Transcript Sex-Linked Disorder

Human Inheritance Notes
Ch. 14 - Part II
Sex-Linked and Autosomal
Genetic Disorders
Karyotype Chart
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Once the cells have been gathered, scientists
will take a picture of the chromosomes and
make a karyotype chart.
•Karyotypes show the 23 homologous pairs for
the person in which the cells were taken.
•The pairs are put in order from longest to shortest
and numbered from 1 to 23.
Pairs 1-22 are called autosomes.
Pair 23 are the sex chromosomes.
• Karyotype interactive
Is this a
Male or
Female
Karyotype
chart?
Female- XX
Down Syndrome
•Cells contain one extra copy of
chromosome # 21.
This results in:
•characteristic facial features
• short stature
• mental retardation
•shorter life-span
•Increased risk for heart
problems, immune system
problems, and cancer.
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Chromosome number disorders link
Down Syndrome link
Klinefelter Syndrome
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This results in a male who has
one extra X chromosome.
47,XXY
This results in
This male can not have
children.
He can show mental
retardation.
Is a true male, but can have
some female characteristics.
Klinefelter Syndrome Link
Sex-linked Inheritance
(carried on the X chromosome)
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Sex-linked disorders are more commonly
seen in males.
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Genes carried on the X chromosome are
said to be sex-linked traits.
A carrier is a person who is heterozygous
for a disorder, but does not show the
disorder
 Carriers of sex-linked traits are female.
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Sex-linked Disorders
(carried on the X chromosome)
Example 1 : Hemophilia
a recessive allele written as Xh
 What is hemophilia? “free bleeders”
 A clotting protein is missing.
 Uncontrolled bleeding can occur from
wounds and bruises.
 Affects 1 out of every 10,000 males
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Phenotype
Genotype
Normal Female
XHXH
Carrier Female
XHXh
Female with
Hemophilia
XhXh
Normal Male
XHY
Male with Hemophilia
XhY
Problem 1: A normal man marries a woman
who is a carrier.
Create a Punnett square to determine the probability of
passing on the allele to their offspring.
XH
XH
Y
XHXH XHY
What are their chances of having a
daughter with hemophilia? 0%
a daughter who is a carrier? 25%
Xh
XHXh XhY
a normal son? 25%
a son with hemophilia? 25%
Sex-linked Disorders
(carried on the X chromosome)
Example 1 : Colorblindness
a recessive allele written as Xc
 What is the most common form of
colorblindness?
 Condition where the person cannot
distinguish between the colors red
and green.
 Affects 1 out of every 10 males and 1
and 100 females.
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Problem 1: A color-blind man marries a
woman who is a carrier.
Create a Punnett square to determine the probability of
passing on the allele to their offspring.
Xc
Y
XC+ XC+Xc XC+Y
What are their chances of having a
daughter with normal vision 25%
a daughter who is a carrier? 25%
Xc
XcXc XcY
a normal son? 25%
a color-blind son? 25%
How can we tell if a genetic
disorder is sex-linked?
 Is
a sex-linked trait more often seen
in males or females? Why?
 Sex-linked
traits are seen more often
in males because males only have to
inherit one copy of the trait on the X
chromosome he received from his
mom!
Sex-linked Disorders
(carried on the X chromosome)
Example 3 : Duchenne Muscular Dystrophy
 a recessive allele written as Xd
 What is muscular dystrophy?
 A disorder that causes the muscles to
slowly weaken and eventually leads to
death.
 Affects 1 out of every 3,000 males
Autosomal Disorders
These are disorders carried on the
autosomes, rather than the sexchromosomes.
 Most genetic disorders are recessive.
 Recessive disordered alleles produce
proteins that either malfunction or no
longer function.
 A heterozygous carrier has a normal
phenotype because the one “normal” allele
produces enough of the required protein.
Recessive Autosomal Disorders
Albinism
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Caused by a recessive allele
on the chromosome 11.
Individuals who are
homozygous recessive are
unable to produce melanin,
the pigment responsible for
human skin color.
They have no pigment in
their hair or skin, and are
sensitive to light.
Recessive Autosomal Disorders
Cystic Fibrosis
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Caused by a recessive allele
on chromosome 7.
Found in 1 in every 2,500
Caucasian births. 1 in 25 are
carriers.
Individuals who are
homozygous recessive make
a defective cell membrane
protein.
This creates extra mucus
that clogs the lungs and
breathing passageways leads
to bacterial infections.
Two carriers have
a 25% chance of
having a child with
cystic fibrosis.
Dominant Autosomal Disorders
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Dominant disorders are typically lethal and
are extremely rare.
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This is because if a person inherits it he or
she dies before they can have children.
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Exceptions:
Huntington Disease
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This caused by a rare dominant allele on
chromosome 4. 1/25,000 affected.
Symptoms start with a loss of muscle control.
*No symptoms until age 30 to 40. Mortality
within 15 years after systems first appear.
The gene for Huntington disease is dominant,
so that every child of a person with the disease
has a 50% chance of inheriting the gene and
having the disease.
*This is a lethal disorder, homozygous
dominant fetus, the baby miscarries.
Huntington Disease
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*This is a lethal disorder, which means if a
person is homozygous dominant the
baby miscarries.
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*No symptoms until age 30 to 40.
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*Why could not showing the disease until
later in life be a problem? A person who
doesn’t know they have the disorder can
pass it on to their children before they
show the disease.
Sickle Cell Anemia
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(codominant disorder) It is found in 1
out of every 1,600 African Americans.
It is an anemia due to the sickled allele
producing an alternate from of
hemoglobin (red blood protein), which
causes cells to sickle or become
crescent shaped.
Individuals with two sickle cell alleles
are said to have sickle cell anemia.
It can lead to serious medical problems
with sickled cells forming blood clots.
Pedigree Charts
The family tree of genetics
What is a pedigree?
Constructing a pedigree
Interpreting a pedigree
What is a Pedigree Chart?
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A Pedigree chart traces the inheritance
of a particular trait through several
generations.
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One GOAL of using a pedigree chart is
to figure out who are carriers of the
trait, because this information is typically
unknown.
Constructing a Pedigree
 Female
 Male
Constructing a Pedigree
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Married Couple
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Siblings
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Horizontal Line
Vertical line
More than one Sibling:
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a horizontal line is drawn
with a vertical line coming
down for each sibling.
Constructing a Pedigree
 Fraternal twins Two line branching from the
same point
 two eggs and two sperm cells.
 Identical twins Also called Maternal Twins
 Same as fraternal twins but a
horizontal line is added.
 one egg and one sperm unite
and later splits to form two
babies
Constructing a Pedigree
Roman numerals to the left of the pedigree
show the generations.
Birth Order: children are listed in birth order
with oldest on left and youngest on the right.
I
II
III
Constructing a Pedigree
Two horizontal lines
from the same person
represent two
marriages / matings.
Example:
This man first had a girl
with the lady on the left,
then had a boy and girl
with the lady on the
right.
More Symbols in a Pedigree Chart
 Full
Shaded:
 Affected
person who
shows a disorder
 Half
shaded:
 Autosomal
 Circle
carrier
with dot:
 X-linked
carrier –always
female
 Deceased
Predicting using Pedigree Charts
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Pedigrees are used to find out:
who are carriers of the disorder &
 the probability of having a future child with
the disorder.
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To begin to interpret a pedigree, first
determine if the disorder is:
Autosomal dominant
 Autosomal recessive
 Sex-linked (carried on the X chromosome)
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Interpreting a Pedigree Chart
First ask:
Is it a Sex-linked or Autosomal Disorder?
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If there is a much larger number of
males than females who are affected
then the disorder is Sex-linked.
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If there is a 50/50 ratio between males
and females who are affected then the
disorder is autosomal.
Interpreting a Pedigree Chart
If it is autosomal disorder then ask:
Is it dominant or recessive?
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If two parents do not show the trait
and their children do show it, then it is
an autosomal recessive disorder
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(parents are heterozygous)
If the disorder is autosomal
dominant, then at least one of the
parents must show the disorder.
Practice Examples
Does this pedigree show a
Sex-linked or Autosomal disorder?
Answer:
Sex-Linked Disorder
much larger number of males are affected
Practice Examples
Does this pedigree show a
Sex-linked or Autosomal disorder?
Answer:
Autosomal Disorder
50/50 ratio between males and females