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Chapter 25
Genetics and Genetic
Diseases
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Lesson 25.1
Mechanisms of Genetics and Mutations
Explain how genes can cause disease.
2. Distinguish between dominant and recessive genetic
traits.
3. Describe sex-linked inheritance and explain how
genetic mutations may occur.
1.
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2
Genetics and Human Disease
 Genetics, begun by Mendel more than 140 years ago, is
the scientific study of inheritance
 Inherited traits can produce disease
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3
Mechanisms of Gene Function
 Gene—independent genetic units (DNA
segments) that carry the genetic code
 Genes dictate the production of enzymes and
other molecules, which in turn dictate the
structure and function of a cell
 Genes are active in the chromatin (strand) form
and inactive when DNA is in the chromosome
(compact) form
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4
Human Genome
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The Human Genome
 Genome—entire set of human chromosomes (46
in nucleus of each cell, 1 mitochondrial
chromosome)
 Rough draft of entire human genome (nearly all
nucleotides in sequence) published in 2001
 Contains about 30,000 genes and pseudogenes
which are large amounts of noncoding DNA
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Terms
 Genomics—analysis of the sequence contained in
the genome
 Proteomics—analysis of the entire group of proteins
encoded by the genome, called the human proteome
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Expression of Genomic Information
 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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Distribution of Chromosomes to Offspring
 Meiotic cell division produces gametes with 23
chromosomes each
 At conception, two gametes join and produce a zygote with
46 chromosomes—the complete human genome
 Two meiotic divisions
 First meiotic division distributes the chromosome pair into
separate cells
 Second meiotic division separates the strands of the
duplicated chromosome and distributes to an individual
gamete
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Meiosis
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Chromosomes


Twenty-two pairs of chromosomes are called
autosomes; each member of a pair
resembles its partner
The chromosomes in the remaining pair of
chromosomes (pair 23) are called sex
chromosomes
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Genetic Variation
 Genetic variation among offspring is increased
by:
 Independent assortment of chromosomes during
gamete formation
 Crossing-over of genes or linked groups of genes
between chromosome partners during meiosis
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Crossing-Over
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Hereditary Traits
 Dominant genes have effects that appear in the
offspring (dominant forms of a gene are often
represented by uppercase letters)
 A genetic carrier is a person who carries a
recessive gene but does not show its effects
because of masking effect of a dominant gene
 Codominant genes are two or more genes that
are all dominant and when they appear together
produce a combined effect in offspring
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Recessive Genes
 Recessive genes have effects that do not appear
in the offspring when they are masked by a
dominant gene (recessive forms of a gene are
represented by lowercase letters)
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Genes and Chromosomes
Dominant and Recessive Alleles
 Gene pairs
 Homozygous—both genes are same
 Heterozygous—the two genes differ
 Dominant allele
 Express effect whether homozygous or heterozygous
 Need to inherit from one parent only
 Recessive allele
 Only expressed if homozygous
 Need to inherit from both parents
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Genes and Chromosomes (cont.)
Dominant and Recessive Alleles (cont.)
 Phenotype: Any characteristic that can be observed or
tested for
 Example: Eye color, blood type
 Genotype: A person’s genetic makeup
 Example: Heterozygous dominant Bb (a carrier)
Homozygous dominant BB
Homozygous recessive bb
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Inheritance of Albinism
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Sex-Linked Traits
 The large X chromosome (“female
chromosome”) contains genes for female sexual
characteristics and many other traits
 The small Y chromosome (“male chromosome”)
contains only genes for male sexual
characteristics
 Normal males have XY as pair 23; normal females
have XX as pair 23
 Nonsexual traits carried on sex chromosomes are
sex-linked traits; most are X-linked traits
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Sex Determination
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Sex-Linked Inheritance
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Genetic Mutations
 Mutation means change; genetic mutation is a
change in the genetic code
 Can result in abnormalities in the genetic code
that cause disease
 Most mutations are believed to be caused by
mutagens
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Lesson 25.2
Genetic Diseases, Prevention, and Treatment
4. Explain the mechanisms of genetic disease and list
some important inherited diseases.
5. Describe how nondisjunction can result in trisomy or
monosomy and list some disorders that result from it.
6. List some tools used in genetic counseling and explain
how they are used to help clients.
7. Describe how genetic disorders can be treated.
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Genetic Diseases
Congenital versus Hereditary Diseases
 Congenital means present at the time of birth
 Hereditary means genetically transmitted
 May not manifest until later in life
Causes of Congenital Disorders
 Often not known
 Certain infections and toxins transmitted from mother
(e.g., German measles)
 Teratogen (agent, i.e., drug)
 Ionizing radiation
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Genetic Diseases (cont.)
Causes of Congenital Disorders
 Alcohol intake
 Fetal alcohol syndrome (FAS)
 Cigarette smoking
 Poor nutrition
 Spina bifida related to inadequate folic acid
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Mechanisms of Genetic Disease
 Single-gene diseases result from individual
mutant genes (or groups of genes) that are
passed from generation to generation
 Some diseases result from the combination of
inheritance and environmental factors
 Epigenetics (imprinting) is the science that
describes how environmental factors may result
in offspring with genetic traits that cannot be
explained by genes alone
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Chromosomal Diseases
 Chromosomal diseases result from chromosome
breakage or from nondisjunction (failure of a
chromosome pair to separate during gamete
formation)
 Trisomy—a chromosome triplet (instead of the
usual pair), caused by nondisjunction
 Monosomy—a single chromosome (instead of a
pair)
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Location of Genes Involved in
Genetic Diseases
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Effects of Nondisjunction
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Examples of Single-Gene
Diseases
 Cystic fibrosis—recessive autosomal condition
characterized by excessive secretion of mucus
and sweat, often causing obstruction of the
gastrointestinal or respiratory tracts
 Phenylketonuria (PKU) —recessive autosomal
condition characterized by excess phenylketone
in urine, caused by accumulation of
phenylalanine in tissues; may cause brain injury
and death
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Examples of Single-Gene
Diseases
 Tay-Sachs disease (TSD) is a recessive
condition involving failure to make a subunit of
an essential lipid-processing enzyme,
hexosaminidase
 Causes abnormal lipid accumulation in brain
tissue, leading to retardation and death by age 4
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Epigenetic Conditions
 Some diseases thought to result from epigenetic
DNA changes that alter gene activity
 Some cancers are associated with methylation
(abnormal levels of methyl groups)
 Fragile X syndrome (FXS) is associated with
overmethylation of part of the X chromosome,
causing mental retardation
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Examples of Chromosomal Diseases
 Down syndrome—usually caused by trisomy of
chromosome 21; characterized by mental
retardation and multiple structural defects
 Klinefelter syndrome —caused by the presence
of two or more X chromosomes in a male (usually
trisomy XXY); characterized by long legs, enlarged
breasts, low intelligence, small testes, sterility,
chronic pulmonary disease
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Down
Syndrome
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Klinefelter
Syndrome
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Turner Syndrome
 Turner syndrome—caused by monosomy of the X
chromosome (XO); characterized by immaturity of
sex organs (resulting in sterility), short stature,
webbed neck, cardiovascular defects, and learning
disorders
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Turner
Syndrome
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Genetic Counseling
 Professional consultations with families
regarding genetic diseases
 Pedigree—chart illustrating genetic
relationships over several generations
 Punnett square—grid used to determine
the probability of inheriting genetic traits
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Pedigree
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Punnett Square
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Inheritance of sex-linked traits
What is the genotype of a carrier female?
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Hereditary Traits
Mitochondrial Inheritance
 Mitochondria contain some DNA
 Multiplies independently
 Can mutate, resulting in disease
 Passed only from the mother to offspring
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Karyotype
 Arrangement of chromosome photographs used to
detect abnormalities
 Amniocentesis—involves collection of fetal cells
floating in the amniotic fluid (via a syringe needle
through the uterine wall)
 Chorionic villus sampling—involves collection of
embryonic cells from outside of chorionic tissue (via
tube through cervical opening)
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Amniocentesis
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Prenatal testing
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Treating Genetic Diseases
 Most current treatments for genetic
diseases are based on relieving or
avoiding symptoms rather than
attempting a cure
 Gene therapy—manipulates genes to cure
genetic problems; most forms of gene
therapy have just begun in humans
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Types of Gene Therapy
 Gene replacement therapy—abnormal genes in
existing body cells are replaced by therapeutic
genes
 Gene augmentation therapy—cells carrying
normal genes are introduced into the body to
augment production of a needed protein
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Gene Therapy
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Leber Congenital Amaurosis
and Cystic Fibrosis Treatment
 Leber congenital amaurosis (LCA) is a form of
blindness due to defective chromosome
 Using viruses with the therapeutic gene added and
compounds to manufacture the missing proteins in
the eye have shown early promise
 Cystic fibrosis (CF) therapies include putting
therapeutic genes in cold viruses or sending
plasmids with the therapeutic gene into the
respiratory tract
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RNA Interference
 RNAi therapy—RNA interference—silences
individual genes that cause disease
 In laboratory studies, RNAi can turn off one gene at
a time
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