Transcript Non-disjunction

CHROMOSOME
ABNORMALITIES
Atypical Chromosome Numbers
• Some developmental disabilities are caused by gene or
chromosome abnormalities.
– Atypical chromosome numbers occur when too many, or
too few, chromosomes appear in an embryo’s cells.
• Effects depend on which chromosome is affected.
• Usually atypical numbers result in miscarriage.
• Trisomy = 3 identical chromosomes, instead of pair
– Trisomy 21 = Down syndrome
Non-disjunction
• Non-disjunction occurs when sex cells are produced with an
extra chromosome.
– homologous chromosomes don’t separate properly
during anaphase I or II
Meiosis I:
Meiosis II:
Normal vs
nondisjunction
Normal vs
nondisjunction
• one gamete has an extra chromosome, and the other
is missing a chromosome.
• If one of these joins normal gamete in fertilization,
diploid zygote will have an atypical number of
chromosomes.
– Mitosis then copies this error in all cells
– Down syndrome is the result of non-disjunction
• Incidence increases greatly with age of mother
• Eggs formed at birth but do not complete meiosis until
released, 1 per month
• Delay in release can cause cell damage
Chromosome
Number
Condition
Traits
Patau
syndrome
• 1 in 16 000 live births.
• severe physical and intellectual disability
• have heart defects, brain or spinal cord abnormalities, extra
fingers and/or toes, a cleft lip,
• Only 5 –10% of babies survive past their first year.
Trisomy 18
Edwards
syndrome
• 1 in 5000 live births
• severe intellectual disability and low birth weight
• small, head, jaw and mouth; clenched fists; heart and organ
defects
• Only 5 –10% of babies survive past their first year.
Trisomy 21
Down
syndrome
• 1 in 800 live births.
• mild to moderate intellectual disability,
Klinefelter
syndrome
• 1 in 500 to 1 in 1000 males.
• affects male sexual development
• small testes that do not produce enough testosterone.
• Testosterone injections help treat the condition.
Trisomy 13
XXY
Patau
syndrome
Edwards
syndrome
Klinefelter syndrome
Down
syndrome Cracking the Code: Understanding Rare
Chromosome Disorders - YouTube
Damage to Chromosome Structure
• A mutation is any damage that occurs to a
chromosome.
– may be as small as a few base pairs
– or as large as the entire structure of the
chromosome.
• 4 different ways chromosome structure can be
damaged
1. DELETION
• part of the chromosome
is actually lost.
• causes include viruses,
radiation and chemicals
• may carry a specific
gene which may have a
large effect on the host.
2. DUPLICATION
• gene sequence is
repeated one or more
times within a
chromosome.
• At some point, too
many repeats can
affect the function of
the gene.
3. INVERSION
• gene segment
momentarily becomes
free from its
chromosome and
then reinserts in the
opposite order.
• This can completely
alter the gene’s
activities.
4. TRANSLOCATION
• part of a chromosome
changes place with
another part of either
the same or a nonhomologous
chromosome.
• Translocations can
result in some cancers
and leukemia.
Genetic Testing
• Examining genes can allow for the diagnosis, treatment, and
prevention of genetic illnesses.
• several types of testing:
– Karyotype analysis - discovers chromosome abnormalities
BiologySource
– Carrier testing - tests parental genes before conception
• Cystic fibrosis (CF) and Tay-Sachs caused by gene
mutations passed on to offspring by both parents.
– Presymptomatic (predictive) testing - searches for specific
genetic diseases that run in families.
• Usually done for disorders that appear later in life
– Diagnostic genetic testing - confirms a diagnosis
• This type of test can be done at any point in a person’s life.
– Prenatal testing - detects chromosomal problems in a fetus.
• embryo becomes a fetus about 7 weeks.
• Down syndrome and spina bifida common tests
• Amniocentesis - genetically testing of fetus between weeks
14 to 20 of pregnancy.
• Chorionic villus sampling is also used.
– Tissue surrounding the fetus is removed and tested.
• These tests have risks and are used only if concerns arise
– Newborn screening - tests for some genetic disorders shortly
after birth.
• Phenylketonuria (PKU) testing is done this way.
PATTERNS OF INHERITANCE
Albinism is a genetic condition
in humans, in which the skin,
eyes, and hair have no
pigment, due to varying
amounts of melanin. It is
very rare, and is not lifethreatening.
Other genetic disorders,
though, cause severe
medical problems.
Mutations in chromosomes
constantly create new
alleles, both harmful and
harmless.
TAY-SACHS DISEASE
Tay-Sachs disease appears
around the age of eight
months. It is known by the
deterioration of brain cells
and spinal cords.
By their first birthday,
children are blind,
mentally handicapped,
and have little muscular
activity.
They will likely die before the
age of 5.
TAY-SACHS DISEASE
It is caused by the absence of a specific enzyme in the
lysosomes of brain cells. The recessive allele does not code
for the production of the enzyme that breaks down lipids.
As the lipids build up, they will eventually destroy the brain
cells that house them.
There is no treatment for Tay-Sachs disease, though there is
now a blood test to identify heterozygous carriers. Can you
think of any implications of this?
PHENYLKETONURIA (PKU)
Phenylalanine is an amino acid essential for human growth
and development. Tyrosine is used to make body hormones
and melanin. An enzyme in our bodies converts
phenylalanine to tyrosine.
Children with PKU will have phenylalanine broken down
abnormally. They will appear normal at birth, but within a
few months, they will be severely handicapped.
Infants are tested for PKU now, and a positive test will result in
a special diet that prevents harmful products from
accumulating. They will lead healthy lives once the nervous
system is fully developed.
SICKLE-CELL ANEMIA
Sickle-cell anemia is the
result of a defect in
hemoglobin in red blood
cells. It leads to blood
clots and reduced blood
flow to vital organs.
As a result, people have
reduced energy, suffer
various illnesses, and are
in constant pain.
SICKLE-CELL ANEMIA
When oxygen is released
from hemoglobin, sickle
cells begin to clump and
block blood vessels,
preventing oxygen from
getting to various parts of
the body.
DUCHENNE MUSCULAR DYSTROPHY
Duchenne muscular
dystrophy results in
individuals becoming
engorged with fat
deposits.
It is the wasting away of
muscle tissue.
Unfortunately, there is no
cure for this disease.
HUNTINGTON’S DISEASE
Huntington’s disease is a lethal disorder where the brain
deteriorates over a period of about 15 years.
Symptoms include irritability and mild memory loss, followed by
involuntary muscle movements.
As the brain deteriorates, symptoms become more severe,
leading to loss of coordination, memory, and speech.
Most people die in their 40’s or 50’s, not knowing if their
children have the mutant allele.