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BIOLOGY
A GUIDE TO THE NATURAL WORLD
FOURTH EDITION
DAVID KROGH
Units of Heredity:
Chromosomes and Inheritance
Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.
12.1 X-linked Inheritance in Humans
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
X-linked Inheritance
• Certain human conditions, such as red-green
color blindness and hemophilia, are called Xlinked conditions.
• They stem from a variant form of gene (an
allele) that is dysfunctional and that is located
on the X chromosome.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
X-linked Inheritance
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
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.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
X-linked Inheritance
• A woman with a dysfunctional blood-clotting
allele on one of her X chromosomes usually
will be protected from hemophilia by a
functional allele on her second X chromosome.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
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.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
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.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
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
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
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.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
X-linked Inheritance
PLAY
Animation 12.1: X-linked Recessive Traits
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
12.2 Autosomal Genetic Disorders
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
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.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Autosomal Genetic Disorders
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
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.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
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.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Autosomal Genetic Disorders
(a) Sickle-cell anemia: transmission of a recessive disorder.
mother
not
sick
S
s
egg
S
SS
father
not
sick
Ss
sperm
Ss
ss
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 percent
chance that any given offspring will
inherit the condition.
25% probability of
inheriting the disorder
s
(b) Huntington disease: transmission of a dominant disorder.
mother
not
sick
h
h
egg
H
Hh
father
sick
Hh
sperm
hh
hh
50% probability of
inheriting the disorder
In Huntington disease, if only a single
parent has a Huntington allele there is
a 50 percent chance that a son or
daughter will inherit the condition.
h
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Figure 12.4
Some Human Genetic Disorders
PLAY
Animation 12.2: Some Human Genetic Disorders
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
12.3 Tracking Traits with Pedigrees
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
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.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Pedigrees
I
Aa
Aa
?
?
A?
A?
female
male
normal
II
?
aa
A?
?
Aa
Aa
carrier
A?
albino
III
?
?
A?
A?
?
aa
A?
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Figure 12.5
12.4 Aberrations in Chromosomal Sets:
Polyploidy
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
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)
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
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.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
12.5 Incorrect Chromosome Number:
Aneuploidy
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
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.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Aneuploidy
• A small proportion of embryos survive
aneuploidy, but the children who result from
these embryos are born with such conditions as
Down syndrome.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
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.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
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
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
22
24
50% abnormal
Figure 12.7
Aneuploidy
• Aneuploidy can come about in regular cell
division (mitosis) as well as in meiosis.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
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.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Aneuploidy and Cancer
1
2
3
6
7
8
13
14
15
19
20
4
9
21
10
5
11
12
16
17
18
22
X
Y
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Figure 12.9
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.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Chromosomal Aberrations
• Chromosomal aberrations include:
–
–
–
–
deletions
inversions
translocations
duplications
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Chromosomal Aberrations
Inversion
Deletion
Translocation
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Duplication
Figure 12.11