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Chapter 24
Genetics and Genomics
Genetics – study of inheritance of characteristics
Genome – complete set of genetic instructions
Genomics – field in which the body is studied in terms of
multiple, interacting genes
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Chromosomes and Genes Come in
Pairs
23 pairs of chromosomes
•  pairs 1-22 are autosomes
•  pair 23 are sex chromosomes
Normal karyotype
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Genotype and Phenotype
Genotype
•  particular combination of genes
•  alleles are variant forms of the same gene
•  homozygous – identical alleles
•  heterozygous – different alleles
•  wild type allele – produces most common or normal
phenotype
Phenotype
•  way that genes are expressed
•  blue eyes, presence of a protein, etc
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Dominant and Recessive
Inheritance
Dominant allele masks the phenotype of the recessive allele
Recessive allele is expressed only if in a double dose (homozygous)
Autosomal conditions are carried on a nonsex chromosome
Sex-linked conditions are carried on a sex chromosome
X-linked conditions are carried on the X chromosome
Y-linked conditions are carried on the Y chromosome
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Autosomal Recessive Disorder
•  cystic fibrosis is an
example
•  sexes are affected with
equal frequencies
•  offspring probabilities
•  25% homozygous
dominant
•  50% heterozygous
•  25% homozygous
recessive
•  punnet square and a
pedigree are useful ways
to express genetic
information
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Autosomal Dominant Disorder
•  Huntington
disease is an
example
•  a person with one
HD allele develops
the disease
•  both sexes are
equally affected
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2
Different Dominance Relationships:
Incomplete Dominance
•  heterozygote has a
phenotype
intermediate between
homozygous
dominant and
homozygous recessive
•  familial
hypercholesterolemia
is an example
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Different Dominance Relationships:
Codominance
•  different alleles are both expressed
•  ABO blood type is an example
•  three alleles of ABO blood typing are IA, IB, I
•  a person with type A may have the genotype IA i or IA IA
•  a person with type B may have the genotype IB i or IB IB
•  a person with type AB must have the genotype IA IB
•  a person with type O blood must have the genotype ii
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Gene Expression:
Penetrance and Expressivity
Complete penetrance
•  everyone who inherits the disease causing alleles
has some symptoms
Imcomplete penetrance
•  some individuals do not express the phenotype
even though they inherit the alleles (example
polydactyly)
Variable expression
•  symptoms vary in intensity in different people
•  two extra digits versus three extra digits in
polydactyly
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3
Gene Expression:
Pleiotrophy and Genetic
Heterogeneity
Pleiotropy
•  single genetic disorder producing several symptoms
•  Marfan syndrome is an example
•  people affected produce several symptoms that vary
Genetic Heterogeneity
•  same phenotype resulting from the actions of different
genes
•  hereditary deafness is an example
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Complex Traits
Most if not all characteristics and disorders considered
“inherited” actually reflect input from the environment
as well as genes.
Polygenic traits
•  determined by more than one gene
•  height, skin color, eye color
Multifactorial traits
•  traits molded by one or more genes plus
environmental factors
•  height and skin color
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Variations in Height
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4
Variations in Skin Color
Mid-range colors are more common
13
Sex Determination
•  an egg contributes
an X chromosome
•  a sperm contributes
either an X or a Y
chromosome
•  A gene on the Y
chromosome, SRY,
determines sex
14
Sex Chromosomes and Their
Genes
X chromosome
•  has over 1,500 genes
•  most genes on the X chromosome do not have
corresponding alleles on the Y chromosome
Y chromosome
•  has only 231 protein-encoding genes
•  some genes are unique only to the Y chromosome
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5
Sex-linked Genes
•  Y-linked genes are transmitted only from father to son
•  X-linked genes are transmitted from father to daughter
or from mother to daughter or son
•  Hemophilia A is a sex-linked disorder
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Hemophilia A
•  passed from mother (heterozygote) to son
•  each son has a 50% chance of receiving the recessive allele from the
mother
•  each son with one recessive allele will have the disease
•  each son has no allele on the Y chromosome to mask the recessive allele
•  each daughter has a 50% chance of receiving the recessive allele from the
mother
•  each daughter with one recessive allele will be a carrier
•  each daughter with one recessive allele does not develop the disease
because she has another X chromosome with a dominant allele
17
Gender Effects on Phenotype
Sex-limited trait
•  affects a structure or function of the body that is
present in only males or only females
•  examples are beards or growth of breasts
Sex-influenced inheritance
•  an allele is dominant in one sex and recessive in the
other
•  baldness is an example
•  heterozygous males are bald but heterozygous females
are not
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6
Chromosomal Disorders
Polyploidy
•  extra set of chromosomes
•  most embryos die
Aneuploidy
•  missing a chromosome or having an extra chromosome
•  results from nondisjunction
•  trisomy is the condition of having an extra chromosome
•  monosomy is the condition of missing a chromosome
Euploid is a normal chromosome number
19
Causes of Aneuploidy
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Prenatal Tests
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Prenatal Tests
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Gene Therapy
•  group of experimental techniques that alter, replace, silence,
or augment a gene’s function to improve, delay or prevent
symptoms
•  heritable gene therapy
•  introduces the genetic change into a sperm, egg, or
zygote
•  changes passed to future generations
•  common in plants; not done in humans
•  nonheritable gene therapy
•  targets only affected cells
•  changes not passed to future generations
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Gene Therapy Targets
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Clinical Application
Down Syndrome
•  most common autosomal aneuploid
•  trisomy 21
•  signs and symptoms include
•  short stature
•  mental retardation
•  protruding tongue
•  heart defects
•  kidney defects
•  suppressed immune systems
•  digestive disorders
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