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Chapter 34
Genetics and Heredity
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Slide 1
Science of Genetics
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Genetics—scientific study of inheritance;
developed to explain how normal biological
characteristics are inherited
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Directly inherited diseases are often called
hereditary diseases
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Slide 2
Chromosomes and Genes
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Mechanism of gene function
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Genetic code is transmitted via genes, which is a segment of DNA
DNA (deoxyribonucleic acid) (Figure 34-1)
• Compact form of DNA is a chromosome—which exists only during
cell division
• Strand form of DNA is chromatin—made up of subunits called
nucleosomes, which are like small spools of DNA wound around
proteins called histones
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Each gene is a sequence of nucleotide bases in the DNA
molecule, which the cell transcribes to an RNA molecule
Each mRNA molecule associates with a ribosome, which
translates the code to form one or more specific polypeptide
molecules
Genes determine the structure and function of the human body by
producing a set of specific regulatory RNA and protein
molecules—along with specific structural proteins
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Slide 3
Chromosomes and Genes

The human genome (Figure 34-2)
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Genome—entire set of human chromosomes (46 in nucleus of each cell,
1 mitochondrial chromosome)
• Map of the entire human genome (nearly all nucleotides in sequence) was
completed in 2003
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• Contains about 20,000 to 25,000 genes and large amounts of noncoding DNA
Genomics—analysis of the sequence contained in the genome
Transcriptomics—analysis of the mRNA codes actually transcribed from
genes in the genome
Proteomics—analysis of the entire group of proteins encoded by the
genome and transcriptome, a group of called the human proteome
Genomic information can be expressed in various ways:
• Ideogram—cartoon of a chromosome showing the centromere as a
constriction and the short segment (p-arm) and long segment (q-arm)
• Genes are often represented as their actual sequence of nucleotide bases
expressed by the letters a, c, g, and t
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Slide 4
Chromosomes and Genes

Distribution of chromosomes to offspring

Meiosis (Figure 34-3)
• Produces gametes with the haploid number of
chromosomes
• When a sperm and an ovum unite at conception, they form
a zygote with 46 chromosomes
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Slide 5
Chromosomes and Genes

Distribution of chromosomes to offspring (cont.)

Principle of independent assortment
• As sperm and ovum are formed, chromosome pairs separate and
the maternal and paternal chromosomes get mixed up and
redistributed independently of the other chromosome pairs,
resulting in each gamete having a different set of 23 chromosomes
• Genetic variation—independent assortment of chromosomes
ensures that each offspring from a single set of parents is
genetically unique
• Applies to individual genes or groups of genes
• Crossing over—during one phase of meiosis, pairs of matching
chromosomes line up along the equator and exchange genes with
one another (Figure 34-4)
• Gene linkage—sometimes an entire group of genes stays together
and crosses over as a single unit
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Slide 6
Gene Expression

Hereditary traits

Dominant and recessive traits
• Each inherited trait is controlled by two sets of similar genes, one
from each parent
• Each autosome in a pair matches its partner in the type of gene it
contains
• Different types of genes (Figures 34-5 and 34-6)
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Dominant gene—effects are seen; capable of masking the effects of a
recessive gene for the same trait
Recessive gene—effects are masked by the effects of a dominant
gene for the same trait
• Genotype—combination of genes within the cells of an individual
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Homozygous—genotype with two identical forms of a gene
Heterozygous—genotype with two different forms of a gene
• Phenotype—manner in which genotype is expressed; how an
individual looks as a result of genotype
• Carrier—person who possesses the gene for a recessive trait but
does not exhibit the trait
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Slide 7
Gene Expression
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Hereditary traits (cont.)
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Polygenic traits—when more than one gene is
involved in producing a particular trait; a “combined
trait” because it results from a combination of genes
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Codominant traits—when two different dominant
genes occur together, each will have an equal effect

Abnormal “disease” genes that persist in a population
often provide some biological advantage, as in the
case of the sickle-gene that protects against malaria
(Figure 34-7)
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Slide 8
Gene Expression

Sex-linked traits (Figures 34-8 and 34-9)
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X chromosome—“female chromosome”; larger than
Y chromosome; includes genes that determine
female sexual characteristics, as well as nonsexual
characteristics (Figure 34-10)
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Y chromosomes—“male chromosome”; smaller
than X chromosome; contains few genes other than
male sexual characteristics
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Sex-linked traits—carried on sex chromosomes;
also known as X-linked traits
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Slide 9
Gene Expression
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Genetic mutations
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Mutation—change in the genetic code
• Deletion—missing information in the genetic code
• Insertion—extra information in the genetic code
• Insertions and deletions result in a failure to make the usual
protein encoded by a particular gene
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Mutations can occur without outside influence
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Mutagens—agents that cause most genetic mutations
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Slide 10
Medical Genetics
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Mechanisms of genetic disorders
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Nuclear inheritance
• Single-gene diseases—caused by individual mutant genes in
nuclear DNA that pass from one generation to the next
• Genetic predisposition—disease occurring because of combined
effects of inheritance and environmental factors
• Chromosomal genetic diseases—congenital conditions such as
trisomy and monosomy that produce life-threatening abnormalities;
trisomic and monosomic individuals die before they can reproduce
(Figure 34-11)
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Mitochondrial inheritance
• Mitochondrial DNA (mDNA)—each mitochondrion has its own DNA
molecule (Figure 34-12)
• Inheritance of mDNA occurs through one’s mother because sperm
does not contribute mitochondria to the ovum during fertilization
• mDNA contains the only genetic code for several important enzymes
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Slide 11
Medical Genetics
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Single-gene diseases
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Cystic fibrosis
• Results because of recessive genes in chromosome pair 7
• Impairment of chloride ion transport across cell membranes
causes exocrine cells to secrete thick mucus and concentrated
sweat; thickened mucus may obstruct respiratory and
gastrointestinal tracts, leading to death
• Treatment—use of drugs and other therapies
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Phenylketonuria (PKU)
• Results from recessive genes that fail to produce phenylalanine
hydroxylase
• Phenylalanine cannot be converted into tyrosine and thus
accumulates
• High concentrations of phenylalanine destroy brain tissue
• Treatment—diets low in phenylalanine
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Slide 12
Medical Genetics
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Single-gene diseases (cont.)
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Tay-Sachs disease
• Recessive condition involving failure to make an essential lipidprocessing enzyme
• Abnormal lipids accumulate in the brain, causing severe retardation
and death by age 4
• No specific therapy available
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Osteogenesis imperfecta (Figure 34-13)
• Dominant genetic disorder of connective tissues resulting in
imperfect bone formation
• Bones are very brittle
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Multiple neurofibromatosis
• Dominant inherited disorder
• Characterized by multiple benign tumors of glial cells that surround
nerve fibers
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Slide 13
Medical Genetics
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Chromosomal diseases—genetic disorders resulting from
nondisjunction during formation of the gametes; produce
either trisomy or monosomy
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Trisomy 21—triplet of chromosomes 21 rather than a pair;
characterized by Down syndrome’s mental retardation and
multiple defects (Figure 34-14)
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Klinefelter syndrome occurs in males with a Y chromosome and
at least two X chromosomes; characteristics include long legs,
enlarged breasts, low intelligence, small testes, sterility, and
chronic pulmonary disease (Figure 34-15)
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Turner syndrome—XO syndrome occurs in females with a single
X chromosome; characterized by failure of ovaries and other
organs to mature, sterility, cardiovascular defects, dwarfism,
webbed neck, and learning disorders; symptoms can be reduced
by hormone therapy (Figure 34-16)
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Slide 14
Medical Genetics
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Genetic basis of cancer
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Oncogenes—abnormal genes thought to cause
some forms of cancer
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Tumor suppressor genes regulate cell division so
it proceeds normally; when nonfunctional because
of a genetic mutation, it allows cells to divide
abnormally
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Also, genetic abnormalities may inhibit the cell’s
cancer-preventing mechanisms
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Slide 15
Prevention and Treatment
of Genetic Diseases
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Genetic counseling—professional consultations
with families regarding genetic diseases
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Pedigree—chart that illustrates genetic relationships
in a family over several generations; helpful in
determining the possibility of producing offspring with
certain genetic disorders (Figure 34-17)
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Punnett square—grid used to determine the
mathematical probability of inheriting genetic traits
(Figure 34-18)
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Karyotype—ordered arrangement of photographs of
chromosomes from a single cell; used in genetic
counseling to identify chromosomal disorders
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Slide 16
Prevention and Treatment
of Genetic Diseases
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Treating genetic diseases (Figure 34-20)
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Gene therapy involves changing the genetic code of
cells to replace normal proteins that are absent in
genetic disorders
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Gene replacement—abnormal, disease-causing
proteins are replaced by “therapeutic” genes; goal is
to genetically alter existing body cells in the hope of
eliminating the cause of a genetic disease
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Gene augmentation—normal genes are introduced
to augment the production of the needed protein
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Slide 17