Human Biology
Sylvia S. Mader
Michael Windelspecht
Chapter 20
Patterns of
Genetic
Inheritance
Lecture Outline
Part 4
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
1
20.4 Beyond Simple Inheritance Patterns
Incomplete dominance
• Occurs when the heterozygote phenotype
is intermediate between phenotypes of the
2 homozygotes
• Example:
(curly hair) CC
x
SS (straight hair)
CS (wavy hair)
2
20.4 Beyond Simple Inheritance Patterns
Familial hypercholesterolemia
• 2 _________ alleles lack LDL-cholesterol receptors.
• 1 mutated allele has half the normal number of
receptors.
• 2 normal alleles have the usual number of receptors.
• When receptors are completely absent, excessive
cholesterol is deposited in various places in the body,
including under the skin.
3
20.4 Beyond Simple Inheritance Patterns
Familial hypercholesterolemia
Plasma cholesterol (milligrams/deciliter)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Homozygote
1000
900
800
700
600
Heterozygote
500
400
300
Normal
cholesterol
deposits
200
100
0
© Mediscan/ Medical-On-Line
Figure 20.17 The inheritance of familial hypercholesterolemia.
4
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)
5
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 ___________________.
• A and B are ______________ alleles.
• The O allele is recessive to both A and B;
therefore, to have this blood type, you must have
2 recessive alleles.
6
20.4 Beyond Simple Inheritance Patterns
Multiple allele inheritance
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
7
20.4 Beyond Simple Inheritance Patterns
Blood type inheritance
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Parents
×
IBi
IAi
sperm
eggs
IA
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
8
Figure 20.18 The inheritance of ABO blood types.
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.
9
20.5 Sex-Linked Inheritance
X-linked inheritance: Color blindness
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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
XbY
Males 1:1 1
1
Offspring
Figure 20.19 Results of an X-linked cross.
10
20.5 Sex-Linked Inheritance
X-linked disorders
• These are more often found in ________ than
__________ because recessive alleles are
always expressed.
• Most X-linked disorders are recessive.
– Color blindness is most often characterized by redgreen color blindness.
– Duchenne muscular dystrophy is characterized by
wasting of muscles and death by age 20.
– Fragile X syndrome is the most common cause of
inherited mental impairment .
– Hemophilia is characterized by the absence of
particular clotting factors; blood clots very slowly or
not at all.
11
20.5 Sex-Linked Inheritance
Duchenne muscular dystrophy
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
fibrous
tissue
abnormal
muscle
normal
tissue
(left, right): Courtesy Dr. Rabi Tawil, Director, Neuromuscular Pathology Laboratory, University of Rochester Medical
Center; (center): Courtesy Muscular Dystrophy Association
Figure 20.21 Muscular dystrophy.
12
20.5 Sex-Linked Inheritance
X-linked disorders
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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
Figure 20.20 X-linked
recessive disorder pedigree.
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.
13
20.5 Sex-Linked Inheritance
X-linked disorders: Hemophilia
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Unaffected male
Unaffected female
Victoria Edward
Hemophiliac male
Carrier female
Victoria
1
Alice
Louis IV
3 Alexandra
?
Olga
Albert
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
Kate
Figure 20B The royal
families’ X-linked
pedigree.
William
11
13
16
14
15
10. Alexandra
11. Charles
12. Diana
13. Andrew
14. Edward
15. Anne
16. Sarah
Harry
(queen): © Stapleton Collection/Corbis; (prince): © Huton Archive/Getty Images
14