Human Genetic Traits

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Human Genetic Traits
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Sex Chromosomes

• In most animal species, chromosomes can be
  categorized
• as two types
• Autosomes non sex chromosomes
• Sex chromosomes determine if the individual is
  male
• or female.
• Sex chromosomes in the human female are XX, male
  XY.
• Males produce X- containing and Y-containing
  gametes
• therefore males determine the sex of the
  offspring.
• Besides genes that determine sex, sex chromosomes
  carry
• many genes for traits unrelated to sex.
• X- linked gene is any gene located on X
  chromosome.

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Autosomes and Sex Chromosomes
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Disorders
Downs Syndrome (chrom 21)
Alzheimers (chrom 1, 10, 14, 19, 21)
Huntingtons (chrom 4)
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X- Linked Alleles
Work with fruit flies by Thomas Hunt Morgan
confirmed that Genes were on chromosomes. X-
linked alleles are designated as superscripts to
X Chromosome. Heterozygous females are carriers
but do not show the trait But can pass it
on. Males are never carriers but express the one
allele on the X Chromosome. One form of colour
blindness is X- linked recessive.
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Importance of genetics

• Understanding hereditary diseases and to develop
  new treatments
• Donor matches
• Paternity
• Forensics
• Evolution

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Genetic Testing

• Would you want to know?
• Ethical concerns
• Cost
• Insurance companies
• see GATTACA

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Genes- genetic material on a chromosome that
codes for a specific trait Genotype- the genetic
makeup of the organism Phenotype- the expressed
trait Allele- an alternative form of a gene
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• Dominance Mechanism
• Two alleles are carried for each trait
• In true-breeding individuals, both alleles are
  the same (homozygous).
• Hybrids, on the other hand, have one of each kind
  of allele (heterozygous).
• One trait is dominant, the other trait is
  recessive

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Allele Example
Gene eye color Alleles brown blue green lavend
er
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Allele Examples
appearance
eye colorhomozygous
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Allele Examples
appearance
eye colorheterozygous, brown dominant over blue
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Genotype vs Phenotype
genotype phenotype
homozygous(dominant) heterozygous homozygous (r
ecessive)
appearance
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Punnett Square
If male female are heterozygous for eye color
male
female
X
brown 3/4 offspring blue 1/4 offspring
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Red-Green Color Blindness Sex-linked trait
Y
XC
XC
Y
XC
XC
XC
Normal male
XC
Y
X
XC
Xc
XC
Xc
Xc
Xc
Y
Normal female recessive gene
Possible outcomes XCXC XCXc XCY XcY
Normal female
Normal Female (carrier)
Normal male
Color-blind male
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Dominance
Most traits show complete dominance Blending
unexpected
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allele
gene
E unconnected earlobe e connected earlobe
unconnected
P
EE x ee
connected
E e
gametes
F1
Ee
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F1
Ee x Ee
1/2 E 1/2 e1/2 E 1/2 e
gametes
E
e
E
EE
Ee
PunnettSquare
e
Ee
ee
F2
1 EE 2 Ee 1 ee
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generation genotypes unconnected
Ee
P
EE, ee
50
11
F1
Ee
100
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F2
EE, 2 Ee, ee
75
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phenotypes
ratio of alleles in the population
Basis of the Castle-Hardy-Weinberg Law
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Human Genetic Traits
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Tongue Roller
R Tongue Rollerr Unable to Roll Tongue
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Widows Peak
W Widows Peakw Lack of Widows Peak
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Free Ear Lobe
Attached Ear Lobe
E Free Ear Lobee Attached Ear Lobe
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Hitchhikers Thumb
Hi Straight Thumbhi Hitchhikers Thumb
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Bent Little Finger
Bf Bent Little Fingerbf Straight Little
Finger
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Mid-digital Hair
M Mid-Digital Hairm Absence of Mid-Digital
Hair
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Dimples
D Dimplesd Absence of Dimples
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Short Hallux
Ha Short Halluxha Long Hallux
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Now complete Human Traits Assignment
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Predicting Offspring
Genetic disorders are medical conditions caused
by alleles inherited from parents. These
disorders are mapped on a Pedigree. A carrier is
a heterozygous individual who has no
apparent abnormality but can pass on an allele
for a recessive inherited genetic disorder.
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Autosomal Dominant and Autosomal Recessive
Characteristics of autosomal dominant
disorders 1. Affected children usually have an
affected parent. 2. Heterozygotes are affected.
Two affected parents can produce unaffected
child two unaffected parents will not have
affected children. Characteristics of autosomal
recessive disorders 1. Most affected children
will have normal parents since heterozygotes
have a normal phenotype. 2. Two affected parents
always produce an affected child. 3. Close
relatives who reproduce together are more likely
to have affected children.
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Chance has no memory!
Each child born to heterozygous parents has a 25
chance of having a disorder regardless of prior
siblings conditions.
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Tracking Human Traits
A pedigree is a diagram of family relationships
that symbols to represent people and lines to
represent genetic relationships. These diagrams
make it easier to visualize relationships within
families, particularly large extended families.
Pedigrees are often used to determine the mode
of inheritance (dominant, recessive, etc.) of
genetic diseases. .
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In a pedigree, squares represent males and
circles represent females. Horizontal lines
connecting a male and female represent mating.
Vertical lines extending downward from a couple
represent their children. Subsequent generations
are therefore written underneath the parental
generations and the oldest individuals are found
at the top of the pedigree. If the purpose of a
pedigree is to analyze the pattern of
inheritance of a particular trait, it is
customary to shade in the symbol of all
individuals that possess this trait. In the
pedigree , the grandparents had two children, a
son and a daughter. The son had the trait in
question. One of his four children also had the
trait.
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Autosomal Dominant Disorders
Achondroplasia - Achondroplasia is inherited as
an autosomal dominant disorder. It is diagnosed
in the first years of life and affects 1 in
26,000 births the majority of cases are
sporadic. This is the classic circus dwarf with
disproportionate shortness of the limbs and large
head, but apparently normal trunk. Huntington's
disease - Huntington's disease is a disorder
passed down through families in which nerve
cells in certain parts of the brain waste away,
or degenerate. Marfan's syndrome - Marfan
syndrome is a genetic disorder that affects the
body's connective tissue - the tissue that makes
up our tendons, ligaments, joints, and muscles,
including the heart, blood vessels, and
eyes. People with this condition are generally
very tall and slim with long arms and fingers.
It's quite rare - roughly 1 in 5,000 people have
it. Neurofibromatosis - is a disorder
characterized by the growth of noncancerous
tumors called neurofibromas. They usually form
on or just underneath the skin, as well as in the
brain and peripheral nervous system. But they
can also develop in other parts of the body, such
as the eye.
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Autosomal Recessive Disorders
Cystic fibrosis (CF) Cystic fibrosis is one of
the most common inherited single gene disorders
in Caucasians. About one in 2500 Caucasian
babies is born with CF and about one in 25
Caucasians of northern European descent carries
the gene for CF. People with CF secrete abnormal
body fluids, including unusual sweat and a thick
mucus which prevents the body from properly
cleansing the lungs. The mucus interrupts the
function of vital organs and leads to chronic
infections. Classic CF also involves the pancreas
and causes decreased absorption of essential
nutrients. Life expectancy has improved, but,
ultimately, death most often occurs from
respiratory failure. Other people with variants
of CF may have only lung involvement, sinusitis,
or infertility. Tay Sachs Disease Tay Sachs
disease is a fatal disorder in children (usually
by age 5) that causes a progressive degeneration
of the central nervous system. It is caused by
the absence of an enzyme called hexosaminidase A
(or hex A). Without hex A, a fatty substance
builds up on the nerve cells in the body,
particularly the brain. The process begins early
in pregnancy when the baby is developing, but is
not apparent until several months after the
birth. To date, there is no cure for Tay Sachs.
Dr. Tay and Dr. Sachs, who originally described
this condition, noted that most Tay Sachs babies
were usually of eastern European Jewish origin.
About one in 30 persons of Ashkenazi Jewish
ancestry carries the Tay Sachs gene.
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Sickle cell anemia (SC) Sickle cell anemia is
one of the most common, inherited single gene
disorders in African-Americans. About one in 600
African-American babies is born with SC, and
about one in 12 African-American people carries
the gene for SC. SC involves the red blood cells,
or hemoglobin, and their ability to carry
oxygen. Normal hemoglobin cells are smooth,
round, and flexible, like the letter "O", so
they can move through the vessels in our bodies
easily. Sickle cell hemoglobin cells are stiff
and sticky, and form into the shape of a sickle,
or the letter "C" when they lose their oxygen.
These sickle cells tend to cluster together and
cannot easily move through the blood vessels. The
cluster causes a blockage and stops the movement
of healthy, normal oxygen carrying blood. This
blockage is what causes the painful and damaging
complications of sickle cell disease.
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phenylketonuria (PKU) Phenylketonuria (commonly
known as PKU) is an inherited disorder that
increases the levels of a substance called
phenylalanine in the blood. Phenylalanine is a
building block of proteins (an amino acid) that
is obtained through the diet. It is found in all
proteins and in some artificial sweeteners. If
PKU is not treated, phenylalanine can build up to
harmful levels in the body, causing intellectual
disability and other serious health problems.
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Complexities
Multiple genes for one trait Example eye
color Blended traits (incomplete
dominance) Influence of the environment
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Many human traits are controlled by single genes.
True for disorders, too. Cystic fibrosis,
deafness, Tay Sachs. Hitchhiker thumb.
Pedigrees used to predict potential disorders in
children, help make a reproductive strategy.
Some people may chose to not have children but
adopt instead.
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• Family pedigrees are used to determine
  patterns of inheritance and individual genotypes

Dd Joshua Lambert
Dd Abigail Linnell
D_ JohnEddy
?
D_ HepzibahDaggett
?
D_ Abigail Lambert
Dd Elizabeth Eddy
?
dd JonathanLambert
dd
Dd
Dd
Dd
dd
Dd
Dd
Female
Male
Deaf
Hearing
Figure 9.8B
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Inherited Genetic Disorders

• Most mutations usually involve recessive alleles
• Phenylketonuria, PKU
• Tay-Sachs disease
• Cystic fibrosis

• A few are caused by dominant alleles
  Achondroplasia, Huntingtons disease

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Sex-linked disorders affect mostly males

• Most sex-linked human disorders are due to
  recessive alleles
• Ex hemophilia, red-green color blindness
• These traits appear mostly in males. Why?
• If a male receives a single X-linked recessive
  allele from his mother, he will have the
  disorder while a female has to receive the
  allele from both parents to be affected

Figure 9.23A
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Pedigree Chart Inheritance Pattern for an
X-linked Recessive Disease
Figure 19.12
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• A high incidence of hemophilia has plagued the
  royal families of Europe

QueenVictoria
Albert
Alice
Louis
CzarNicholas IIof Russia
Alexandra
Alexis inherited hemophilia from his mother
Alexandra, a condition that could be traced back
to her maternal grandmother Queen Victoria.
Alexis
Figure 9.23B
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Hemophilia in European royalty

• Hemophilia figured prominently in the history of
  European royalty in the 19th and 20th centuries.
  Britain's Queen Victoria, through two of her five
  daughters including Princess Alice and Princess
  Beatrice, passed the mutation to various royal
  houses across the continent, including the royal
  families of Spain, Germany and Russia.

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General Pedigree
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Autosomal Dominant Pedigree
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• Look for
• Trait in every generation
• Once leaves the pedigree does not return
• Every person with the trait must have a parent
  with the trait
• Males and females equally affected

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Autosomal Recessive Pedigree
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• Look for
• Skips in generation
• Unaffected parents can have affected children
• Affected person must be homozygous
• Males and females affected equally

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Sex linked Dominant pedigrees

• Look for
• More males being affected
• Affected males passing onto all daughter
  (dominant) and none of his sons
• Every affected person must have an affected parent

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Sex linked recessive pedigrees

• More males being affected
• Affected female will pass onto all her sons
• Affected male will pass to daughters who will be
  a carrier (unless mother also affected)
• Unaffected father and carrier mother can produce
  affected sons

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Sex linked recessive
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If one of the parents is homozygous for a
dominant allele, all the children will be affected
If one parent is heterozygous for a dominant
allele and the other is homozygous recessive,
there is a chance that half their children
will be affected
If both parents are heterozygous for a recessive
allele, there is a chance that one in four of
their children will be affected