Transcript Chap 12 PP

Biology
A Guide to the Natural World
Chapter 12 • Lecture Outline
Units of Heredity: Chromosomes and Inheritance
Fifth Edition
David Krogh
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12.1 X-linked Inheritance in Humans
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X-linked Inheritance
• Certain human conditions, such as redgreen color blindness and hemophilia, are
called X-linked conditions.
• They stem from a variant form of gene
(an allele) that is dysfunctional and that is
located on the X chromosome.
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X-linked Inheritance
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Figure 12.1
X-linked Inheritance
• Men are more likely than women to suffer
from these conditions because men have
only a single X chromosome.
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X-linked Inheritance
• A woman with a dysfunctional bloodclotting allele on one of her X chromosomes
usually will be protected from hemophilia
by a functional allele on her second X
chromosome.
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X-linked Inheritance
• Hemophilia and red-green color blindness
are examples of recessive genetic
conditions, meaning conditions that will not
exist in the presence of even a single
functional allele.
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X-linked Inheritance
• Given the nature of recessive genetic
conditions, persons who do not themselves
suffer from such conditions may still
possess an allele for it, which they can pass
on to their offspring.
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X-linked Inheritance
mother not
color-blind
functional redgreen allelles
X
X
nonfunctional redgreen allelles
egg
X
father not
color-blind
XX
XX
daughters are
not color-blind
XY
XY
one son is
color-blind
sperm
Y
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Figure 12.2
X-linked Inheritance
• Such persons, referred to as carriers, are
heterozygous for the condition.
• The alleles they have for the trait differ: one
is functional, the other is dysfunctional.
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X-linked Inheritance
Animation 12.1: X-linked Recessive Traits
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12.2 Autosomal Genetic Disorders
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Autosomal Genetic Disorders
• Sickle-cell anemia is an example of an
autosomal recessive disorder.
• It is autosomal because the genetic defect
that brings it about involves neither the X
nor Y chromosome.
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Autosomal Genetic Disorders
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Figure 12.3
Autosomal Genetic Disorders
• It is recessive because persons must be
homozygous for the sickle-cell allele to
suffer from the condition—they must have
two alleles that code for the same sickle-cell
hemoglobin protein.
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Autosomal Genetic Disorders
• Some genetic disorders are referred to as
dominant disorders, meaning those in which
a single allele can bring about the condition
regardless of whether a person also has a
normal allele.
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(a) Sickle-cell anemia: transmission of a recessive disorder.
mother not
sick
s
S
egg
S
father
not
sick
SS
Ss
sperm
Ss
ss
s
Sickle-cell anemia is a recessive autosomal
disorder; both the mother and father must
carry at least one allele for the trait in order
for a son or a daughter to be a sickle-cell victim.
When both parents have one sickle-cell allele,
there is a 25% chance that any given offspring
will inherit the condition.
25% probability of inheriting the disorder
(b) Huntington disease: transmission of a dominant disorder.
mother not
sick
h
h
egg
H
Hh
father
sick
Hh
sperm
h
hh
hh
50% probability of inheriting the disorder
In Huntington disease, if only a single
parent has a Huntington allele there is
a 50% chance that a son or daughter
will inherit the condition.
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Figure 12.4
Some Human Genetic Disorders
Animation 12.2: Some Human Genetic Disorders
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12.3 Tracking Traits with Pedigrees
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Pedigrees
• In tracking inherited diseases, scientists
often find it helpful to construct medical
pedigrees, which are genetic familial
histories that normally take the form of
diagrams.
• Pedigrees allow experts to make deductions
about the genetic makeup of several
generations of family members.
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Pedigrees
I
Aa
Aa
?
?
A?
A?
female
male
normal
II
?
aa
A?
?
Aa
Aa
carrier
A?
albino
III
?
?
A?
A?
?
aa
A?
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Figure 12.5
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12.4 Aberrations in Chromosomal
Sets: Polyploidy
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Polyploidy
• Human beings and many other species have
diploid or paired sets of chromosomes.
• In human beings, this means 46
chromosomes in all:
• 22 pairs of autosomes
• And either an XX chromosome pair (for
females) or an XY pair (for males)
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Polyploidy
• The state of having more than two sets of
chromosomes is called polyploidy.
• Many plants are polyploid, but the condition
is inevitably fatal for human beings.
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12.5 Incorrect Chromosome
Number: Aneuploidy
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Aneuploidy
• Aneuploidy is a condition in which an
organism has either more or fewer
chromosomes than normally exist in its
species’ full set.
• Aneuploidy is responsible for a large
proportion of the miscarriages that occur in
human pregnancies.
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Aneuploidy
• A small proportion of embryos survive
aneuploidy, but the children who result
from these embryos are born with such
conditions as Down syndrome.
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Nondisjunction
• The cause of aneuploidy usually is
nondisjunction, in which homologous
chromosomes or sister chromatids fail to
separate correctly in meiosis
• This leads to eggs or sperm that have one
too many or one too few chromosomes.
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Nondisjunction
Normal
Abnormal
Abnormal
Nondisjunction
in meiosis I
Nondisjunction
in meiosis II
23
23
23
23
100% of gametes get
normal number of
chromosomes
24
24
22
22
100% of gametes get
abnormal number of
chromosomes
23
23
50% normal
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22
24
50% abnormal
Figure 12.7
Aneuploidy
• Aneuploidy can come about in regular cell
division (mitosis) as well as in meiosis.
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Aneuploidy and Cancer
• A number of cancer researchers believe that
mitotic aneuploidy can be a cause of cancer
rather than an effect of it, as previously
believed.
• Recent evidence indicates that, at the least,
such aneuploidy appears prior to the
initiation of some forms of cancer.
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Aneuploidy and Cancer
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Figure 12.9
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12.6 Structural Aberrations in
Chromosomes
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Chromosomal Aberrations
• Harmful aberrations can occur within
chromosomes, with many of these
aberrations coming about because of
mistakes in chromosomal interactions.
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Chromosomal Aberrations
• Chromosomal aberrations include:
•
•
•
•
deletions
inversions
translocations
duplications
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Chromosomal Aberrations
Inversion
Deletion
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Translocation
Duplication
Figure 12.11
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