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Meiosis Review
Terminology
• What is the difference between a chromosome, homologous
chromosomes, sister chromatids, and a tetrad?
GAMETE
FORMATION
GAMETOGENESIS
Gametogenesis: the
production of
gametes (sex cells)
through the process
of meiosis.
SPERMATOGENESIS
• Meiosis in males
• occurs in the testes
• Starts with a diploid cell called a
spermatogonium
• produces four non-identical
haploid sperm cells (spermatids)
• Following meiosis the sperm
cells develop into mature sperm
(spermatozoa) –the nucleus
forms into a head, and a
flagellum is formed for
locomotion.
SPERMATOGENESIS
Occurs all the time from puberty until death.
It takes approximately 68-74 hours for a sperm to
be created.
Meiosis produces approximately 250 000 000
sperm every day in males!
OOGENESIS
• Meiosis in females
• Occurs in the ovaries and
oviducts
• Starts with a diploid cell
called an oogonium.
• Egg production starts
before a female is born, but
pauses in Meiosis I before
the cells (primary oocyte)
divide.
• The meiotic process
resumes at puberty with
ovulation (and fertilization),
for 1 cell (ovum) every
month.
OOGENESIS
• Cytokinesis is not equal in meiosis of egg cells, one of
the cells receives the majority of the cytoplasm, resulting
in one egg cell and three polar bodies.
• The purpose of the unequal division is to allow the egg
cell to have sufficient nutrients to support a zygote
immediately after fertilization.
ERRORS IN MEIOSIS
• Changes in chromosomes are known as mutations.
• Mutations can be beneficial, neutral or harmful.
• Because chromosomes are copied during interphase, all
daughter cells (sperm or egg) will carry the mutation.
• If that sperm or egg is part of fertilization, the new organism
will carry that error in all of its cells.
Atypical Chromosome Numbers
• 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.
• For example:
• Trisomy = 3 identical chromosomes, instead of pair
– Trisomy 21 = Down syndrome
Non-disjunction
• The failure of chromosomes or tetrads to separate properly
during anaphase is called non-disjunction. This results in
the addition or deletion of a chromosome in a gamete.
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.
NON-DISJUNCTION
• If it happens in meiosis I, all the resulting cells will
be affected. If it happens in meiosis II, only half
will be affected.
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
NON-DISJUNCTION
If a gamete with an extra
chromosome is fertilized by a
normal gamete, the zygote
will have an extra
chromosome, called trisomy.
If a gamete missing a
chromosome is fertilized by a
normal gamete, the zygote
will have only one copy of a
chromosome, called
monosomy.
TRISOMY
• Trisomy 21 results in
Down Syndrome.
• Some effects include
mental delay,
weakened
cardiovascular
system, shortened
limbs, widely-spaced
eyes, receding
forehead/chin, and
protruding tongue.
TRISOMY
• Klinefelter’s Syndrome
occurs when an
individual receives two X
chromosomes and a Y
chromosome.
• Result: infertile male with
varying degrees of
femininity.
MONOSOMY
• Turner Syndrome: the
individual only gets one sex
chromosome, an X, from their
mother.
• Result: infertile female with a
broad chest, poor breast
development, low set ears,
short stature and poor hearing
amongst other things.
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
CHROMOSOME
ABNORMALITIES
Damage to Chromosome Structure
• A mutation is any damage that occurs to a chromosome
– Can occur spontaneously, or by radiation or exposure
to certain chemicals
– Changes may be as small as a few base pairs or as
large as the entire structure of the chromosome.
• There are 4 different ways chromosome structure can be
damaged:
DELETION
• In deletion, part of the
chromosome is actually
lost. Viruses, radiation
and chemicals can
cause a piece of a
chromosome to become
dislocated.
• This piece may carry a
specific gene which may
have a large effect on
the host.
DUPLICATION
In duplication, a gene
sequence is repeated
one or more times
within a chromosome.
At some point, too many
repeats can affect the
function of the gene.
INVERSION
In inversion, a gene
segment momentarily
becomes free from its
chromosome and
then reinserts in the
opposite order.
This can completely
alter the gene’s
activities.
TRANSLOCATION
• In translocation, part of
a chromosome
changes place with
another part of either
the same or a nonhomologous
chromosome.
• Translocations can
result in some cancers,
Down Syndrome, 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.
GENETIC TESTING
– 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.
Genetic Testing In-utero