Download NOTES: 14.1 -14.2 HUMAN HEREDITY

NOTES: 14.1 -14.2
HUMAN HEREDITY
• Key Terms
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Autosomal Recessive
Autosomal Dominant
Pedigree
Tay-Sachs
Cystic Fibrosis
Phenylketonuria
• Key Concepts
– How to read and interpret a
pedigree
– How diseases are caused
Recessive and Dominant
Alleles:
• Some common genetic disorders are autosomal
recessive
– This means that you need two recessive alleles (on
any of the 44 chromosomes—NOT the sex
chromosomes) to express the disease
• EX: Cystic Fibrosis
• Other genetic disorders are autosomal dominant
– Only one allele is needed for the trait to be
expressed
• EX: Huntington’s Disease
Autosomal Recessive Disorders:
• In order to develop an autosomal
recessive trait, an individual must have the
genotype: “aa”
• To be born with a homozygous recessive
genotype, both parents must be
heterozygotes (“carriers”)…or
homozygous recessive themselves
(although with most disorders, that is not
the case)…WHY NOT?
From gene to molecule:
• In both cystic fibrosis and sickle cell
anemia, a small change in the DNA of a
single gene affects the structure of a
protein, causing a serious genetic disorder
Cystic Fibrosis:
• Caused by a recessive allele on chromosome
#7
– It is an autosomal genetic disorder
• Causes digestive and respiratory problems
• Death around 20-30 years of age
• How does it happen?
– Three bases are deleted from the protein, which
removes one amino acid
– The protein cannot fold properly anymore, and is
destroyed
– Result: airway is clogged with mucus
CF Example:
• Cystic fibrosis heterozygotes (Ff)
– just one copy of the normal (dominant) allele is
enough to supply the cell with the proper proteins to
function.
– Because of this, the normal allele is considered
dominant over the recessive allele
• Therefore, a person who is heterozygous does
not suffer from Cystic Fibrosis
Sickle Cell Anemia:
One DNA base has been changed
• Amino acid is valine, instead of glutamic
acid
• Result = abnormal hemoglobin
• The abnormal hemoglobin forms crystallike structures that change the shape of
the red blood cells
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Sickle Cell Anemia:
• The abnormal red blood cells are shaped
like a “sickle” or a half-moon;
• These RBCs slow blood flow, block small
blood vessels, and result in tissue damage
and pain.
Sickle Cell Anemia:
• Three genotypes possible:
1) HH = all healthy hemoglobin
2) Hh = ½ healthy hemoglobin; ½
abnormal hemoglobin;
**CODOMINANCE!!
3) hh = all abnormal hemoglobin
Sickle Cell Anemia:
• Individuals who are Hh do not have
serious health problems and can lead
relatively normal lives, but• They do show some signs of sickle cell
anemia if the availability of oxygen is
reduced
(i.e. high altitude;
strenuous exercise)
Pedigrees
• Graphical
representation
recording the line of
ancestors
– Family tree
– Used for breeding
animals (e.g. dogs,
cats, horses)
– Trace genetic
disorders
Pedigree Chart:
• Shows how a trait is transmitted
from generation to generation
• Each row is a generation
• Circles represent females
• Squares represent males
– Shaded in: person expresses
that trait
– Half shaded in: person is only a
carrier
– Clear: person does not carry or
express that trait
Symbols to Pedigrees
Tay-Sachs Disease:
• Autosomal recessive disorder
• Recessive allele results in the absence of
an enzyme that normally breaks down
lipids in the central nervous system
• Without this enzyme, the lipids accumulate
in the nervous system and the affected
individual dies
Phenylketonuria (PKU):
• Autosomal recessive disorder;
• Absence of an enzyme to break down the amino
acid phenylalanine
• The accumulation of phenylalanine causes
damage to the nervous system
• By avoiding phenylalanine in
the diet, affected infants
can avoid the symptoms
of the disorder
Autosomal Recessive Traits &
Pedigrees:
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May skip a generation
Affected individuals are born to 2 carriers
Males and females affected equally
KNOWN carriers will be half-shaded in…it
is not always possible to know if an
individual is a carrier
What about autosomal dominant
human traits?
• A single dominant allele inherited from 1
parent is all that is needed for a person to
show the dominant trait.
Simple Dominant Traits:
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Tongue rolling
Unattached earlobes
Hitchhiker’s thumb
Hair in the middle section of fingers
Ability to taste PTC
Autosomal Dominant Traits &
Pedigrees:
• Typically seen in every generation,
affecting multiple people
• Affected individuals are born to affected
parent(s)
• Males and females affected equally
• There are NO “carriers”!
(you either have it – AA or Aa –
or you don’t – aa)
Autosomal Dominant Disorder:
HUNTINGTON’S DISEASE
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Rare, but lethal, dominant allele;
Results in a breakdown of parts of the brain;
Onset between the ages of 30 and 50;
No known treatment or cure;
There is a test available to see if you have it –
may help with the decision to start a family;
• Every child of an affected individual has a 50/50
chance of being affected (and passing it down)
Sex-Linked Traits and Pedigrees:
• only (or mostly) males are affected;
• affected males are born to “carrier”
females;
• typically not seen in all generations
Queen Victoria’s Legacy in
Royal Families of Europe