Transcript chapter14_Sections 1

Cecie Starr
Christine Evers
Lisa Starr
www.cengage.com/chemistry/starr
Chapter 14
Human Inheritance
(Sections 14.1 - 14.4)
Albia Dugger • Miami Dade College
Variation in Human Skin Color
• Fraternal twins Kian and Remee inherited different alleles of
genes for skin color from their mixed-race parents, who must
be heterozygous for those alleles
14.1 Shades of Skin
• Like most human traits, skin color has a genetic basis
• Minor differences in alleles for melanin synthesis and
deposition of melanosomes affect skin color
• Differences probably evolved as a balance between vitamin D
production and protection against harmful UV radiation, which
increases risk of birth defects
Variation in Human Skin Color
• More than 100 gene products are
involved in melanin synthesis, and
melanosome formation and deposition
• Light-skinned people of European
descent carry a mutation in gene
SLC24A5 that encodes a transport
protein in melanosome membranes
All in the Family: Mixed Race Twins
14.2 Human Genetic Analysis
• Geneticists study inheritance patterns in humans by tracking
genetic disorders and abnormalities through families
• A genetic abnormality is an uncommon version of a heritable
trait that does not result in medical problems
• A genetic disorder is a heritable condition that sooner or later
results in mild or severe medical problems
Pedigrees
• Charting genetic connections with pedigrees reveals
inheritance patterns of certain traits:
• Dominant and recessive alleles
• Alleles on autosomes or sex chromosomes
• Probability that a trait will recur a family or population
• pedigree
• Chart showing the pattern of inheritance of a trait through
generations in a family
Charting Pedigrees
• Standard symbols used
in pedigrees
Example: Polydactyly
• Polydactyly is
characterized by extra
fingers, toes, or both
• Black numbers on the
pedigree = number of
fingers on each hand
• Red numbers = number
of toes on each foot
Pedigree for Polydactyly
Charting
Pedigrees
male
female
marriage/mating
offspring
individual showing
trait being studied
sex not
specified
generation
Gene not expressed in this carrier.
Fig. 14.2ab, p. 204
Animation: Pedigree diagrams
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Pedigree for Huntington’s Disease
Types of Genetic Variation
• Single genes that follow Mendelian inheritance patterns
govern more than 6,000 genetic abnormalities and disorders
• Most human traits are polygenic (influenced by multiple
genes) and often have environmental factors as well
• Alleles that give rise to severe genetic disorders are rare
Types of Genetic Variation
• We will look at 6 main patterns of inheritance for genetic
abnormalities and disorders:
• Autosomal dominant inheritance pattern
• Autosomal recessive inheritance pattern
• X-linked recessive inheritance pattern
• X-linked dominant inheritance pattern
• Changes in chromosome number
• Changes in chromosome structure
Key Concepts
• Tracking Traits in Humans
• Inheritance patterns in humans are determined by
following traits through generations of family trees
• Types of traits followed in such studies include genetic
abnormalities or syndromes associated with a genetic
disorder
ANIMATION: Human Sex Determination
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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 homozygous and heterozygous
people
• An allele is inherited in an autosomal recessive pattern if the
trait it specifies appears only in homozygous people
The Autosomal Dominant Pattern
• An autosomal dominant trait appears in every generation.
• When one parent is heterozygous, and the other is
homozygous recessive, each child has a 50% chance of
inheriting the dominant allele and displaying the trait
Autosomal Dominant Inheritance
• A dominant allele (red)
is fully expressed in
heterozygous people
Autosomal
Dominant
Inheritance
normal
mother
affected
father
meiosis
and gamete
formation
affected child
normal child
disorder-causing
allele (dominant)
Fig. 14.3, p. 206
Autosomal
Dominant
Inheritance
normal
mother
affected
father
meiosis
and gamete
formation
affected child
normal child
disorder-causing
allele (dominant)
Stepped Art
Fig. 14.3, p. 206
ANIMATION: Autosomal-recessive
inheritance
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Autosomal Dominant Disorders
Disorder
Main Symptoms
Achondroplasia
One form of dwarfism
Aniridia
Defects of the eyes
Camptodactyly
Rigid, bent fingers
Hypercholesterolemia High cholesterol level
Huntington’s disease Degeneration of nervous system
Marfan syndrome Abnormal connective tissue
Polydactyly
Extra fingers, toes, or both
Progeria
Drastic premature aging
Neurofibromatosis Tumors of nervous system, skin
Achondroplasia
• Achondroplasia
interferes with formation
of the embryonic
skeleton
Hutchinson–Gilford Progeria
• A mutation causes
defects in transcription,
mitosis, and division
• Symptoms of premature
aging begin before age
two
The Autosomal Recessive Pattern
• An autosomal allele is inherited in a recessive pattern if it is
expressed only in homozygous people, so recessive traits
may skip generations
• People heterozygous for the allele are carriers – they have
the allele but not the trait
• Each child of two carriers has a 25% chance of being
homozygous and having the trait
Autosomal Recessive Inheritance
• Two parents who are
carriers of a recessive
autosomal allele (red)
• Each child has a 25%
chance of being
homozygous for the trait
Autosomal
Recessive
Inheritance
carrier mother
carrier father
meiosis
and gamete
formation
affected child
carrier child
normal child
A
disorder-causing
allele (recessive)
Fig. 14.5a, p. 207
Autosomal
Recessive
Inheritance
carrier mother
carrier father
meiosis
and gamete
formation
affected child
carrier child
normal child
disorder-causing
allele (recessive)
Stepped Art
Fig. 14.5a, p. 207
Animation: Autosomal-dominant inheritance
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Autosomal Recessive Disorders
Disorder
Albinism
Methemoglobinemia
Cystic fibrosis
damage
Ellis–van Creveld
Fanconi anemia
Galactosemia
Hemochromatosis
Phenylketonuria (PKU)
Sickle-cell anemia
Tay–Sachs disease
Main Symptoms
Absence of pigmentation
Blue skin coloration
Abnormal glandular secretions
leading to tissue and organ
Dwarfism, heart defects, polydactyly
Abnormalities, bone marrow failure
Brain, liver, eye damage
Iron overload , joint & organ damage
Mental impairment
Adverse pleiotropic effects
Deterioration of mental and physical
abilities; early death
Albinism
• Albinism, a lack of
melanin, occurs in
people homozygous for
recessive alleles that
code for a defective
form of the enzyme
tyrosinase
Tay–Sachs Disease
• Mutations cause
gangliosides to
accumulate to toxic
levels in nerve cells
• Affected children, such
as Conner Hopf, die
before age five
Key Concepts
• Autosomal Inheritance
• Many human traits can be traced to dominant or recessive
alleles on autosomes
• These alleles are inherited in characteristic patterns:
dominant alleles tend to appear in every generation;
recessive ones can skip generations
14.4 X-Linked Inheritance Patterns
• An allele is inherited in an X-linked pattern when it occurs on
the X chromosome
• Most X-linked inheritance disorders are recessive, because Xlinked dominant alleles tend to be lethal in male embryos
XX and XY
• X-linked recessive disorders tend to appear in men more
often than in women
• Men (XY) have only one X chromosome
• Women have two X chromosomes (XX), so they can be
heterozygous for a recessive allele
• Men can transmit an X-linked allele to daughters, but not to
sons – only a woman can pass an X-linked allele to a son
X-Linked Recessive Inheritance
• In this case, the mother
carries a recessive
allele on one of her two
X chromosomes (red)
X-Linked
Recessive
Inheritance
carrier mother
normal father
meiosis
and gamete
formation
normal daughter or son
carrier daughter
affected son
recessive allele
on X chromosome
Fig. 14.6, p. 208
X-Linked
Recessive
Inheritance
carrier mother
normal father
meiosis
and gamete
formation
normal daughter or son
carrier daughter
affected son
recessive allele
on X chromosome
Stepped Art
Fig. 14.6, p. 208
Animation: X-linked inheritance
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X-Linked Recessive Disorders
Disorder
Main Symptoms
Androgen insensitivity XY individual but having some
syndrome
female traits; sterility
Red-green color
Inability to distinguish red from
blindness
green
Hemophilia
Impaired blood clotting ability
Muscular dystrophies Progressive loss of muscle function
X-linked anhidrotic Mosaic skin (patches with or without
dysplasia
sweat glands); other effects
Red–Green Color Blindness
• Most genes involved in proper function of pigment-containing
receptors in the eyes are on the X chromosome
• Color blindness includes a range of conditions in which an
individual cannot distinguish among some or all colors
• Some types of color blindness confuse red and green colors,
others see green as shades of gray, but perceive blues and
yellows quite well
Red–Green Color Blindness
Tests for Color Blindness
Hemophilia A
• Hemophilia A, an X-linked recessive disorder that interferes
with blood clotting, involves factor VIII, a protein product of a
gene on the X chromosome
• In the 19th century, hemophilia was relatively common in royal
families of Europe and Russia, probably because of the
common practice of inbreeding
Hemophilia in
Descendants of Queen Victoria
Duchenne Muscular Dystrophy
• A gene on the X chromosome encodes dystrophin, a protein
essential in muscle and nerve cells
• Boys with DMD are in a wheelchair by age 12, and die from a
heart disorder or respiratory failure before age 30
Key Concepts
• Sex-Linked Inheritance
• The X chromosome holds about 10 percent of all human
genes, so many traits are affected by alleles on this
chromosome
• Inheritance patterns of such X-linked alleles tend to differ
between males and females