Download Chapt20 Lecture 13ed Pt 2

Human Biology
Sylvia S. Mader
Michael Windelspecht
Chapter 20
Patterns of
Genetic
Inheritance
Lecture Outline
Part 2
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1
20.2 One- and Two-Trait Inheritance
Practicing ratios
• _____________ is the number of offspring
with the same genotype.
• _____________ is the number of offspring
with the same outward appearance.
2
20.2 One- and Two-Trait Inheritance
Practicing ratios
• What is the phenotypic
ratio?
3: 1 (3 with freckles
and 1 with no freckles)
eggs
M/F
sperm
• What is the genotypic
ratio?
1: 2: 1 (1 FF: 2 Ff: 1 ff)
F
f
F
FF
Ff
f
Ff
ff
3
20.2 One- and Two-Trait Inheritance
Monohybrid crosses
Monohybrid cross – an experimental cross in which
parents are identically heterozygous at 1 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.
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
Offspring
Figure 20.4 Determining if a dominant phenotype is homozygous or heterozygous.
4
20.2 One- and Two-Trait Inheritance
Possible gametes for 2 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
Figure 20.5 Meiosis results in genetic diversity of gametes.
s
W
S
w
W
s
w
wS
s
s
W
W
Ws
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20.2 One- and Two-Trait Inheritance
Dihybrid cross (a type of twotrait cross)
• Dihybrid cross – an
experimental cross
usually involving
parents who are
______________ for
different alleles of 2
genes
×
P generation
wwss
WWSS
P gametes
ws
WS
F1generation
WwSs
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– Results in a 9:3:3:1
genotypic ratio for the
offspring
Figure 20.6 Expected results of a dihybrid cross.
6
20.2 One- and Two-Trait Inheritance
Punnett square for a dihybrid cross
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WWSS
P gametes
wwss
ws
WS
F1 generation
WwSs
oocytes
WS
F1 gametes
Ws
wS
ws
WS
F2 generation
WWSS
WWSs
WwSS
WwSs
WWSs
WWss
WwSs
Wwss
WwSS
WwSs
wwSS
wwSs
Wwss
wwSs
wwss
Ws
sperrm
• What would the
Punnett square look
like for a dihybrid
cross between a
male who is WWSS
and a female who is
wwss?
P generation
wS
ws
WwSs
Offspring
Allele Key
Figure 20.6 Expected results of a dihybrid cross.
W = Widow’s peak
w = Straight hairline
S = Short fingers
s = Long fingers
Phenotypic Ratio
9
3
3
1
Widow’ speak, short fingers
Widow’ speak, long fingers
Straight hairline, short fingers
Straight hairline, long fingers
7
20.2 One- and Two-Trait Inheritance
Two-trait cross
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Parents
×
WwSs
wwss
eggs
ws
WS
WwSs
sperm
Ws
Wwss
wS
wwSs
ws
wwss
Offspring
Key
W
w
S
s
= Widow’s peak
= Straight hairline
= Short fingers
= Long fingers
Phenotypes
9
3
3
1
Widow’ speak, short fingers
Widow’ speak, long fingers
Straight hairline, short fingers
Straight hairline, long fingers
Figure 20.7 Two-trait cross.
8
20.2 One- and Two-Trait Inheritance
Phenotypic ratios of common crosses
Table 20.1 Phenotypic ratios of common crosses
9
20.2 One- and Two-Trait Inheritance
Preimplantation genetic diagnosis
• If prospective parents carry an allele for a genetic
disorder, they may seek assurance that their offspring
will be free of the disorder
• Following in vitro fertilization (IVF), the zygote divides.
• When the embryo has 8 cells, 1 may be removed for
genetic testing.
• Only embryos that will not have the genetic disorders
of interest are placed in the uterus to continue
developing.
10
20.2 One- and Two-Trait Inheritance
Preimplantation genetic diagnosis
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Woman is heterozygous.
8-cell embryo
egg
Polar body
has
genetic
defect.
Embryonic cell
is removed.
Egg is genetically healthy.
egg nucleus
sperm nucleus
Cell is genetically
healthy.
Embryo develops
normally in uterus.
a. Testing the embryo
Embryo develops
normally in uterus.
b. Testing the egg
(both): © Brand X/SuperStock RF
Figure 20A The process of preimplantation genetic diagnosis.
11
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.
Figure 20.8 Autosomal
recessive disorder
pedigree.
• Close relatives who reproduce are more likely to have
affected children.
• Both males and females are affected with equal frequency.
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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
Figure 20.9
Autosomal
dominant disorder
pedigree.
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.
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