiO)
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 (IAIA or IAiO)
Type B
BB or BO (IBIB or IBiO)
Type AB
AB (IAIB)
Type O
OO (iAiB)