Chromosomes and Human Inheritance · Constructing a Pedigree for Polydactyly . 12.8 Prospects in...
Transcript of Chromosomes and Human Inheritance · Constructing a Pedigree for Polydactyly . 12.8 Prospects in...
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Chromosomes and
Human Inheritance
Chapter 12
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Impacts, Issues:
Strange Genes, Tortured Minds
Exceptional creativity often accompanies
neurobiological disorders such as schizophrenia,
autism, chronic depression, and bipolar disorder
• Examples: Lincoln, Woolf, and Picasso
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12.1 Human Chromosomes
In humans, two sex chromosomes are the
basis of sex – human males have XY sex
chromosomes, females have XX
All other human chromosomes are autosomes
– chromosomes that are the same in males and
females
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Sex Determination in Humans
Sex of a child is determined by the father
• Eggs have an X chromosome; sperm have X or Y
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Sex Determination in Humans
The SRY gene on the Y chromosome is the
master gene for male sex determination
• Triggers formation of testes, which produce the
male sex hormone (testosterone)
• Without testosterone, ovaries develop and
produce female sex hormones (estrogens)
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Sexual Development in Humans
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Fig. 12-2a, p. 186
diploid
germ cells
in female
diploid
germ cells
in male
meiosis, gamete
formation in both
female and male:
eggs sperm
X × Y
× X X
fertilization:
X X
X XX XX
Y XY XY
sex chromosome combinations
possible in the new individual
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Karyotyping
Karyotype
• A micrograph of all metaphase chromosomes in a
cell, arranged in pairs by size, shape, and length
• Detects abnormal chromosome numbers and
some structural abnormalities
Construction of a karyotype
• Colchicine stops dividing cells at metaphase
• Chromosomes are separated, stained,
photographed, and digitally rearranged
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Karyotyping
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12.1 Key Concepts
Autosomes and Sex Chromosomes
All animals have pairs of autosomes –
chromosomes that are identical in length, shape,
and which genes they carry
Sexually reproducing species also have a pair of
sex chromosomes; the members of this pair
differ between males and females
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12.2 Autosomal Inheritance Patterns
Many human traits can be traced to autosomal
dominant or recessive alleles that are inherited
in Mendelian patterns
Some of those alleles cause genetic disorders
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Autosomal Dominant Inheritance
A dominant autosomal allele is expressed in
homozygotes and heterozygotes
• Tends to appear in every generation
• With one homozygous recessive and one
heterozygous parent, children have a 50%
chance of inheriting and displaying the trait
• Examples: achondroplasia, Huntington’s disease
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Autosomal Recessive Inheritance
Autosomal recessive alleles are expressed only
in homozygotes; heterozygotes are carriers and
do not have the trait
• A child of two carriers has a 25% chance of
expressing the trait
• Example: galactosemia
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Autosomal Inheritance
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Galactosemia
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Neurobiological Disorders
Most neurobiological disorders do not follow
simple patterns of Mendelian inheritance
• Depression, schizophrenia, bipolar disorders
Multiple genes and environmental factors
contribute to NBDs
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12.3 Too Young to be Old
Progeria
• Genetic disorder that results in accelerated aging
• Caused by spontaneous mutations in autosomes
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12.2-12.3 Key Concepts
Autosomal Inheritance
Many genes on autosomes are expressed in
Mendelian patterns of simple dominance
Some dominant or recessive alleles result in
genetic disorders
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12.4 Examples of X-Linked Inheritance
X chromosome alleles give rise to phenotypes
that reflect Mendelian patterns of inheritance
Mutated alleles on the X chromosome cause or
contribute to over 300 genetic disorders
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X-Linked Inheritance Patterns
More males than females have X-linked
recessive genetic disorders
• Males have only one X chromosome and can
express a single recessive allele
• A female heterozygote has two X chromosomes
and may not show symptoms
Males transmit an X only to their daughters, not
to their sons
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X-Linked Recessive Inheritance Patterns
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Some X-Linked Recessive Disorders
Hemophilia A
• Bleeding caused by lack of blood-clotting protein
Red-green color blindness
• Inability to distinguish certain colors caused by
altered photoreceptors in the eyes
Duchenne muscular dystrophy
• Degeneration of muscles caused by lack of the
structural protein dystrophin
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Hemophilia A in Descendents
of Queen Victoria of England
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Red-Green Color Blindness
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Fig. 12-9c, p. 191
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Fig. 12-9d, p. 191
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12.4 Key Concepts
Sex-Linked Inheritance
Some traits are affected by genes on the X
chromosome
Inheritance patterns of such traits differ in males
and females
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12.5 Heritable Changes
in Chromosome Structure
On rare occasions, a chromosome’s structure
changes; such changes are usually harmful or
lethal, rarely neutral or beneficial
A segment of a chromosome may be duplicated,
deleted, inverted, or translocated
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Duplication
DNA sequences are repeated two or more
times; may be caused by unequal crossovers in
prophase I
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Deletion
Loss of some portion of a chromosome; usually
causes serious or lethal disorders
• Example: Cri-du-chat
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Deletion: Cri-du-chat
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Inversion
Part of the sequence of DNA becomes oriented
in the reverse direction, with no molecular loss
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Translocation
Typically, two broken chromosomes exchange
parts (reciprocal translocation)
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Does Chromosome Structure Evolve?
Changes in chromosome structure can reduce
fertility in heterozygotes; but accumulation of
multiple changes in homozygotes may result in
new species
Certain duplications may allow one copy of a
gene to mutate while the other carries out its
original function
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Differences Among
Closely Related Organisms
Humans have 23 pairs
of chromosomes;
chimpanzees, gorillas,
and orangutans have
24
• Two chromosomes
fused end-to-end
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Evolution of X and Y Chromosomes
from Homologous Autosomes
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12.6 Heritable Changes in
the Chromosome Number
Occasionally, new individuals end up with the
wrong chromosome number
• Consequences range from minor to lethal
Aneuploidy
• Too many or too few copies of one chromosome
Polyploidy
• Three or more copies of each chromosome
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Nondisjunction
Changes in chromosome number can be caused
by nondisjunction, when a pair of
chromosomes fails to separate properly during
mitosis or meiosis
Affects the chromosome number at fertilization
• Monosomy (n-1 gamete)
• Trisomy (n+1 gamete)
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Nondisjunction
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Autosomal Change and Down Syndrome
Only trisomy 21 (Down syndrome) allows
survival to adulthood
• Characteristics include physical appearance,
mental impairment, and heart defects
Incidence of nondisjunction increases with
maternal age
Can be detected through prenatal diagnosis
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Trisomy 21
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Fig. 12-13b, p. 194
n + 1
n + 1
n − 1
n − 1
chromosome
alignments at
metaphase I
NONDISJUNCTION
AT ANAPHASE I
alignments at
metaphase II
CHROMOSOME
NUMBER
IN GAMETES anaphase II
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Down Syndrome and Maternal Age
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Change in Sex Chromosome Number
Changes in sex chromosome number may
impair learning or motor skills, or be undetected
Female sex chromosome abnormalities
• Turner syndrome (XO)
• XXX syndrome (three or more X chromosomes)
Male sex chromosome abnormalities
• Klinefelter syndrome (XXY)
• XYY syndrome
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Turner Syndrome
XO (one unpaired X
chromosome)
• Usually caused by
nondisjunction in the
father
• Results in females
with undeveloped
ovaries
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12.5-12.6 Key Concepts: Changes in
Chromosome Structure or Number
On rare occasions, a chromosome may undergo
a large-scale, permanent change in its structure,
or the number of autosomes or sex
chromosomes may change
In humans, such changes usually result in a
genetic disorder
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12.7 Human Genetic Analysis
Charting genetic connections with pedigrees
reveals inheritance patterns for certain alleles
Pedigree
• A standardized chart of genetic connections
• Used to determine the probability that future
offspring will be affected by a genetic abnormality
or disorder
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Studying Inheritance in Humans
Genetic studies can
reveal inheritance
patterns or clues to
past events
• Example: A link
between a Y
chromosome and
Genghis Khan?
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Defining Genetic Disorders
and Abnormalities
Genetic abnormality
• A rare or uncommon version of a trait; not
inherently life threatening
Genetic disorder
• An inherited condition that causes mild to severe
medical problems, characterized by a specific set
of symptoms (a syndrome)
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Some Human Genetic Disorders
and Genetic Abnormalities
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Table 12-1, p. 196
Stepped Art
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Recurring Genetic Disorders
Mutations that cause genetic disorders are rare
and put their bearers at risk
Such mutations survive in populations for
several reasons
• Reintroduction by new mutations
• Recessive alleles are masked in heterozygotes
• Heterozygotes may have an advantage in a
specific environment
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A Pedigree for Huntington’s Disease
A progressive degeneration of the nervous
system caused by an autosomal dominant allele
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Constructing a Pedigree for Polydactyly
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12.8 Prospects in Human Genetics
Genetic analysis can provide parents with
information about their future children
Genetic counseling
• Starts with parental genotypes, pedigrees, and
genetic testing for known disorders
• Information is used to predict the probability of
having a child with a genetic disorder
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Prenatal Diagnosis
Tests done on an embryo or fetus before birth to
screen for sex or genetic problems
• Involves risks to mother and fetus
Three types of prenatal diagnosis
• Amniocentesis
• Chorionic villus sampling (CVS)
• Fetoscopy
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Amniocentesis
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Fetoscopy
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Preimplantation Diagnosis
Used in in-vitro fertilization
• An undifferentiated cell is removed from the early
embryo and examined before implantation
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After Preimplantation Diagnosis
When a severe problem is diagnosed, some
parents choose an induced abortion
In some cases, surgery, prescription drugs,
hormone replacement therapy, or dietary
controls can minimize or eliminate symptoms of
a genetic disorder
• Example: PKU can be managed with dietary
restrictions
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Genetic Screening
Genetic screening (widespread, routine testing
for alleles associated with genetic disorders)
• Provides information on reproductive risks
• Identifies family members with a genetic disorder
• Used to screen newborns for certain disorders
• Used to estimate the prevalence of harmful
alleles in a population
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12.7-12.8 Key Concepts
Human Genetic Analysis
Various analytical and diagnostic procedures
often reveal genetic disorders
What an individual, and society at large, should
do with the information raises ethical questions