Human Biology
Sylvia S. Mader
Michael Windelspecht
Chapter 20
Patterns of
Genetic
Inheritance
Lecture Outline
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Points to Ponder
• What is the genotype and the phenotype of an individual?
• What are the genotypes for homozygous recessive and
dominant individuals and a heterozygote individual?
• Be able to draw a punnett square for any cross (1-trait cross,
2-trait cross, and a sex-linked cross).
• What are Tay-Sachs disease, Huntington disease, sickle-cell
disease, and PKU?
• How are each of the above inherited?
• What is polygenic inheritance?
• What is a multifactorial trait?
• What is sex-linked inheritance?
• Name 3 X-linked recessive disorders.
• What is codominance?
• What is incomplete dominance?
• What do you think about genetic profiling?
20.1 Genotype and phenotype
These traits are genetically inherited
Answer these questions about your inheritance:
• Do you have a widow’s peak?
• Are your earlobes attached or unattached?
• Do you have short or long fingers?
• Do you have freckles?
• Can you roll your tongue?
• Do you have Hitchhiker’s thumb?
20.1 Genotype and phenotype
Genotype
Genotype – specific genes for a particular trait written with
symbols
– Alleles: alternate forms of a specific gene at the same position
(locus) on a gene (e.g., allele for unattached earlobes and
attached lobes); alleles occur in pairs
– Dominant gene: will be expressed and will mask a recessive
gene (Tt or TT)
– Recessive allele: allele that is only expressed when a gene has
two of this type of allele
– Homozygous dominant genotype: 2 dominant alleles (TT or
AA)
– Homozygous recessive genotype: 2 recessive alleles (tt or aa)
– Heterozygous genotype: one dominant allele and one recessive
allele (Tt or Aa)
20.1 Genotype and phenotype
Phenotype
Phenotype – the physical or outward expression of
the genotype
Genotype
EE
Ee
ee
Phenotype
unattached earlobe
unattached earlobe
attached earlobe
What are your genotype and phenotype?
20.1 Genotype and phenotype
Understanding genotype & phenotype
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egg
E
e
E
ee
E
E
ee
sperm
fertilization
EE
ee
Ee
growth and
development
EE
unattached earlobe
ee
attached earlobe
Allele Key
E = unattached earlobes
e = attached earlobes
Ee
unattached earlobe
20.2 One-and Two-trait inheritance
What about your inheritance?
20.2 One-and Two-trait inheritance
Crosses
•
One-trait cross – considers the inheritance of one characteristic
e.g. WW x Ww
•
Two-trait cross – considers the inheritance of two characteristics
e.g. WWTT x
•
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WwTT
Gametes only carry one allele, so if an individual has the genotype Ww what are the possible
gametes that this individual can pass on?
Answer: either a W or a w but not both
Another example:
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Parents
no freckles
ff
no freckles
×
ff
meiosis
gametes
Offspring
f
f
ff
no freckles
20.2 One-and Two-trait inheritance
Punnett squares
•
Punnett squares
are the use of a
grid that diagram
crosses between
individuals by
using the possible
parental gametes
These allow one
to figure the
probability that an
offspring will have
a particular
genotype and
phenotype
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Parents
×
Ff
Ff
eggs
Sperm
•
F
f
F
f
FF
Ff
Ff
Offspring
ff
Key
F = Freckles
f = No freckles
Freckles
No freckles
Phenotypic Ratio
3:1
3
Freckles
1
No freckles
20.2 One-and Two-trait inheritance
Practicing punnett squares
F – freckles
f – no freckles
eggs
M/F
sperm
• What would a
punnett square
involving a man (M)
with a genotype Ff
and a woman (F)
with a genotype Ff
look like?
F
f
F
FF
Ff
f
Ff
ff
20.2 One-and Two-trait inheritance
Practicing ratios
•
Genotypic ratio: the number
of offspring with the same
genotype
eggs
•
Phenotypic ratio: the number
of offspring with the same
outward appearance
•
What is the genotypic ratio?
1: 2: 1 (1 FF: 2 Ff: 1 ff)
•
What is the phenotypic ratio?
3: 1 (3 with freckles and 1
with no freckles)
sperm
M/F
F
f
F
FF
Ff
f
Ff
ff
20.2 One-and Two-trait inheritance
Monohybrid crosses
Monohybrid cross – an experimental cross in which parents
are identically heterozygous at one gene pair (e.g., Aa x Aa)
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Parents
ww
Sperm
eggs
W
w
Ww
Ww
Key
W = Widow’s peak
w = Straight hairline
Widow’s peak
Straight hairline
Ww
Offspring
Ww
All
×
ww
eggs
Phenotypic Ratio
W
a.
w
Ww
w
w
W
Ww
Ww
w
ww
Sperm
×
WW
Parents
Widow’s peak
b.
Offspring
ww
Key
W = Widow’s peak
w = Straight hairline
Widow’s peak
Straight hairline
Phenotypic Ratio
1:1
Widow’s peak
1
Straight hairline
1
20.2 One-and Two-trait inheritance
Possible gametes for two traits
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Cell has
two pairs of
homologues.
Allele Key
one pair
W = Widow’s peak
w = Straight hairline
S = Short fingers
s = Long fingers
one pair
either
or
MEIOSIS I
S
Ss
s
S
Ss
s
W
W
w
w
w
W
w
W
MEIOSIS II
S
W
S
S
s
s
S
S
W
w
w
w
w
S
W
s
W
WS
s
w
S
w
ws
s
W
S
w
W
s
w
wS
s
s
W
W
Ws
20.2 One-and Two-trait inheritance
Dihybrid cross (a type of two-trait
cross)
• Dihybrid cross – an
experimental cross
usually involving parents
that are homozygous for
different alleles of two
genes and results in a
9:3:3:1 genotypic ratio for
the offspring
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×
P generation
wwss
WWSS
P gametes
ws
WS
F1generation
WwSs
20.2 One-and Two-trait inheritance
Practicing a punnett square for a 2-trait
cross
eggs
F1gametes
WS
Ws
wS
ws
WWSS
WWSs
WwSS
WwSs
WWSs
WWss
WwSs
Wwss
wwSS
wwSs
WS
F1generation
Ws
sperm
• What would the
punnett square look
like for a dihybrid
cross between a male
that is WWSS and a
female that is wwss?
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wS
WwSS
ws
WwSs
Wwss
wwSs
Offspring
Allele Key
W = Widow’s peak
W = Straight hairline
S = Short fingers
S = Long fingers
9
3
3
1
Phenotypic Ratio
9:3:3:1
Widow’s peak, short fingers
Widow’s peak, long fingers
Straight hairline, short fingers
Straight hairline, long fingers
20.3 Inheritance of genetic disorders
Autosomal recessive disorder
• Individuals must be homozygous recessive to
have the disorder
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I
aa
II
III
IV
A?
A?
Aa
*
Aa
A?
relatives
Aa
Aa
aa
aa
Autosomal recessive disorders
• Affected children can have
unaffected parents.
A?
A?
Key
A?
aa = affected
Aa = carrier (unaffected)
AA = unaffected
A? = unaffected
(one allele unknown)
• Heterozygotes (Aa) have an unaffected phenotype.
• Two affected parents will always have affected children.
• Affected individuals with homozygous unaffected mates will have
unaffected children.
• Close relatives who reproduce are more likely to have
affected children.
• Both males and females are affected with equal frequency.
20.3 Inheritance of genetic disorders
Autosomal dominant disorder
• Individuals that are homozygous dominant and
heterozygous will have the disorder
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Aa
Aa
I
*
II
III
Aa
aa
Aa
Aa
A?
aa
Autosomal dominant disorders
aa
aa
aa
aa
aa
aa
Key
AA = affected
Aa = affected
A? = affected
(one allele unknown)
aa = unaffected
• Affected children will usually have
an affected parent.
• Heterozygotes (Aa) are affected.
• Two affected parents can produce an unaffected child.
• Two unaffected parents will not have affected children.
• Both males and females are affected with equal frequency.
20.3 Inheritance of genetic disorders
Genetic disorders of interest
• Tay-Sachs disease: lack of the enzyme that breaks down lipids in
lysosomes resulting in excess and eventually death of a baby
• Cystic fibrosis: Cl- do not pass normally through a cell membrane
resulting in thick mucus in lungs and other places often causing
infections
• Phenylketonuria (PKU): lack of an enzyme needed to make a certain
amino acid and affects nervous system development
• Sickle-Cell disease: red-blood cells are sickle shaped rather than
biconcave that clog blood vessels
• Huntington disease: huntington protein has too many glutamine
amino acids leading to the progressive degeneration of brain cells
20.3 Inheritance of genetic disorders
Genetic disorders
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
H2O
Cl–
Cl–
Cl
H2O
Cl–
H2O
Cl
nebulizer
many neurons in
normal brain
defective
channel
percussion
vest
loss of neurons in
huntington brain
(both brain tissue slides): Courtesy Dr. Hemachandra Reddy, The Neurological Science Institute, Oregon Health
& Science University; (woman with Huntington): © Steve Uzzell
thick mucus
© Pat Pendarvis
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20.4 Beyond simple inheritance patterns
Polygenic inheritance
• Polygenic traits - two or more sets of alleles govern one
trait
– Each dominant allele codes for a product so these effects are
additive
– Results in a continuous variation of phenotypes
– Environmental effects cause intervening phenotypes
– e.g., skin color ranges from very dark to very light
– e.g., height varies among
• Multifactorial trait – a polygenic trait that is particularly
influenced by the environment
– e.g., skin color is influenced by sun exposure
– e.g., height can be affected by nutrition
20.4 Beyond simple inheritance patterns
Polygenic inheritance
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Genotypes
Phenotypes
AABB
AABb or AaBB
Very dark
AaBb or Aabb or aaBB
Medium brown
Aabb or aaBb
Light
aabb
Very light
Dark
Number of Men
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most
are
this
height
few
62
short
64
few
66
68
70
Height in Inches
Courtesy University of Connecticut/Peter Morenus, photographer
72
74
tall
20.4 Beyond simple inheritance patterns
Demonstrating environmental
influences on phenotype
• Himalayan rabbit’s coat
color influenced by
temperature
• There is an allele
responsible for melanin
production that appears
to be active only at lower
temperatures
• The extremities have a
lower temperature and
thus the ears, nose,
paws, and tail are dark in
color
20.4 Beyond simple inheritance patterns
Incomplete dominance
• Occurs when the heterozygote is
intermediate between the 2 homozygotes
• Example:
(curly hair) CC
x
SS (straight hair)
CS (wavy hair)
20.4 Beyond simple inheritance patterns
Codominance
• Occurs when the alleles are equally
expressed in a heterozygote
• Example:
(Type A blood) AA
x
BB (Type B blood)
AB (Type AB blood that has characteristics
of both blood types)
20.4 Beyond simple inheritance patterns
Multiple allele inheritance
• The gene exists in several allelic forms
• A person only has 2 of the possible alleles
• A good example is the ABO blood system
• A and B are codominant alleles
• The O alleles is recessive to both A and B therefore to
have this blood type you must have 2 recessive alleles
20.4 Beyond simple inheritance patterns
Multiple allele inheritance
Based on what you know what type of blood
would each of the following individuals have in a
cross between Ao and Bo?
possible genotypes:
AB
Bo
Ao
oo
phenotypes:
Type AB blood
Type B blood
Type A blood
Type O blood
20.4 Beyond simple inheritance patterns
Blood type inheritance
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Parents
×
IBi
IAi
sperm
eggs
IB
i
IB
IAIB
I Bi
i
IAi
ii
Offspring
Key
Blood type A
Blood type B
Blood type AB
Blood type O
Phenotypic Ratio
1:1:1:1
1
1
1
1
20.5 Sex-linked inheritance
Sex-linked inheritance
• Traits are controlled by genes on the sex
chromosomes
 X-linked inheritance: the allele is carried on
the X chromosome
 Y-linked inheritance: the allele is carried on
the Y chromosome
 Most sex-linked traits are X-linked
20.5 Sex-linked inheritance
X-linked inheritance: Color blindness
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Parents
XBY
Possible offspring:
XBXB normal vision female
XBXb normal vision female
XBY normal vision male
XbY normal vision male
XBY
×
XBXb
eggs
XB
XB
Xb
XBXB
XBXb
Key
XB = Normal vision
Xb = Color-blind
Normal vision
Color-blind
Phenotypic Ratio
Females All
sperm
Cross:
XBXb x
Y
XBY
Offspring
XbY
Males 1:1 1
1
20.5 Sex-linked inheritance
X-linked disorders
• More often found in males than females
because recessive alleles are always expressed
• Most X-linked disorders are recessive:
– Color blindness: most often characterized by redgreen color blindness
– Muscular dystrophy: characterized by wasting of
muscles and death by age 20
– Fragile X syndrome: most common cause of inherited
mental impairment
– Hemophilia: characterized by the absence of
particular clotting factors that causes blood to clot
very slowly or not at all
20.5 Sex-linked inheritance
X-linked disorders
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XBXB
XBY
XbY
XBXb daughter
grandfather
XBY
XbXb
XbY
XBY
XBXB
XBXb
XbY
grandson
Key
XBXB = Unaffected female
XBXb = Carrier female
XbXb = Color-blind female
XBY = Unaffected male
XbY = Color-blind male
X-linked Recessive
Disorders
• More males than females are affected.
• An affected son can have parents who have the
normal phenotype.
• For a female to have the characteristic, her father
must also have it. Her mother must have it or be a
carrier.
• The characteristic often skips a generation from the
grandfather to the grandson.
• If a woman has the characteristic, all of her sons will
have it.
20.5 Sex-linked inheritance
X-linked disorders: Hemophilia
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Unaffected female
Unaffected male
Victoria Edward
Carrier female
Hemophiliac male
Albert
Victoria
1
Alice Louis IV
3
Alexandra
?
Olga
2
Nicholas II
?
?
10
4
Leopold
Beatrice
Mary
?
7
?
Helena
8
?
Marie
Alexi
Tatiana
Anastasia
?
9
?
?
?
Juan Carlos
5
All were assassinated
6
1. Victoria
2. Edward VII
3. Irene
4. George V
6. Margaret
7. Victoria
8. Alfonso XIII
9. Juan
Philip Elizabeth II
12
William
11
13
16
14
15
Harry
(queen): © Stapleton Collection/ Corbis; (prince): © Huton Archive/Getty Images
10. Alexandra
11. Charles
12. Diana
13. Andrew
14. Edward
15. Anne
16. Sarah