Transcript Genetics

Human Genetics
How many pairs of chromosomes are in each of
our body cells?
A.
B.
C.
D.
32
23
46
16
23 pairs! This is 46 total; we get 23 from
mom & 23 from dad.
Body cells are also known as what?
A.
B.
C.
D.
Somatic Cells!
Gametes
Histones
Chromatin
Somatic Cells
Which cells in our bodies do NOT have all 46
chromosomes?
A.
B.
C.
D.
Gametes
Histones
Chromatin
Somatic Cells
Gametes! Gametes are also known as sex cells.
THESE ARE SPERM & EGG CELLS!
How many chromosomes do human sperm or
egg cells contain?
A.
B.
C.
D.
32
23
46
16
23! They do NOT get 2 copies of each
chromosome; they just get 1!
Gametes – Sperm & Egg Cells
• All gametes are haploid.
• In humans, that means each egg cell & each sperm cell
has 1 copy of each chromosome (23 total chromosomes).
– Egg Cells: all human egg cells carry 23 chromosomes, 1 of
which is a single X chromosome
– Sperm Cells: in males, there are 2 types of sperm cells – 1
carries an X chromosome & 1 carries a Y chromosome
The 2 Types of Chromosomes
• Autosomes: non-sex chromosomes
– Of the 46 chromosomes, 44 of them (22 pairs) are
autosomes.
• Sex Chromosomes: the last 2 chromosomes; they
determine the sex of the person
– Females have 2 X chromosomes (XX).
– Males have 1 X and 1 Y chromosome (XY).
Is this person male or female?
When a sperm & egg cell
combine, half the time the
fertilized eggs (zygotes) are
female & half of the time
they’re male.
FUN FACT!
There are actually slightly more men on Earth than women.
WHY? Shouldn’t it be half & half?
Since the Y chromosome is so much
smaller, it’s lighter. That means that
the sperm cells that carry the Y
chromosome can swim a little faster!
This gives them an advantage & they
can reach the egg cell more quickly!
Karyotypes – what’s that?
Karyotypes
• Scientists can analyze an individual’s chromosomes by
taking a picture of cells during mitosis.
Why does this need to be done during mitosis?
• It’s easiest to view chromosomes during mitosis,
because they are condensed!
Karyotypes
• From a picture of chromosomes, scientists can cut & paste
to arrange the chromosomes in pairs to form a karyotype.
The last pair (#23) is
the sex chromosomes.
A. Man
B. Woman
Woman!
There are 2 X
chromosomes, but
no Y chromosome!
A. Man
B. Woman
Man!
It’s not labeled, but you can see that 1 of the sex chromosomes
is much smaller than the other (the X & the Y).
Why is karyotyping important?
Using Karyotypes
• Genetic disorders can be identified by looking at an
individual’s karyotype.
Down Syndrome can
be confirmed even
before the baby is
born!
Human Chromosome Disorders
How often do they occur?
• Relatively high frequency in humans, but most
embryos are spontaneously aborted
– Developmental problems result from biochemical
imbalances.
About 60% of first trimester
miscarriages are due to
chromosome disorders.
Between 5-10% of stillborn
deliveries are due to
chromosome disorders.
Human Chromosome Disorders
• Some conditions are survivable.
How do chromosome disorders
occur?
Chromosome Abnormalities
• Incorrect number of chromosomes/mutations within
the chromosome
– Nondisjunction: chromosomes don’t separate properly
during meiosis
– Chromosome Mutations: deletion, inversion, duplication,
translocation
Remember!
• During meiosis, a diploid cell (2 copies of each
chromosome) divides & produces 4 haploid gametes
(sperm or egg).
– Gametes only have 1 copy of each chromosome!
Nondisjunction
• Sometimes, chromosomes
may not separate properly
during meiosis; this is called
nondisjunction.
• If nondisjunction occurs,
abnormal numbers of
chromosomes (1 extra or 1
missing) are found in
gametes & chromosomal
disorders may result.
Nondisjunction
• Homologous
chromosomes don’t
separate properly
during Meiosis 1.
OR
• Sister chromatids fail
to separate during
Meiosis 2.
Nondisjunction
• Trisomy: cells have 3
copies of a chromosome
– In trisomies, the gamete
of one parent donated 2
of one type of
chromosome to the child
& the gamete of the other
parent donated 1 (like
normal).
Nondisjunction
• Monosomy: missing 1 chromosome
– In monosomies, the gamete of one parent donated 1
chromosome & the other parent did not donate any.
Down Syndrome
• Trisomy 21
– Every 1 in 700 children born in the US have Trisomy 21.
Chromosome 21 is the
smallest, but an
abnormality still has
severe effects.
Down Syndrome vs. Age of Mother
Mother’s age
Incidence of Down
Syndrome
Under 30
<1 in 1000
30
1 in 900
35
1 in 400
36
1 in 300
37
1 in 230
38
1 in 180
39
1 in 135
40
1 in 105
42
1 in 60
44
1 in 35
46
1 in 20
48
1 in 16
49
1 in 12
Women are born with all the egg
cells they’ll ever have.
So, a 35 year old woman has 35
year old egg cells.
Other Examples of Disorders caused by
Nondisjunction
• Klinefelter’s Syndrome – XXY (male)
• Turner’s Syndrome – XO (female)
As long as there’s at least 1 Y
chromosome, the karyotype is male!
Klinefelter’s Syndrome
• XXY
– 1/2000 live births
– Have male sex organs, but
are sterile
– Feminine characteristics
• Some breast
development
• Lack of facial hair
– Tall
– Normal intelligence
Turner’s Syndrome
• XO (1 X chromosome)
–
–
–
–
–
1/5000 births
Varied degrees of effects
Webbed neck
Short stature
Sterile
Jacob’s Syndrome
• XYY
–
–
–
–
–
1/1000 live births
Extra Y chromosome
Slightly taller than average
More active
Normal intelligence, slight
learning disabilities
– Delayed emotional maturity
– Normal sexual development
Some men will never know
they have Jacob’s Syndrome!
Trisomy X
• XXX
– 1/2000 live births
– Produces healthy females!
• All but 1 X chromosome are inactivated.
Most mammals leave only 1 X
chromosome activated in their cells.
The inactivated chromosome(s) are
called barr bodies!
Remember! Chromosomal Mutations
• Error in replication:
– Deletion: loss of a
chromosomal segment
– Duplication: repeat a
segment
• Error in crossing over:
– Inversion: reverses a
segment
– Translocation: move
segment from 1
chromosome to another
How do we know if a growing embryo has a
chromosome abnormality?
Genetic Testing!
• Amniocentesis: a syringe takes a sample of the embryo cells,
then stains & photographs the chromosomes
– Analysis of the karyotype
The Human Genome Project
• The genome of many organisms has been
determined, including the human genome!
• Now, we are analyzing the code, finding genes
coding for different traits, & discovering ways to
isolate them & manipulate them.
• In gene therapy, an absent gene or faulty gene is
replaced by a normal, working gene!
Video – Blind Dog Gene Therapy
Pedigrees – what are those?
Pedigrees
• Similar to a family tree;
both are used to show
relationships in a family
• Used to demonstrate how
traits are passed from 1
generation to another
• Genetic counselors use
pedigrees to follow how
genetic disorders are
inherited.
Pedigrees
• People who are heterozygous for a recessive genetic
disorder (they are unaffected) are called carriers.
– Sometimes, carriers of traits may be represented as a halfshaded shape or a shape with a dot in the middle.
Pedigree Symbols
• Square: male
• Circle: female
• Shaded: affected individual
• Open: unaffected
• Horizontal Line: marriage
• Vertical Line & Bracket:
descendants of a marriage
female
marriage
male
parents
children
Most genetic disorders are caused by mutations on
autosomes (non-sex chromosomes).
• Autosomal Recessive
Disorders: albinism, cystic
fibrosis, tay-sachs disease
• Autosomal Dominant
Disorders: achondroplasia
(common form of dwarfism)
• Codominant Disorders
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•
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•
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Could this trait be inherited as a simple autosomal recessive?
Could this trait be inherited as a simple autosomal dominant?
Could this trait be inherited as a simple X-linked recessive?
Could this trait be inherited as a simple X-linked dominant?
Could this trait be inherited as a simple Y-linked trait?
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•
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Could this trait be inherited as a simple autosomal recessive?
Could this trait be inherited as a simple autosomal dominant?
Could this trait be inherited as a simple X-linked recessive?
Could this trait be inherited as a simple X-linked dominant?
Could this trait be inherited as a simple Y-linked trait?
Draw the Pedigree!
• An unaffected man marries a woman who is a carrier for
Duchenne Muscular Dystrophy, which is attributed to an
X-linked gene.
• They have 4 children, 1 son with Duchenne, 1 carrier
daughter and a daughter & son who are unaffected.
• The child with Duchenne Muscular Dystrophy dies in
childhood.
• The carrier daughter marries and has 3 children of her
own, 2 of which are carriers and 1 daughter of which is
unaffected.