Transcript Autosomal Recessive Inheritance

Human Inheritance
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
12.1 Human Chromosomes
 Males have XY sex chrom, females have XX
 All other chromosomes are autosomes –
chromosomes that are same in males and
females
Sex Determination in Humans
 Sex of a child is determined by the father
• Eggs have an X chromosome; sperm have X or Y
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
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
Autosomal Inheritance
Fig. 12-4a, p. 188
Fig. 12-4b, p. 188
Galactosemia
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
12.3 Too Young to be Old
 Progeria
• Genetic disorder that results in accelerated aging
• Caused by spontaneous mutations in autosomes
12.4 Examples of X-Linked Inheritance
 X chrom 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
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
X-Linked Recessive Inheritance Patterns
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
Hemophilia A in Descendents
of Queen Victoria of England
Nondisjunction
 Changes in chro number cause nondisjunction,
when a pair of chrom fails to separate during
meiosis
 Affects the chromosome number at fertilization
• Monosomy (n-1 gamete)
• Trisomy (n+1 gamete)
Nondisjunction
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
Trisomy 21
Down Syndrome and Maternal Age
Change in Sex Chromosome Number
 Changes in sex chrom number may impair
learning or motor skills
 Female sex chrom abnormalities
• Turner syndrome (XO)
• XXX syndrome (three or more X chromosomes)
 Male sex chromosome abnormalities
• Klinefelter syndrome (XXY)
• XYY syndrome
Turner Syndrome
 XO (one unpaired X
chromosome)
• Usually caused by
nondisjunction in the
father
• Results in females
with undeveloped
ovaries
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
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)
Some Human Genetic Disorders
and Genetic Abnormalities
Constructing a Pedigree for Polydactyly
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
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
Amniocentesis
Fetoscopy
Preimplantation Diagnosis
 Used in in-vitro fertilization
• An undifferentiated cell is removed from the early
embryo and examined before implantation
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
Genetic Screening
 Genetic screening (widespread, routine testing
for alleles associated with genetic disorders)
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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