Chapter 9
Patterns of
Genetic
Inheritance
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Gregor Mendel Deduced
Laws of Inheritance
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9.1 A blending model
of inheritance existed
prior to Mendel
 Austrian monk Gregor Mendel developed the
fundamental laws of heredity after performing a
series of experiments with pea plants
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Figure 9.1 Gregor Mendel examining a pea plant
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9.2 Mendel designed his
experiments well
Figure 9.2A Garden pea anatomy and the
cross-pollination procedure Mendel used
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Figure 9.2B Garden pea traits and crosses studied by
Mendel
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Single-Trait Crosses Reveal
Units of Inheritance and the Law
of Segregation
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9.3 Mendel’s law of segregation
describes how gametes
pass on traits
 The law of segregation states:
 Each individual has two factors for each trait
 The factors segregate (separate) during the
formation of the gametes
 Each gamete contains only one factor from each pair
of factors
 Fertilization gives each new individual two factors for
each trait
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Figure 9.3
Monohybrid
cross done by
Mendel
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9.4 The units of inheritance are
alleles of genes
 Traits are controlled by alleles - alternate forms
of a gene
 Found on homologous chromosomes at a particular
gene locus
 The dominant allele masks the expression of
the other allele - the recessive allele
 Genotype refers to the alleles an individual
receives at fertilization
 Homozygous - an organism has two identical alleles
at a gene locus
 Heterozygous - an organism has two different alleles
at a gene locus
 Phenotype - the physical appearance of the
individual.
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Figure 9.4 Occurrence of alleles on homologous chromosomes
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Two-Trait Crosses Support the Law
of Independent Assortment
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9.5 Mendel’s law of independent
assortment describes inheritance
of multiple traits
 The law of independent assortment states the
following:
 Each pair of factors separates (assorts)
independently (without regard to how the others
separate)
 All possible combinations of factors can occur in the
gametes
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Figure 9.5
Dihybrid cross
done by Mendel
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9.6 Mendel’s results are consistent
with the laws of probability
Figure 9.6 Use of Punnett square to calculate probable events
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9.7 Testcrosses support Mendel’s
laws and indicate the genotype
 Testcross - intentional breeding in order to
determine underlying genotypes
 One-trait Testcross - When a heterozygous
individual is crossed with one that is homozygous
recessive, the results are always a 1:1 phenotypic
ratio
 Two-trait Testcross - when an individual is
heterozygous for two traits is crossed with one that is
recessive for the traits, the offspring have a 1:1:1:1
phenotypic ratio
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Figure 9.7A One-trait testcross, when the individual with the
dominant phenotype is heterozygous
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Figure 9.7B One-trait testcross when the individual with the
dominant phenotype is homozygous
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Mendel’s Laws Apply to Humans
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9.8 Pedigrees can reveal the
patterns of inheritance
 Some genetic disorders are medical conditions
inherited from parents
 Some may be due to the inheritance of abnormal
alleles on autosomal chromosomes - all the
chromosomes except the sex chromosomes
 Carriers - those individuals that carry the abnormal
allele but do not express it
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Figure 9.8A Autosomal recessive pedigree
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Figure 9.8B Autosomal dominant pedigree
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9.9 Some human genetic disorders
are autosomal recessive
 Tay-Sachs Disease - uncontrollable seizures,
and paralysis prior to dying
 Results from a lack of the enzyme Hex A
 Cystic Fibrosis - most common lethal genetic
disease of Caucasians in the U.S.
 Genetic testing for the recessive allele is possible
 Phenylketonuria - most commonly inherited
metabolic disorder affecting nervous system
 Many diet products have warnings that they contain
phenylalanine
 Sickle-cell Disease - genotype HbS HbS has
many symptoms from anemia to heart failure
 Individuals who are HbA HbS have sickle-cell trait
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9.10 Some human genetic disorders
are autosomal dominant
 Neurofibromatosis - many children with
neurofibromatosis have learning disabilities and
are hyperactive
 Huntington disease - a neurological disorder
that leads to progressive degeneration of brain
cells
 Achondroplasia - a common form of dwarfism
associated with a defect in the growth of long
bones
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APPLYING THE CONCEPTS—HOW BIOLOGY IMPACTS OUR LIVES
9.11 Genetic disorders may now
be detected early on
 Testing Fetal Cells
 Amniocentesis - long needle withdraws a small amount of the
fluid that surrounds the fetus and contains a few fetal cells
 Chorionic Villi Sampling (CVS) - tube is inserted through the
vagina into the uterus and fetal cells are obtained by suction
 Testing the Embryo
 A single cell can be removed from the 8-celled embryo and
subjected to preimplantation genetic diagnosis (PGD)
 Testing the Egg
 Polar bodies (nonfunctional cells produced during egg
formation) receive a haploid number of chromosomes
 When a woman is heterozygous for a recessive genetic disorder,
about half the polar bodies have received the mutated allele,
while the egg has received the normal allele
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Figure 9.11A Prepregnancy testing of an embryo
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Figure 9.11B Prepregnancy testing of an egg
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Complex Inheritance Patterns
Extend the Range of
Mendelian Analysis
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9.12 Incomplete dominance still
follows the law of segregation
 Incomplete dominance - heterozygote has an
intermediate phenotype between that of either
homozygote
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Figure 9.12 Incomplete dominance
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9.13 A gene may have
more than two alleles
 Multiple alleles - gene has several allelic forms
 Example: blood type is determined by multiple alleles
 IA = A antigen on red blood cells
 IB = B antigen on red blood cells
 i = Neither A nor B antigen on red blood cells
 Possible phenotypes and genotypes for blood type:
 This is an example of codominance because both IA
and IB are fully expressed
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9.14 Several genes and the
environment can influence a single
multifactorial characteristic
 Polygenic inheritance occurs when a trait is
governed by two or more genes
 Multifactorial traits - controlled by polygenes subject
to environmental influences
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Figure 9.14 Polygenic
inheritance: Dark dots
stand for dominant
alleles; the shading
stands for
environmental
influences
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9.15 One gene can influence
several characteristics
 Pleiotropy - when a single gene has more than
one effect
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Figure 9.15A Marfan syndrome illustrates the multiple effects a
single human gene can have
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Chromosomes Are the
Carriers of Genes
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9.16 Traits transmitted via the X
chromosome have a unique
pattern of inheritance
 X-linked alleles have a different pattern of
inheritance than autosomal alleles
 The Y chromosome cannot offset the inheritance of
an X-linked recessive allele
 Affected males always receive their X-linked
recessive mutant allele from the female parent
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Figure 9.16
X-linked inheritance
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9.17 Humans have
X-linked disorders
 Color Blindness - the alleles for the red- and
green-sensitive proteins are on the X
chromosome
 Muscular Dystrophy - occurs in males but the
recessive allele remains in the population
through passage from mother to daughter
 Hemophilia - 1 in 10,000 males is affected by
both external and internal bleeding
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Figure 9.17 X-linked
recessive pedigree
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9.18 The genes on one
chromosome form a linkage group
 Gene linkage - the existence of several genes
on the same chromosome
 Genes on a single chromosome form a linkage
group because they tend to be inherited together
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Figure 9.18 A simplified map of the genes on chromosome 2 of
Drosophila
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9.19 Frequency of recombinant
gametes maps the chromosomes
 A linkage map can also be called a chromosome
map because it tells the order of gene loci on
chromosomes
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Figure 9.19
Example of
incomplete
linkage
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APPLYING THE CONCEPTS—HOW SCIENCE PROGRESSES
9.20 Thomas Hunt Morgan is
commonly called “the fruit fly guy”
 In 1908, Morgan began experimenting with the fruit
fly (Drosophila melanogaster)
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Connecting the Concepts:
Chapter 9
 The first of Mendel’s laws tells us that an
individual has two alleles, but the gametes have
only one allele for every trait
 The second law tells us that the gametes have all
possible combinations of alleles
 Polygenic inheritance and X-linked inheritance
extend the range of Mendelian analysis
 Males are more apt than females to display an Xlinked disorder
 Genes do have loci on the chromosomes, but
today we know that genes are composed of
DNA
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