Download Genetic Diseases (cont.)

Chapter 25
Heredity and
Hereditary Diseases
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Key Terms
allele
genetic
pedigree chart
amniocentesis
genotype
phenotype
autosome
heredity
progeny
carrier
heterozygous
recessive
chromosome
homozygous
sex-linked
congenital
karyotype
teratogen
dominant
meiosis
trait
familial
mutagen
gene
mutation
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Genes and Chromosomes
Genes
• Segments of DNA contained in the chromosomes
• Control manufacture of protein synthesis
• An allele is a specific version of a given gene
Chromosomes
• During mitosis, the DNA that makes up the
chromosomes is replicated and distributed to daughter
cells
• 46 chromosomes in humans
– 22 autosome pairs
– One sex chromosome pair
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Genes and Chromosomes (cont.)
Figure 25-1 Genes and chromosomes.
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Genes and Chromosomes (cont.)
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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Genes and Chromosomes (cont.)
Distribution of Chromosomes to Offspring
• Reproductive cells produced by meiosis
• 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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Genes and Chromosomes (cont.)
Figure 25-2 Meiosis.
How many cells are produced in one complete meiosis?
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Genes and Chromosomes (cont.)
Punnett Squares
A grid showing all the
combinations of alleles
that can result from a
given parental cross
• Capital letter
represents the
dominant allele
• Lower-case letter
represents the
recessive allele
What percentage of children from this cross will show the recessive
phenotype? What percentage will be heterozygous?
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Genes and Chromosomes (cont.)
Sex Determination
• Sex chromosomes not matched in size or appearance
– Female (X) chromosome larger
– Male (Y) chromosome smaller
• Sex
– Females: Two X chromosomes—XX
– Males: An X and a Y chromosome—XY
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Genes and Chromosomes (cont.)
Figure 25-4 Sex determination.
What percentage of children from this cross will show the
recessive phenotype? What percentage will be heterozygous?
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Genes and Chromosomes (cont.)
Sex-Linked Traits
• Traits carried on sex chromosomes (mostly X
chromosome)
• Most are recessive
• Usually males exhibit trait
• Heterozygous females are carriers inherited from mother
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Genes and Chromosomes (cont.)
Figure 25-5 Inheritance of sex-linked traits.
What is the genotype of a carrier female?
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Hereditary Traits (cont.)
• Observable hereditary traits are skin, eye, and hair color
• Less clearly defined traits are weight, body build, life
span, and susceptibility to disease
• Single-gene inheritance; less common
• Multifactorial (multiple gene) inheritance; most common
‒ Produces wide range of variation
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Hereditary Traits (cont.)
Gene Expression
• Influenced by:
– Sex
– Other genes
• Codominance
• Incomplete dominance
– The environment
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Hereditary Traits (cont.)
Genetic Mutation
A change in a gene or chromosome
• May be caused by:
– Chromosomal breakage or loss
– Gene fragment rearrangement
• May occur during cell division
– Spontaneous
– Induced by a mutagen
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Hereditary Traits (cont.)
Figure 25-6 Genetic exchange.
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Hereditary Traits (cont.)
Mitochondrial Inheritance
• Mitochondria contain some DNA
– Multiplies independently
– Can mutate, resulting in disease
• Passed only from the mother to offspring
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Genetic Diseases (cont.)
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 (cont.)
• Alcohol intake
– Fetal alcohol syndrome (FAS)
• Cigarette smoking
• Poor nutrition
– Spina bifida related to inadequate folic acid
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Genetic Diseases (cont.)
Figure 25-7 Congenital and hereditary diseases.
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Genetic Diseases (cont.)
Figure 25-8 Fetal alcohol syndrome.
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Genetic Diseases (cont.)
Figure 25-9 Spina bifida.
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Genetic Diseases (cont.)
Chromosomal Disorders
• Down syndrome (trisomy 21)
• Klinefelter syndrome
• Turner syndrome
• Dominant gene disorders
– Huntington disease
– Marfan syndrome
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Genetic Diseases (cont.)
Chromosomal Disorders (cont.)
• Recessive gene disorders
– Phenylketonuria (PKU)
– Sickle cell anemia
– Cystic fibrosis
– Tay-Sachs disease
– Progressive muscular atrophies
– Albinism
– Fragile X syndrome
– Osteogenesis imperfecta
– Neurofibromatosis
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Genetic Diseases (cont.)
Figure 25-10 Chromosomal disorders.
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Treatment and Prevention of Genetic
Diseases (cont.)
• More than 4,000 genetic diseases identified
• List is growing as science advances
Genetic Counseling
• Team approach
• Those who might consider genetic counseling:
– Prospective parents over 35 years of age
– Family history of genetic disorders
– Considering some form of fertility treatment
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Treatment and Prevention of Genetic
Diseases (cont.)
Genetic Counseling (cont.)
• Family history
– Pedigree chart
• Laboratory studies
– First-trimester prenatal screening:
• Nuchal transparency test
• Pregnancy-associated plasma protein test
• Human chorionic gonadotropin test
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Treatment and Prevention of Genetic
Diseases (cont.)
• Laboratory studies (cont.)
– Second-trimester screening
• Alpha-fetoprotein (AFP) screening
• Estriol test
• Inhibin test
‒ Amniocentesis
• Chorionic villus sampling
• Karyotyping
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Treatment and Prevention of Genetic
Diseases (cont.)
Figure 25-11 A pedigree (family history) showing three
generations (F1–F3).
What are the possible genotypes of the two normal children
in the F3 generation?
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Treatment and Prevention of Genetic
Diseases (cont.)
Figure 25-12 Prenatal testing.
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Treatment and Prevention of Genetic
Diseases (cont.)
Figure 25-13 Karyotype.
Look closely at this karyotype. How many chromosomes are
present? What is the gender of the baby, and what genetic
disorder is represented?
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Treatment and Prevention of Genetic
Diseases (cont.)
Counseling Prospective Parents
• Counselors have pertinent facts from family history and
laboratory studies
• Prospective parents may use information to make
decisions
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Treatment and Prevention of Genetic
Diseases (cont.)
Progress in Medical Treatment
• Dietary control for certain diseases
– Examples
• Maple syrup urine disease
• Wilson disease
• Phenylketonuria
• Vitamins
• Hormones
• Future: Possible treatment or correction of genetic
disorder with genetic engineering
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Case Study
Role of a Genetic Counselor
• Works as part of a team along with physicians, nurses,
laboratory, and social service professionals
• Collects all of the pertinent facts from the family history
and laboratory studies
• Has knowledge of genetic inheritance patterns
• Presents information and discuss with prospective
parents/parents the inheritance of their children
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Word Anatomy (cont.)
Word Part
Meaning
Example
Genes and Chromosomes
aut/o
self
Autosomes are all the chromosomes
aside from the two that determine
sex.
chrom/o
color
Chromosomes color darkly with stains.
heter/o
other, different
Heterozygous paired genes (alleles)
are different from each other.
homo-
same
Homozygous paired genes (alleles) are
the same.
phen/o
to show
Traits that can be observed or tested
for make up a person’s phenotype.
Heredity Traits
multi-
many
Multifactorial traits are determined by
multiple pairs of genes.
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Word Anatomy (cont.)
Word Part
Meaning
Example
Genetic Diseases
-cele
swelling
In spina bifida, the meninges can
protrude through the spine as a
meningocele.
con-
with
A congenital defect is present at the
time of birth.
dacty/o
digit (finger or
toe)
In polydactyly, there is an extra finger
on the hand.
terat/o
malformed fetus
A teratogen is an agent that causes
birth defects.
Treatment and Prevention of Genetic Diseases
-centesis
tapping,
perforation
Amniocentesis is a tap of the amniotic
fluid.
kary/o
nucleus
A karyotype is an analysis of the
chromosomes contained in the nucleus
of a cell.
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