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Patterns of Inheritance
Chapter 9
Part 2
9.4 Complex Variations in Traits
 A trait that is influenced by the products of
multiple genes often occurs in a range of small
increments of phenotype (continuous variation)
 The more genes and other factors that influence
a trait, the more continuous the distribution of
phenotype
Continuous Variation
 Continuous variation
• A range of small differences in a shared trait
 Bell curve
• Curve that typically results when range of
variation in a continuous trait is plotted against
frequency in a population
Continuous Variation (Bell Curve)
Fig. 9-12a, p. 165
Fig. 9-12b, p. 165
Animation: Continuous variation in
height
Environmental Effects on Phenotype
 Enzymes and other gene products control steps
of most metabolic pathways
 Mutations, interactions among genes, and
environmental conditions can affect one or more
steps, and contribute to variation in phenotypes
Environmental Effects
on Animal Phenotypes
 Snowshoe hare in summer and winter
 Daphnia without and with predators
Fig. 9-13a, p. 166
Fig. 9-13b, p. 166
Fig. 9-13c, p. 166
Environmental Effects
on Plant Phenotypes
Height (centimeters)
60
60
60
0
0
0
A Plant grown
at high elevation
(3,060 meters
above sea level)
B Plant grown
at mid-elevation
(1,400 meters
above sea level)
C Plant grown
at low elevation
(30 meters
above sea level)
Fig. 9-14, p. 166
Animation: Coat color in the Himalayan
rabbit
9.5 Human Genetic Analysis
 Inheritance patterns in humans are studied by
following inherited genetic disorders through
generations and graphing results as pedigrees
 Pedigree
• Chart showing the pattern of inheritance of a gene
in a family
Pedigree: Polydactyly
male
female
marriage/mating
offspring
individual showing
trait being studied
sex not
specified
generation
A Standard symbols used in
pedigrees
*Gene not expressed in this carrier.
B A pedigree for polydactyly, which is characterized by extra
fingers, toes, or both. The black numbers signify the number of
fingers on each hand; the blue numbers signify the number of
toes on each foot.
Fig. 9-15, p. 167
male
female
marriage/mating
offspring
individual showing
trait being studied
sex not
specified
generation
A) Standard symbols used in
pedigrees
*Gene not expressed in this carrier.
B) A pedigree for polydactyly, which is characterized by extra
fingers, toes, or both. The black numbers signify the number of
fingers on each hand; the blue numbers signify the number of
toes on each foot.
Stepped Art
Fig. 9-15, p. 167
Animation: Pedigree diagrams
Genetic Abnormalities and Disorders
 Genetic abnormality
• An uncommon version of a heritable trait that does
not result in medical problems
 Genetic disorder
• A heritable condition that results in a syndrome of
mild or severe medical problems
 Syndrome
• Set of symptoms characterizing a genetic disorder
9.6 Human Genetic Disorders
 Some dominant or recessive alleles on
autosomes or the X chromosome are associated
with genetic abnormalities or disorders
 An autosomal dominant allele is expressed in
homozygotes and heterozygotes
 An autosomal recessive allele is expressed only
in homozygotes
Some Autosomal Dominant Disorders
Autosomal Dominant Inheritance
normal
mother
affected
father
meiosis
and gamete
formation
affected child
normal child
disorder-causing
allele (dominant)
Fig. 9-16a, p. 168
Fig. 9-16b, p. 168
Animation: Autosomal-dominant
inheritance
Some Autosomal Recessive Disorders
Autosomal Recessive Inheritance
carrier mother
carrier father
meiosis
and gamete
formation
affected child
carrier child
normal child
disorder-causing
allele (recessive)
Fig. 9-17a, p. 169
Animation: Autosomal-recessive
inheritance
X-Linked Recessive Disorders
 Alleles on the X chromosome are inherited and
expressed differently in males and females
 Males cannot transmit a recessive X-linked allele
to their sons
 Females pass X-linked alleles to male offspring
• Example: Queen Victoria of England and
hemophilia
Some X-Linked Recessive Disorders
X-Linked Recessive Inheritance
Fig. 9-18 (top), p. 170
Fig. 9-18 (a-d), p. 170
Animation: X-linked inheritance
An X-Linked Trait:
Queen Victoria and Hemophilia
Animation: Morgan’s reciprocal crosses
Video: ABC News: Genetic testing:
Screening embryos for disease
9.7 Changes in Chromosome Number
 Chromosome number can change permanently,
usually resulting from nondisjunction
 Nondisjunction
• Failure of duplicated chromosomes (sister
chromatids) or homologous chromosomes to
separate during meiosis or mitosis
Nondisjunction During Meiosis
Metaphase I
Anaphase I
Telophase I
Metaphase II Anaphase II Telophase II
Fig. 9-20a, p. 172
Metaphase I
Anaphase I
Telophase I
Metaphase II Anaphase II Telophase II
Stepped Art
Fig. 9-20b, p. 172
Aneuploidy
 Aneuploidy
• A chromosome abnormality in which a cell has
too many or too few copies of a particular
chromosome (trisomy, monosomy)
 The most common aneuploidy in humans,
trisomy 21, causes Down syndrome
Some Disorders Caused by Aneuploidy
Polyploidy
 Autosomal aneuploidy is usually fatal in humans,
but many flowering plants, and some insects,
fishes and other animals are polyploid
 Polyploid
• Having three or more of each type of
chromosome characteristic of the species
Autosomal Change and Down Syndrome
 Trisomy 21 (Down syndrome)
• The only autosomal trisomy that allows humans
to survive to adulthood
• Affected individuals tend to have certain physical
features and impairments
 Nondisjunction leading to trisomy 21 increases
with age of the mother
Autosomal Change and Down Syndrome
Fig. 9-20b, p. 172
Fig. 9-20c, p. 172
Sex Chromosome Abnormalities
 Usually associated with learning difficulties,
speech delays, and motor skill impairment
 Female sex chromosome abnormalities:
• Turner syndrome (XO), XXX syndrome
 Male sex chromosome abnormalities:
• Klinefelter syndrome (XXY); XYY syndrome
Turner’s Syndrome (XO)
 Affects development of ovaries, sexual maturity
Animation: Nondisjunction
9.8 Some Prospects in Human Genetics
 Geneticists estimate the chance that a couple’s
offspring will inherit a genetic abnormality or
disorder
 Potential parents who may be at risk of
transmitting a harmful allele to offspring have
screening or treatment options
Prenatal Diagnosis
 Tests performed before birth
• Amniocentesis
• Chorionic villi sampling (CVS)
• Fetoscopy
 Carry risks of miscarriage or injury to fetus
Amniocentesis
 Risks of puncture, infection, loss of amniotic fluid
Animation: Amniocentesis
Fetoscopy
 Increased risk of miscarriage
Preimplantation Diagnosis
 A single cell taken from an embryo produced by
in vitro fertilization is tested before implantation
Genetic Screening
and Phenotypic Treatments
 Genetic screening
• Hospitals routinely screen newborns for certain
genetic disorders such as PKU (Phenylketonuria)
 Phenotypic treatments
• Surgery, prescription drugs, hormone
replacement therapy, or dietary controls can
minimize symptoms of some genetic disorders
9.9 Impacts/Issues Revisited
 The cystic fibrosis (CF) allele is very common in
some populations
 The CF allele is lethal in homozygotes, but offers
heterozygotes some protection against bacterial
diseases such as typhoid fever
Digging Into Data:
Cystic Fibrosis and Typhoid Fever