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PowerPoint® Lecture Slide Presentation by Vince Austin
Human Anatomy & Physiology
FIFTH EDITION
Elaine N. Marieb
Chapter 30
Heredity
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Genetics
• The study of the mechanism of heredity
• Nuclei of all human cells (except gametes) contain
46 chromosomes
• Sex chromosomes determine the genetic sex (XX =
female, XY = male)
• Karyotype – the diploid chromosomal complement
displayed in homologous pairs
• Genome – genetic (DNA) makeup represents two
sets of genetic instructions – one maternal and the
other paternal
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Alleles
• Matched genes at the same locus on homologous
chromosomes
• Homozygous – two alleles controlling a single trait
are the same
• Heterozygous – the two alleles for a trait are
different
• Dominant – an allele masks or suppresses the
expression of its partner
• Recessive – the allele that is masked or suppressed
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Genotype and Phenotype
• Genotype – the genetic makeup
• Phenotype – the way one’s genotype is expressed
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Segregation and Independent Assortment
• Chromosomes are randomly distributed to daughter
cells
• Members of the allele pair for each trait are
segregated during meiosis
• Alleles on different pairs of homologous
chromosomes are distributed independently
• The number of different types of gametes can be
calculated by this formula:
2n, where n is the number of homologous pairs
• In a man’s testes, the number of gamete types that
can be produced based on independent assortment is
223, which equals 8.5 million possibilities
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Segregation and Independent Assortment
Figure 30.2
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Crossover
• Homologous chromosomes synapse in meiosis I
• One chromosome segment exchanges positions with
its homologous maternal counterpart
• Crossing over occurs, forming a chiasma
• Genetic information is exchanged between
homologous chromosomes
• Two recombinant chromosomes are formed
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Crossover
Figure 30.3.1
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Crossover
Figure 30.3.2
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Random Fertilization
• A single egg is fertilized by a single sperm in a
random manner
• Considering independent assortment and random
fertilization, an offspring represents one out of 72
trillion (8.5 million  8.5 million) zygote
possibilities
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Dominant-Recessive Inheritance
• Reflects the interaction of dominant and recessive
alleles
• Punnett square – diagram used to predict the
probability of having a certain type of offspring
with a particular genotype and phenotype
• Example: probability of different offspring from
mating two heterozygous parents
T = tongue roller and t = cannot roll tongue
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Dominant-Recessive Inheritance
Figure 30.4
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Dominant and Recessive Traits
• Examples of dominant disorders: achondroplasia
(type of dwarfism) and Huntington’s disease
• Examples of recessive conditions: albinism, cystic
fibrosis, and Tay-Sachs disease
• Carriers – heterozygotes who do not express a trait
but can pass it own to their offspring
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Incomplete Dominance
• Heterozygous individuals have a phenotype
intermediate between homozygous dominant and
homozygous recessive
• Sickling is a human example when aberrant
hemoglobin (Hb) is made from the recessive allele (s)
SS = normal Hb is made
Ss = sickle-cell trait (both aberrant and normal
Hb is made)
ss = sickle-cell anemia (only aberrant
Hb is made)
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Multiple-Allele Inheritance
• Genes that exhibit more than two alternate alleles
• ABO blood grouping is an example
• Three alleles (IA, IB, i) determine the ABO blood
type in humans
• IA and IB are codominant (both are expressed if
present), and i is recessive
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Sex-Linked Inheritance
• Inherited traits determined by genes on the sex
chromosomes
• X chromosomes bear over 2500 genes; Y carries
about 15
• X-linked genes are:
• Found only on the X chromosome
• Typically passed from mothers to sons
• Never masked or damped in males since there is no
Y counterpart
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Polygene Inheritance
• Depends on several different gene pairs at different
loci acting in tandem
• Results in continuous phenotypic variation between
two extremes
• Examples: skin color, eye color, and height
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Polygenic Inheritance of Skin Color
• Alleles for dark skin
(ABC) are incompletely
dominant over those for
light skin (abc)
• The first generation
offspring each have three
“units” of darkness
(intermediate
pigmentation)
• The second generation
offspring have a wide
variation in possible
pigmentations
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Figure 30.5
Environmental Influence on Gene Expression
• Phenocopies – environmentally produced
phenotypes that mimic mutations
• Environmental factors can influence genetic
expression after birth
• Poor nutrition can effect brain growth, body
development, and height
• Childhood hormonal deficits can lead to abnormal
skeletal growth
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Genomic Imprinting
• The same allele can have different effects depending
upon the source parent
• Deletions in chromosome 15 result in:
• Prader-Willi syndrome if inherited from the father
• Angelman syndrome if inherited from the mother
• During gametogenesis, certain genes are methylated
and tagged as either maternal or paternal
• Developing embryos “read” these tags and express
one version or the other
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Genomic Imprinting
Figure 30.6
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Extrachromosomal (Mitochondrial) Inheritance
• Some genes are in the mitochondria
• All mitochondrial genes are transmitted by the
mother
• Unusual muscle disorders and neurological problems
have been linked to these genes
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Genetic Screening, Counseling, and Therapy
• Newborn infants are screened for a number of
genetic disorders: congenital hip dysplasia,
imperforate anus, and PKU
• Genetic screening alerts new parents that treatment
may be necessary for the well-being of their infant
• Example: a woman pregnant for the first time at age
35 may want to know if her baby has trisomy-21
(Down syndrome)
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Genetic Screening, Counseling, and Therapy
Figure 30.7
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Carrier Recognition
• Identification for the heterozygote state for a given
trait
• Two major avenues are used to identify carriers:
pedigrees and blood tests
• Pedigrees trace a particular genetic trait through
several generations; helps to predict the future
• Blood tests and DNA probes can detect the presence
of unexpressed recessive genes
• Sickling, Tay-Sachs, and cystic fibrosis genes can be
identified by such tests
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Fetal Testing
• Is used when there is a known risk of a genetic
disorder
• Amniocentesis – amniotic fluid is withdrawn after
the 14th week and sloughed fetal cells are examined
for genetic abnormalities
• Chorionic villi sampling (CVS) – chorionic villi are
sampled and karyotyped for genetic abnormalities
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Fetal Testing
Figure 30.9
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Human Gene Therapy
• Genetic engineering has the potential to replace a
defective gene
• Defective cells can be infected with a genetically
engineered virus containing a functional gene
• The patient’s cells can be directly injected with
“corrected” DNA
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings