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Chapter 14
Chromosomes and
Human Inheritance
Albia Dugger • Miami Dade College
14.1 Shades of Skin
• Variations in skin color may have evolved as a balance
between vitamin D production and UV protection
• More than 100 gene products are involved in the synthesis of
melanin, and the formation and deposition of melanosomes
• Mutations in some of these genes may have contributed to
regional variations in human skin color
Variation in Human Skin Color
14.1 Human Chromosomes
• Geneticists study inheritance patterns in humans by tracking
genetic disorders and abnormalities through families
• Charting genetic connections with pedigrees reveals
inheritance patterns of certain traits
Pedigrees
• Inheritance patterns in humans are typically studied by
tracking observable traits in families over generations
• A standardized chart of genetic connections (pedigree) is
used to determine the probability that future offspring will be
affected by a genetic abnormality or disorder
• Pedigree analyses also reveals whether a trait is associated
with a dominant or recessive allele, and whether the allele is
on an autosome or a sex chromosome
Standard Symbols Used in Pedigrees
male
female
marriage/mating
offspring
individual showing trait being studied
sex not specified
generation
Polydactyly
A Pedigree for Polydactyly
A Pedigree for Huntington’s Disease
Genetic Abnormalities and Disorders
• A genetic abnormality is an uncommon version of a trait that
is not inherently life-threatening,
• A genetic disorder causes medical problems that may be
severe
• A genetic disorder is often characterized by a specific set of
symptoms (a syndrome)
Types of Genetic Variation
• Single genes on autosomes or sex chromosomes govern
more than 6,000 genetic abnormalities
• Most human traits, including skin color, are polygenic
(influenced by multiple genes) and some have epigenetic
contributions or causes
Patterns of Inheritance
• Based on variations in single genes (Mendelian patterns)
• Autosomal dominant
• Autosomal recessive
• X-linked recessive
• X-linked dominant
• Based on variations in whole chromosomes
• Changes in chromosome number
• Changes in chromosome structure
Table 14-1 p221
Table 14-1 p221
Table 14-1 p221
Table 14-1 p221
Recurring Genetic Disorders
• Mutations that cause genetic disorders are rare and put their
bearers at risk
• Such mutations survive in populations for several reasons
• Reintroduction by new mutations
• Recessive alleles are masked in heterozygotes
• Heterozygotes may have an advantage in a specific
environment
Take-Home Message: How do we study
inheritance patterns in humans?
• Inheritance patterns in humans are often studied by tracking
traits through generations of families
• A genetic abnormality is a rare version of an inherited trait; a
genetic disorder is an inherited condition that causes medical
problems
• Some human genetic traits are governed by a single gene
and inherited in a Mendelian fashion; many others are
influenced by multiple genes and epigenetics
ANIMATED FIGURE: Pedigree diagrams
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14.3 Autosomal Inheritance Patterns
• An allele is inherited in an autosomal dominant pattern if the
trait it specifies appears in heterozygous people
• An allele is inherited in an autosomal recessive pattern if the
trait it specifies appears only in homozygous people
Autosomal Dominant Inheritance
• A dominant autosomal allele is expressed in homozygotes
and heterozygotes
• Tends to appear in every generation
• With one homozygous recessive and one heterozygous
parent, children have a 50% chance of inheriting and
displaying the trait
• Examples:
• Achondroplasia
• Huntington’s disease
• Hutchinson–Gilford progeria
normal
mother
affected
father
meiosis
and gamete
formation
affected child
normal child
disorder-causing
allele (dominant)
Stepped Art
Figure 14-3a p222
Figure 14-3b p222
Figure 14-3c p222
Autosomal Recessive Inheritance
• Autosomal recessive alleles are expressed only in
homozygotes
• Heterozygotes are carriers and do not have the trait
• A child of two carriers has a 25% chance of expressing the
trait
• Examples:
• Albinism
• Tay-Sachs didease
carrier mother
carrier father
meiosis
and gamete
formation
affected child
carrier child
normal child
disorder-causing
allele (recessive)
Stepped Art
Figure 14-4a p223
Figure 14-4b p223
INTERACTION: Autosomal-dominant
inheritance
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INTERACTION: Autosomal-recessive
inheritance
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Take-Home Message: How do we know a trait
is associated with an allele on an autosome?
• With an autosomal dominant inheritance pattern, persons
heterozygous for an allele have the associated trait; the trait
appears in every generation
• With an autosomal recessive inheritance pattern, only
persons who are homozygous for an allele have the
associated trait, which can skip generations
12.4 Examples of X-Linked Inheritance
• Traits associated with recessive alleles on the X chromosome
appear more frequently in men than in women
• A man cannot pass an X chromosome allele to a son
• Mutated alleles on the X chromosome cause or contribute to
over 300 genetic disorders
X-Linked Recessive Pattern
• More males than females have X-linked recessive genetic
disorders
• Males have only one X chromosome and can express a
single recessive allele
• A female heterozygote has two X chromosomes and may not
show symptoms
• Males transmit an X only to their daughters, not to their sons
carrier mother
normal father
meiosis
and gamete
formation
normal daughter or son
carrier daughter
affected son
recessive allele
on X chromosome
Stepped Art
Figure 14-6a1 p224
Some X-Linked Recessive Disorders
• Red-green color blindness
• Inability to distinguish certain colors caused by altered
photoreceptors in the eyes
• Duchenne muscular dystrophy
• Degeneration of muscles caused by lack of the structural
protein dystrophin
• Hemophilia A
• Bleeding caused by lack of blood-clotting protein
Figure 14-6b1 p224
You may have one form of red–green
color blindness if you see a 7 in this
circle instead of a 29.
You may have another form of red–
green color blindness if you see a 3
instead of an 8 in this circle.
Figure 14-6c1 p224
INTERACTION: X-linked inheritance
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Hemophilia A in Descendents
of Queen Victoria of England
Take-Home Message:
Is a trait associated with
an allele on an X chromosome?
• Men who have an X-linked recessive allele have the trait
associated with the allele; heterozygous women do not, they
have a normal allele on their second X chromosome – the
trait appears more often in men
• Men transmit an X-linked allele to their daughters, but not to
their sons
14.5 Heritable Changes
in Chromosome Structure
• On rare occasions, a chromosome’s structure changes; such
changes are usually harmful or lethal, rarely neutral or
beneficial
• A segment of a chromosome may be duplicated, deleted,
inverted, or translocated
Duplication
• DNA sequences that
are repeated two or
more times
• Duplication may be
caused by unequal
crossovers in prophase
Deletion
• Loss of some portion of
a chromosome
• Usually causes serious
or lethal disorders
• Example: Cri-du-chat
Inversion
• Part of the sequence of
DNA becomes oriented
in the reverse direction,
with no molecular loss
Translocation
• If a chromosome breaks, the broken part may get attached to
a different chromosome, or to a different part of the same one
• Most translocations are reciprocal, or balanced, which means
that two chromosomes exchange broken parts
• A reciprocal translocation between chromosomes 8 and 14 is
the usual cause of Burkitt’s lymphoma
Translocation
D With a translocation, a broken piece of a chromosome gets reattached in
the wrong place. This example shows a reciprocal translocation, in which
two chromosomes exchange chunks.
Chromosome Changes in Evolution
• Changes in chromosome structure can reduce fertility in
heterozygotes; but accumulation of multiple changes in
homozygotes may result in new species
• Certain duplications may allow one copy of a gene to mutate
while the other carries out its original function
• Example: X and Y chromosomes were once homologous
autosomes in reptile-like ancestors of mammals
Evolution of X and Y Chromosomes
from Homologous Autosomes
(autosome pair)
Ancestral reptiles
>350 mya
Figure 14-9a p227
Y
X
SRY
Ancestral reptiles
350 mya
Figure 14-9b p227
Y
X
area that
cannot cross
over
Monotremes
320–240 mya
Figure 14-9c p227
Y
X
Marsupials
170–130 mya
Y
X
Monkeys
130–80 mya
Y
X
Humans 50–
30 mya
Figure 14-9d p227
Differences Among
Closely Related Organisms
• Humans have 23 pairs
of chromosomes;
chimpanzees, gorillas,
and orangutans have 24
telomere
sequence
• Two chromosomes
fused end-to-end
human
chimpanzee
Take-Home Message:
Does chromosome structure change?
• A segment of a chromosome may be duplicated, deleted,
inverted, or translocated
• Such a change is usually harmful or lethal, but may be
conserved in the rare circumstance that it has a neutral or
beneficial effect
ANIMATION: Deletion
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ANIMATION: Duplication
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ANIMATION: Translocation
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14.6 Heritable Changes
in Chromosome Number
• Occasionally, abnormal events occur before or during
meiosis, and new individuals end up with the wrong
chromosome number
•
Consequences range from minor to lethal changes in form
and function
Polyploidy and Aneuploidy
• Many flowering plant species, and some insects, fishes, and
other animals, are polyploid – they have three or more
complete sets of chromosomes
• Trisomy and monosomy are examples of aneuploidy, in
which an individual’s cells have too many or too few copies of
a chromosome
Nondisjunction
• Changes in chromosome number can be caused by
nondisjunction, when a pair of chromosomes fails to
separate properly during mitosis or meiosis
• Affects the chromosome number at fertilization
• Monosomy (n-1 gamete)
• Trisomy (n+1 gamete)
Nondisjunction
Metaphase I
Anaphase I
Telophase I
Metaphase II
Anaphase II
Telophase II
Stepped Art
Autosomal Change and Down Syndrome
• Only trisomy 21 (Down syndrome) allows survival to
adulthood
• Characteristics include physical appearance, mental
impairment, and heart defects
• Incidence of nondisjunction increases with maternal age
• Can be detected through prenatal diagnosis
Trisomy 21: Genotype
Trisomy 21: Phenotype
Change in Sex Chromosome Number
• Changes in sex chromosome number may impair learning or
motor skills, or be undetected
• Female sex chromosome abnormalities
• Turner syndrome (XO)
• XXX syndrome (three or more X chromosomes)
• Male sex chromosome abnormalities
• Klinefelter syndrome (XXY)
• XYY syndrome
Take-Home Message: What
are the effects of
chromosome number changes?
• Nondisjunction can change the number of autosomes or sex
chromosomes in gametes; such changes usually cause
genetic disorders in offspring
• Sex chromosome abnormalities are usually associated with
some degree of learning difficulty and motor skill impairment
ANIMATION: Sources of genotype variation
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14.7 Genetic Screening
• Our understanding of human inheritance can provide
prospective parents with information about the health of their
future children
Detecting Genetic Disorders
• Surgery, prescription drugs, hormone replacement therapy,
and dietary controls can minimize and in some cases
eliminate the symptoms of a genetic disorder
• Some disorders can be detected early enough to start
countermeasures before symptoms develop
• Example: Most hospitals in the United States now screen
newborns for mutations that cause phenylketonuria (PKU)
Family Planning
• Couples may choose to know if their future children face a
high risk of inheriting a severe genetic disorder
• Genetic analysis starts with parental karyotypes, pedigrees,
and genetic testing for known disorders
• Information is used to predict the probability of having a child
with a genetic disorder
Prenatal Diagnosis
• In prenatal diagnosis, an embryo or fetus is tested before birth
to screen for sex or genetic problems
• Noninvasive techniques include obstetric sonography
• Invasive procedures in which samples of tissue or blood are
taken involve risks to mother and fetus
• Fetoscopy
• Amniocentesis
• Chorionic villus sampling (CVS)
Ultrasound
Conventional ultrasound
4D ultrasound
Fetoscopy
Fetoscopy
amniotic sac
chorion
Figure 14-14 p230
Preimplantation Diagnosis
• Couples at high risk of having a child with a genetic disorder
may choose in vitro fertilization for preimplantation diagnosis
• An undifferentiated cell is removed from the early embryo and
its genes are examined before implantation
• If the embryo has no detectable genetic defects, it is inserted
into the mother’s uterus to continue developing
Embryo After 3 Mitotic Divisions
Take-Home Message: How do we use what we
know about human inheritance?
• Genetic testing can provide prospective parents with
information about the health of their future children