Download Chapter 11 Complex Inheritance and Human Heredity

Chapter 11 Complex
Inheritance and Human
Heredity
11.1 Basic Patterns of Human
Inheritance
1
Recessive Genetic Disorders
Mendel’s work went unnoticed by the scientific
community for about 30 years then it was
rediscovered in the early 1900s.
 At that time many scientists were interested in
the cause of diseases and noticed that some
diseases “ran in families”.
 Alkaptonuria was the first identified (recessive)
genetic disorder. Alkaptonuria, from an enzyme
deficiency, causes black acidic urine and later in
life affects bones and joints.
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Recessive Genetic Disorders
 A recessive trait is
expressed when the
individual is
homozygous recessive
for the trait.
 Both parents would
need to have at least
one recessive allele.
 Usually the parents
are heterozygous
(carriers) for the
disorder.
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Cystic Fibrosis
Affects the mucus-producing glands,
digestive enzymes, and sweat glands
 Chloride ions are not absorbed into the
cells of a person with cystic fibrosis but
are excreted in the sweat.
 Without sufficient chloride ions in the
cells, a thick mucus is secreted
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Cystic Fibrosis
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Cystic Fibrosis
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Albinism
Caused by altered genes, resulting in the
absence of the skin pigment melanin in
hair and eyes
 White hair.
 Very pale skin
 Pink pupils
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Albinism
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Tay-Sachs Disease
Caused by the absence of the enzymes
responsible for breaking down fatty acids
called gangliosides
 Gangliosides accumulate in the brain,
inflating brain nerve cells and causing
mental deterioration.
 Death by age 2
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Dominant Genetic Disorders
99.9% of population is homozygous recessive for achondroplasia
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Huntington’s Disease
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Affects the nervous
system
Latent disorder
affects age 30 to 50
Gradual loss of brain
function (“holes” in
brain)
Genetic test available
Result of allele
mutation at tip of
chromosome #4
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Achondroplasia
Most common form of
dwarfism
 75% of individuals
born to parents of
average size, result of
new mutation
 Lethal spontaneous
abortion in
homozygous
dominant genotype
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Pedigree Analysis
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Pedigree Analysis
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Are females
Are males
Shaded in circles and
squares are affected
individuals
Roman Numerals (I –
IV) are generations
Lines across
represent mating
Lines down
represent offspring
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Pedigree Analysis
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Count the number of
affected males and affected
females. If most males and
few or no females most
likely sex linked trait.
Look at the affected
individuals. If every
individual with the trait has
a parent with the trait then
this trait is dominant. If
non-affected parents
produce an offspring with
the trait then it is recessive.
Determine the phenotype
and genotype of every
individual
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Pedigree Analysis
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Count the number of
affected males and affected
females. If most males and
few or no females most
likely sex linked trait.
Look at the affected
individuals. If every
individual with the trait has
a parent with the trait then
this trait is dominant. If
non-affected parents
produce an offspring with
the trait then it is recessive.
Determine the phenotype
and genotype of every
individual
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Pedigree Analysis
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Affected males: 1
Affected females: 2
(not sex linked)
No affected individual
has parent with the
trait, means recessive
All affected individuals
would be homozygous
recessive, aa
All parents of affected
individuals would be
heterozygous, Aa
Siblings of affected
individuals would be
heterozygous (Aa) or
homozygous dominant
(AA)
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Pedigree Analysis
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Chapter 11 Complex
Inheritance and Human
Heredity
11.2 Complex Patterns of
Inheritance
20
Incomplete Dominance
Many scientists repeated Mendel’s work
expecting to get similar results with
different species. Carl Correns crossed
white snapdragons with red snapdragons
 According to Mendelian genetics, results
would have yielded 100% dominant
phenotype (red or white)
 Correns found 100% pink
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Incomplete Dominance
Incomplete Dominance is the appearance of an
intermediate phenotype
 Both alleles are expressed producing both the
red and white proteins which gives the
appearance of pink
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Codominance
Both phenotypes of
the two homozygotes
are expressed
 See red and white
(not pink)
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Codominance
Both phenotypes of
the two homozygotes
are expressed
 See red hairs and
white hairs (not pink)

Roan Cattle
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Codominance
Sickle cell disease most
common genetic
disorder in African
Americans (1 in 400).
 In heterozygous
condition both normal
(round) and sickle red
blood cells are
produced
 Few disease symptoms
in heterozygous
individual
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Codominance
Sickle cells do not
effectively carry
oxygen, cause
blockage in blood
vessels, pain and
fatigue.
 Heterozygous
condition protects
against malaria (see
p.303)
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Multiple Alleles
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More than two alleles
Human blood groups
have three alleles: ABO
Blood Types: Genotypes
A: IA IA or IAi
B: IB IB or IBi
AB: IA IB
O: ii
Both IA and IB are dominant
to i and are codominant to
each other
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Multiple Alleles
Multiple alleles can demonstrate a
hierarchy of dominance.
 In rabbits, four alleles code for coat color:
C, cch, ch, and c.

§ C is dominant to cch, ch, and c.
§ cch is dominant to ch and c.
§ ch is dominant to c.
§ c is recessive to all
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Multiple Alleles
Chinchilla
?
Albino
Light gray
Dark gray
Himalayan
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Epistasis
 Variety is the result of one allele hiding the
effects of another allele.
eebb
eeB_
No dark pigment present in fur
E_bb
E_B_
Dark pigment present in fur
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Sex Determination
 Sex chromosomes
determine an
individual’s gender.
 Human males are XY;
females XX
 Autosomes are the
nonsex chromosomes
 Humans have 22
pairs of autosomes
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Dosage Compensation
The X chromosome is more than three
times larger than the Y chromosome; has
more than three times more information
 Females (XX) have two copies of this
information while males (XY) have only
one copy of the X chromosome
 In females one of the X chromosomes
stops working; which X is a random event
and becomes a Barr Body
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Dosage Compensation
Barr bodies form from
the inactivated “extra”
X chromosome in
females
 Colors of the calico
cat are caused by the
random inactivation
of a particular X
chromosome (orange
on one X and black
on the other X
chromosome)
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Sex-Linked Traits
The X and Y chromosomes are not
homologous (do not pair: carry different
information)
 Males only have one X chromosome; only
one allele for each trait; only need one
recessive to show the trait
 Males show the sex linked traits much
more often than females
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Sex-Linked Traits
Red Green Color Blindness
 8% of United States
males affected
 Red and green colors
look like shades of
brown
 Are you color blind?
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Sex-Linked Traits

Hemophilia is
characterized by
delayed blood
clotting.
– Common in Royal
Families of Europe due
to intermarriage
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Polygenic Traits
Poly = many; genic = genes
 Many genes needed to determine a trait
 Most human traits are polygenic
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Environmental Influences
Environment has an influence on
phenotype
 Sunlight: without enough sunlight most
flowering plants do not bear flowers
 Water: many plants lose their leaves in
response to lack of water.
 others: diet, exercise
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Environmental Influences
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Temperature:
Siamese cat’s tail,
feet, ears and nose
are dark in response
to cooler
temperatures; the
black pigment
production is
responsive to
temperature
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Twin Studies
 Helps scientists separate genetic
contributions from environmental
contributions
 Traits that appear frequently in identical
twins are at least partially controlled by
heredity.
 Traits expressed differently in identical
twins are strongly influenced by
environment.
 Interesting information from identical twins
separated at birth in Minnesota Twin
Studies
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Chapter 11 Complex
Inheritance and Human
Heredity
11.3 Chromosomes and Human
Heredity
41
Karyotype Studies
Karyotype is a micrograph (picture) of
chromosomes at metaphase arranged by size in
homologous pairs.
 Twenty-two pairs of autosomes are the same for
males and females.
 Males have one X chromosome and one Y
chromosome (not paired) and females have two
X chromosomes (paired).
 Karyotypes are useful for detecting chromosome
abnormalities
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Karyotype Studies
Chromosome smear
Karyotype
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Telomeres
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Telomeres protective end
caps on the
chromosomes made of
DNA and proteins.
Short repetitive
nonessential DNA
sequences
Prevent one chromosome
form binding to another
chromosome (not sticky)
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Telomeres
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Absolutely essential for
chromosome function
Telomeres decrease in
size (length) as an
organism ages and may
play a role in aging and
cancer.
Cancer cells have
increased length of
telomeres compared to
adult cells.
Telomere function is an
intense field of research
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Nondisjunction
Failure of chromosomes to separate
during cell division
 Results in one cell getting too many
chromosomes and one cell not getting
enough.
 If nondisjunction occurs during meiosis
the resulting gametes could form an
organism with every cell having an
incorrect chromosome number.
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Down Syndrome
Results of parental gamete nondisjunction
resulting in trisomy (three chromosomes )
of chromosome #21.
 Symptoms include: mental retardation,
distinctive facial features, short stature,
heart defects, sexually underdeveloped,
sterile, more likely to develop leukemia
and Alzheimer's, shorter life span
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Down Syndrome
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Down Syndrome
Incidence of Down
Syndrome increases
with increasing age of
the mother
 Risk increases to 6%
for mothers over age
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 Recommended fetal
testing for mother’s
age 35 and older
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Other Trisomys
Patau syndrome: trisomy 13; 1/5000;
serious eye, brain and circulatory
problems; lifespan less than 1 year
 Edward’s syndrome: trisomy 18; 1/10,000;
effects every body organ; lifespan less
than 1 year
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Nondisjunction of Sex
Chromosomes Male
Klinefelter syndrome: (XXY); 1/2000;
males sex organs, testes abnormally
small; sterile; feminine body
characteristics (breast enlargement, fat
deposition); normal intelligence
 (XYY); taller than average male; not
characterized by a “syndrome”; some
suggest increased aggression
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Nondisjunction of Sex
Chromosomes Female
(XXX): 1/1000; normal due to X
inactivitation; only distinguishable by
karyotype
 Turner syndrome Monosomy X (XO):
1/5000; female with no maturation of sex
organs; no secondary sex characteristics;
short; sterile; normal intelligence with
spatial learning disabilities
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Fetal Testing
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Amniocentesis:
removal amniotic fluid
with sloughed fetal
cells at 14th -16th
week; for diagnosis of
chromosomal
abnormalities; 1%
risk of complications
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Fetal Testing
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Chorionic villus
sampling: removal of
placental sample at
8th -10th week;
quicker results; less
accurate; less than
1% risk of
complications
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Fetal Testing

Fetal blood sampling:
sample of fetal blood;
can detect increased
number of genetic
abnormalities;
medication can be
given to fetus before
birth; greater risk to
fetus
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