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Chapter 14
Human Genetics
Human Chromosomes
• Humans have 46 chromosomes
– 22 pairs of our chromosomes are autosomes
• Chromosomes that are not sex chromosomes
– 1 pair of our chromosomes are sex
chromosomes
• XX = girl
XY = boy
• We can organize all
of our 46
chromosomes from
largest to smallest
into a chart called a
karyotype
– The sex
chromosomes are
always listed at the
end
• Karyotypes are useful tools for diagnosing
patients with chromosomal genetic
disorders
– Down Syndrome, Fragile X syndrome,
Turners, etc…
How do we get these
chromosomes?
• Amniocentesis
– Procedure of removing fetal cells from the
amniotic fluid in the womb
Pedigrees
• A pedigree is a tool used to determine the
mode of inheritance of a gene in a
particular family
Pedigree Activity
• This is the story of Grandma and Grandpa
Flipnob and their clan. They were married
way back in 1933, and have been just like
newlyweds ever since.
• From their union, four individuals were
created. Elizabeth, the eldest, was born in
1935. Fred soon followed in 1936. In 1939
Michelle was brought into this world. Mickey
(a surprise to the whole family) was the baby
of the family, not being born until 1950.
• Elizabeth fell in love at a young age, and
wed her high school sweetheart, David in
1954. From this marriage, two bundles of
joy came about (at the same time)
– Identical Twins!! John and Sonny
• It took Fred a little longer to find his
soul mate. Finally in 1970, Fred found
the woman of his dreams, Wilma and
they were married. Since they married
so late in life, they only brought one
new person into this world
– Barney - 1972
• Michelle was a hard working woman, and
never found time in her schedule for love.
She led a very productive and fulfilling life,
but she never did marry and have children.
• Mickey was a wild one!! After a long string
of girlfriends, he finally chose Monica to
spend the rest of his life with. They were
wed in 1975 and brought two girls into this
world.
– Krista 1977
– Janet 1979
Questions
• Not having freckles is a recessive trait.
• Grandpa Flipnob did not have freckles, but his
beautiful bride did.
• Fred and Michelle were the only two of their
siblings to have freckles.
• Of the grandchildren, the twins did not have
freckles, and neither did Barney, but the two girls
did
• Assign two recessive genes to any
person on the pedigree whose symbol
is shaded. Small letters are written
below the person’s symbol
• Assign only one dominant gene to any
person on the pedigree whose symbol
is unshaded. A capital letter is written
above the symbol.
• Us this information to determine the
second alleles for each person with the
dominant phenotype.
Chromosomal disorders & blood
types
Blood Group Genes
• Rh factor is determined by a single gene
– It is dominant so an individual with Rh+/Rh+
or Rh+/Rh- are said to be Rh positive
– A person with two Rh- alleles is said to be Rh
negative
– Rh stands for “Rhesus monkey”
• The A, B, AB and O blood groups are
controlled by multiple alleles
– IAIA and IAi = blood type A
– IBIB and IBi = blood type B
– IAIB = blood type AB
– ii = blood type O
Blood Types - Multiple Alleles and
Codominance
•In humans, there are four blood
types (phenotypes): A, B, AB,
and O
•O is recessive, two O alleles
must be present for the person to
have type O blood
•A and B are codominant. If a
person receives an A allele and a
B allele, their blood type is type
AB
•Crosses involving blood type
often use an I to denote the
alleles - see chart.
to know whether at type A or B person is
heterozygous or homozygous. Type O's are
automatically OO and type AB is automatically
AB. Crosses are performed the same as any
other.
•The blood type determines what antibodies are located within the
blood. Type A blood has type B antibodies. If type B blood is put into
their bodies, their immune system reacts as if it were a foreign
invader, the antibodies clump the blood - can cause death.
•Type AB blood has no antibodies, any blood can be donated to them
- they are called the "universal acceptors"
•Type O blood has no surface markers on it, antibodies in the blood
do not react to type O blood, they are called the "universal donors"
• What happens when you give a person
with type A blood, type B blood?
– Clumping can occur and it is deadly!
Human Genes
• Human genome – our complex set of
genetic information
• Human Genome Project – An international
effort to map all 4 billion base pairs of the
human genome. It is complete and the
information is being used to help those
with genetic disorders
Chromosomal Disorders
• Down Syndrome
– Occurs from nondisjunction
(failure of chromosomes to
separate during meiosis)
– Three copies of chromosome
21
– Mild to severe mental
retardation, increased
susceptibility to many
diseases, other birth defects
Recessive Alleles
• Most genetic disorders are
recessive
– This means the child has to
inherit both alleles for the
trait from the parents
– Example: Cystic Fibrosis
• Excess mucus in lungs,
digestive tract, liver and
increased susceptibility to
infections
• Cystic fibrosis is
autosomal recessive
– Recessive and found
on an autosome
• What is the probability
of having a child with
CF if both parents are
heterozygous for the
trait?
C
c
C
CC
Cc
c
Cc
cc
• Another Autosomal recessive disorder is
PKU (Phenylketonuria)
• Lack of normal skin pigment; mental
retardation
• It is treatable and we are all tested when
we are born
• A woman who is a
Phenylketonuric marries a
man who is normal. They
have a child that is
diagnosed with PKU at
birth. What is the father’s
genotype?
• Possible genotypes?
PP
or
p
p
P
Pp
Pp
P
Pp
Pp
p
p
P
Pp
Pp
p
pp
pp
Pp
Dominant Alleles
• Some genetic disorders are dominant
• Example: Huntington’s Disease
– Progressive loss of muscle coordination and
mental function until death occurs
• A Man who is
heterozygous for
Huntington’s disease
h
has a child with a
woman who does not
carry a gene for
Huntington’s. What is
the probability that they h
will have a child with
Huntington’s?
H
h
Hh
hh
Hh
hh
Codominant Alleles
• Example: Sickle Cell Anemia
– People who have sickle cell anemia have
abnormally shaped red blood cells
– This is due to a defect in the hemoglobin
protein that carries oxygen
– These RBC’s tend to get stuck in narrow
blood vessels which damages cells, tissues
and organs
• A person who is heterozygous for Sickle
cell is said to have Sickle cell trait
• A mild reversible form of sickling may
occur, but may go unnoticed.
• Heterozygous individuals in tropical
regions have an advantage, because this
trait renders them resistant to malaria
• A man and a woman
both heterozygous for
Sickle cell have a child
what are the possible
genotypes and
phenotypes of this
child?
A
S
A
AA
AS
S
AS
SS
Tay-Sachs disease (abbreviated TSD)
•autosomal recessive
•a baby with Tay-Sachs disease
is born without one of those
important enzymes,
Hexosaminidase A (Hex A). So,
as those fatty proteins build up in
the brain, they hurt the baby's
sight, hearing, movement, and
mental development.
Sex linked traits
Sex-linked Traits
• Some genes that code for traits are
located on the X chromosome
• Males are affected more than females
because they only have one X
chromosome
• Example: Color blindness
– Most common form is red-green color
blindness
– If a male has an affected X chromosome, he
is color blind, a female with one affected
chromosome will be normal, but is called a
carrier
• A normal male
marries a carrier
female. What is the
probability that they
will have a son who is
color blind?
X
Y
X
XX
XY
Xc
X Xc
Xc Y
• A colorblind male
and a normal
female are
expecting a child.
What is the
chance they will
have a colorblind
boy?
Xc
Y
X
X Xc
XY
X
X Xc
XY
• Example 2: Hemophilia
– A blood clotting disorder
– A cut or bruise could be deadly
– Proteins called clotting factors now allow
hemophiliacs to lead a fairly normal life
• A Carrier female and
a normal male are
expecting their first
child. What is the
probability that this
child will have
hemophilia? Carry
the gene for
hemophilia?
X
Xh
X
Y
XX
XY
XXh
XhY
• Sex Chromosome disorders
– Too many or not enough sex chromosomes
– Example: Females can have Turner
syndrome (45X)
• Most look normal, may have decrease life span
and are usually sterile
– Male’s can have Klinefelter Syndrome
(47XXY)
• Most look normal, but may be infertile. They may
exhibit some emotional or social problems
Human DNA Analysis
• We can test for the presence of certain
alleles by using gene probes
– Fluorescent tags that attach to the gene of
interest
• DNA fingerprinting
– Using gel electrophoresis, scientists can
identify individuals based on their DNA code
– Everybody has a unique DNA fingerprint
– Paternity cases, Crime investigations
Gene Therapy
• An absent or faulty gene
is replaced by a normal,
working gene
– Sometimes viruses are
used to carry the gene
into the cells. They are
modified so they don’t
cause disease
– Blood disorders may be
treated by bone marrow
transplant with normal
cells