Download NOTES: 14.1-14.2 - Human Heredity / Pedigrees (slideshow)

NOTES: 14.1-14.2:
HUMAN HEREDITY
Human Genes:
● The human genome is the complete set of
genetic information
-it determines characteristics such as eye
color and how proteins function within cells
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
What makes an allele dominant
or recessive?
• Gene = sequence of DNA that codes for a
protein
• Usually:
– Dominant alleles code for the correct production of
that protein
– Recessive alleles code for no / wrong protein
production
– Heterozygous condition — the normal (dominant)
allele will still cause correct protein production
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
●
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)
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
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:
•
•
•
•
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:
•
•
•
•
•
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
•
•
•
•
•
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)
• Can’t trace every human trait through a
pedigree because some genes are
polygenic
–
–
–
–
Shape of your eyes
Shape of your ears
Height
Eye color
• Also, phenotype is influenced by your
environment (ex: nutrition & exercise)
– Average height is 10 cm more than it was in the
1800’s in the US & Europe due to nutritional
improvements
– Genes that are denied a proper environment in
which to reach full expression in 1 generation, can
achieve full potential in a later generation (genes
are inherited, the environment is not)
REVIEW:
Human Blood Groups
Blood Groups
• Human blood comes in a variety of
genetically determined blood groups
• Using the wrong blood during a blood
transfusion can be fatal
• A number of genes help determine blood
type but we will focus on two:
– ABO blood groups
– Rh blood groups
Blood Groups — Rh factor:
• The Rh blood group is determined by a
single gene with 2 alleles — positive and
negative
• The “positive” allele is dominant
• You need two Rh- alleles (Rh-/Rh-) to be
Rh negative
Rh Factor
• The Rh factor genetic information is
also inherited from our parents, but it is
inherited independently of the ABO
blood type alleles.
– There are 2 different alleles for the Rh
factor known as Rh+ and Rh-.
Mother
Father
Child
Rh-
Rh+
Rh+
Rh-
Rh-
Rh-
The “Rh Issue”… Mom = Rh-
Baby #1 = Rh+
ABO Blood Groups
•
•
•
•
•
This is a case of multiple alleles
There are 3 alleles for this gene—A, B, and O.
AND…A and B are CODOMINANT!
O is recessive to A and B
Alleles A and B produce antigens (which are
carbohydrates) on the surface of red blood cells
• O produces NO antigens
ABO Blood Groups—the wrong blood can be
FATAL
• Antigens are recognized by the immune system
and induce an immune response
• If the wrong blood is transfused, the body will
respond to these antigens by producing
antibodies
– Antibodies are named for what they attack
• Antibodies bind to the foreign molecule (the
antigen) and blood clumping will occur, which
leads to blood clotting, which leads to death
ABO Blood Groups
• If you have blood type A, then you have:
– The “A” antigen on the surface or your RBCs
– You have anti-B antibodies
– You can receive type A blood and type O blood
• Remember: Your antibodies are named for
what they attack—so if you received type
AB or type B blood then clumping would
occur.
ABO Blood Groups
• Try this cross
• Cross a person who’s genotype is IAIA with
a person who is IAIB.
• Give the possible genotypes and
phenotypes
• Cross a person who’s genotype is IAIA with
a person who is IAIB.
• Gametes = IA, IA and IA, IB
• Cross:
IA
IA
IA
I AI A
I AI A
IB
I AI B
I AI B
• Genotypes= ½ IAIA, ½ IAIB
• Phenotypes = ½Type A, ½ Type AB
Human Genes & Chromosomes:
Vocabulary:
• Sex-linked gene
• Sex-influenced gene
Key Concepts:
• Why are sex-linked
disorders more common in
males than in females?
Lou Gehrig: died at
37 of ALS
Human Genes & Chromosomes:
• Chromosomes 21 & 22 are the smallest human
autosomes
– Chromosome 22 contains approximately 43 million
DNA bases (approx. 545 genes)
– Chromosome 21 contains approximately 32 million
DNA bases (approx. 225 genes)
• 1 of these genes is associated with ALS (Amyotrophic Lateral
Sclerosis)
• Causes a progressive loss of muscle control due to the
destruction of nerves in the brain and spinal cord
Remember…
• Humans have 46 chromosomes (23 pair)
• 2 of them are sex chromosomes  they
determine what sex you are
– XX = female
– XY = male
• 44 of them are autosomes  they do not
determine what sex you are
Who determines the sex of a
child?
The mother or the father?
THE FATHER!!!!
Why does the father determine the
sex of the offspring???
• Mom is XX, she can donate either one X
chromosome or the other X chromosome
• Dad is XY, he can donate either an X
chromosome or a Y chromosomes.
• If the offspring receives the father’s X, it is
female
• If the offspring receives the father’s Y, it is male
The Y chromosome:
• If a Y chromosome is present, the person
is male
• X chromosomes contain genes necessary
for growth / development
• No cases of a person born with being
45,0Y
– Probably spontaneously aborted (miscarriage)
Sex Linked Genes:
• Genes carried on the X or Y chromosome are
“sex-linked” because they are on the sex
chromosomes
• Many sex-linked genes are found
on the X-chromosome
Sex Linked Genes:
• All X-linked traits are expressed in males
• WHY???????
• Males only have 1 copy of the X chromosome,
while females need 2 copies of the defective
gene
Human Sex-Linked Gene
Disorders:
1. COLORBLINDNESS:
•
•
•
3 human genes associated with color vision
are located on the X-chromosome
In males, a defective version of any one of
these produces colorblindness
Females must receive 2 copies of the allele to
be colorblind
XCXC
XCXc
XcY
XCY
More Human Sex-Linked Gene
Disorders…
2. HEMOPHILIA
• 2 important genes on the X-chromosome that
code for proteins that control blood clotting
• A recessive allele in either of these 2 genes
may lead to hemophilia
-“bleeders disease”
-injections of normal clotting
proteins prevent death
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
Sex-Influenced Genes:
• A person’s phenotype is affected by their sex
(internal environment is different…hormones!)
• Ex: Pattern Baldness
WOMEN:
BB: not bald
MEN:
BB: not bald
Bb: not bald
Bb: bald
bb: bald
bb: bald
X-Chromosome Inactivation:
• Females have 2 X-chromosomes… if 1 is
enough for males, how does the cell
“adjust” to the extra x-chromosome in
females?
• One X-chromosome is randomly
switched off
– Condenses and is called a Barr body
X-Chromosome Inactivation –
an example:
• Tortoise-shell cats:
 XBY = brown male
 XbY = orange male
X-Chromosome Inactivation –
an example:
• Tortoise-shell cats:
 XBXB = brown female
 XbXb = orange female
 XBXb = tortoise-shell female
-Can a male cat have tortoise-shell fur?
Tortoise-shell cats!
(a.k.a. “Torties”)
XBXb