Human Biology
Sylvia S. Mader
Michael Windelspecht
Chapter 20
Patterns of
Genetic
Inheritance
Lecture Outline
Part 1
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Patterns of Genetic
Inheritance
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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 heterozygous individual?
• Be able to draw a Punnett square for a one-trait cross,
two-trait cross, and sex-linked cross.
• What are Tay-Sachs disease, Huntington disease, sicklecell disease, and PKU? How are each of these
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 preimplantation genetic testing?
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20.1 Genotype and Phenotype
These traits are genetically inherited
Answer these questions about your inheritance.
• Do you have a widow’s peak or a straight
hairline?
• Are your earlobes attached or unattached?
• Do you have short or long fingers?
• Do you have freckles?
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20.1 Genotype and Phenotype
Genotype
Genotype – specific genes for a particular trait
written with _________
– ________ are 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.
– A ________ gene will be expressed and will
mask a recessive gene (Tt or TT).
– A ________ allele is only expressed when a
gene has 2 of this type of allele.
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20.1 Genotype and Phenotype
Genotype
– A ___________________ genotype consists
of 2 dominant alleles (TT or AA).
– A ___________________ genotype consists
of 2 recessive alleles (tt or aa).
– A _______________ genotype consists of 1
dominant allele and 1 recessive allele (Tt or
Aa).
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20.1 Genotype and Phenotype
Phenotype
Phenotype – the ________________ expression
of the genotype
Genotype
EE
Ee
ee
Phenotype
unattached earlobe
unattached earlobe
attached earlobe
What are your genotype and phenotype?
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20.1 Genotype and Phenotype
Understanding genotype and 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
Ee
unattached earlobe
Allele Key
Figure 20.1 Genetic inheritance
affects our characteristics.
E = unattached earlobes
e = attached earlobes
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20.2 One- and Two-Trait Inheritance
What about your inheritance?
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a. Widow’s peak: WW or Ww
b. Straight hairline: ww
c. Unattached earlobes: EE or Ee d. Attached earlobes: ee
e. Shortfingers: SS or Ss
Figure 20.2 Common inherited traits
in humans.
f. Long fingers: ss
g. Freckles: FF or Ff
h. No freckles: ff
a: © HFPA, 63rd Golden Globe Awards; b: © Dynamic Graphics/ PictureQuest RF;
c–f: © The McGraw-Hill Companies, Inc. Bob Coyle, photographer;
g: © Jupiterimages/ Getty RF; h: © Creatas/PunchStock RF
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20.2 One-and Two-Trait Inheritance
Crosses
• One-trait cross – considers the inheritance of 1
characteristic
e.g. WW x Ww
• Two-trait cross – considers the inheritance of 2
characteristics
e.g. WWTT x WwTT
• Gametes only carry 1 allele, so if an individual
has the genotype Ww, what are the possible
gametes that this individual can pass on?
Answer:
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20.2 One-and Two-Trait Inheritance
Crosses
Another example:
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Parents
no freckles
ff
no freckles
×
ff
meiosis
gametes
Offspring
f
f
ff
no freckles
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20.2 One- and Two-Trait Inheritance
Punnett squares
• Punnett squares are the use of a grid to
diagram crosses between individuals by using
the ___________________.
• These allow one to determine the __________
that an offspring will have a particular
genotype and phenotype.
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20.2 One- and Two-Trait Inheritance
Punnett squares
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Parents
×
Ff
Ff
eggs
f
FF
Ff
Sperm
F
F
f
Ff
ff
Key
F = Freckles
f = No freckles
Freckles
No freckles
Phenotypic Ratio
3:1
3
Freckles
1
No freckles
Offspring
Figure 20.3 Expected results of a monohybrid cross.
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20.2 One- and Two-Trait Inheritance
Practicing Punnett squares
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
F – freckles
f – no freckles
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