NOTES: 14.1 -14.2 HUMAN HEREDITY
• Key Terms
– Autosomal Recessive
– Autosomal Dominant
– Pedigree
– Tay-Sachs– 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 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 • 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:From gene to molecule:From gene to molecule: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 disorderprotein, 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• 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 – 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 glutamicacid
• Result = abnormal hemoglobin
• The abnormal hemoglobin forms crystal-• The abnormal hemoglobin forms crystal-like structures that change the shape of the red blood cells
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.and pain.
Sickle Cell Anemia:
• Three genotypes possible:
1) HH = all healthy hemoglobin
2) Hh = ½ healthy hemoglobin; ½ 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 • 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• Circles represent females
• Squares represent males
– Shaded in: person expressesthat 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 systemlipids 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 systemdamage to the nervous system
• By avoiding phenylalanine in
the diet, affected infants
can avoid the symptoms
of the disorder
Autosomal Recessive Traits &
Pedigrees:
• May skip a generation
• Affected individuals are born to 2 carriers
• Males and females affected equally
• KNOWN carriers will be half-shaded in…it • 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:
• Tongue rolling
• Unattached earlobes
• Hitchhiker’s thumb
• Hair in the middle section of fingers• 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)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
• 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;• 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• typically not seen in all generations
Queen Victoria’s Legacy in
Royal Families of Europe