genetics
TRANSCRIPT
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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
× XX
fertilization:
X X
X XX XX
Y XY XY
sex chromosome combinations possible in the new individual
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Fig. 12-2bc, p. 186
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Fig. 12-2bc, p. 186
At seven weeks, appearance of “uncommitted” duct system of embryo
At seven weeks, appearance of structures that will give rise to external genitalia
Y chromosome present
Y chromosome absent
Y chromosome present
Y chromosome absent
testes ovaries
10 weeks 10 weeks
ovary
penis vaginal opening
uterus
penis vagina
testis birth approaching
b c
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Animation: Human sex determination
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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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Fig. 12-3a, p. 187
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Fig. 12-3b, p. 187
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Animation: Karyotype preparation
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12.1 Key ConceptsAutosomes 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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Fig. 12-4a, p. 188
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Fig. 12-4b, p. 188
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Animation: Autosomal dominant inheritance
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Animation: Autosomal recessive 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 ConceptsAutosomal 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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Animation: X-linked inheritance
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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. 12-9a, p. 191
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Fig. 12-9b, p. 191
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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 ConceptsSex-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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p. 192
normal chromosome
one segment repeated
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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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p. 192
segment C deleted
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Deletion: Cri-du-chat
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Fig. 12-10a, p. 192
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Fig. 12-10b, p. 192
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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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p. 192
segments G, H, I become inverted
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Translocation
Typically, two broken chromosomes exchange parts (reciprocal translocation)
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p. 192
chromosome
nonhomologous chromosome
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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Fig. 12-11, p. 193
human chimpanzee gorilla orangutan
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Evolution of X and Y Chromosomes from Homologous Autosomes
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Fig. 12-12, p. 193
Ancestral reptiles Ancestral reptiles Y X
Monotremes Y X
Marsupials Y X
Monkeys Y X
Humans Y X(autosome pair)
areas that can cross over
areas that cannot cross over
SRY
A Before 350 mya, sex was determined by temperature, not by chromosome differences.
B SRY gene evolves 350 mya. Other mutations accumulate and the chromosomes of the pair diverge.
C By 320–240 mya, the two chromosomes have diverged so much that they no longer cross over in one region. The Y chromosome begins to degenerate.
D Three more times, 170–130 mya, the pair stops crossing over in another region. Each time, more changes accumulate, and the Y chromosome gets shorter. Today, the pair crosses over only at a small region near the ends.
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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 GAMETESanaphase II
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Fig. 12-13b, p. 194
chromosome alignments at metaphase I
NONDISJUNCTION AT ANAPHASE I
alignments at metaphase II
n + 1
n + 1
n − 1
n − 1
CHROMOSOME NUMBER
IN GAMETESanaphase IIStepped Art
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Down Syndrome and Maternal Age
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Fig. 12-14a, p. 195
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Fig. 12-14b, p. 195
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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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Animation: Pedigree diagrams
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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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Animation: 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 ConceptsHuman 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
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Animation: Deletion
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Animation: Duplication
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Animation: Inversion
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Animation: Morgan’s reciprocal crosses
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Animation: Translocation
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Video: Strange genes, richly tortured minds