Transcript File

4.3 Alterations In Chromosome
Structure and Number
Big Ideas
• Errors in meiosis can lead to errors in chromosome number
in gametes. Embryos that develop from these gametes will
have missing or extra chromosomes
• Duplication, deletion, inversion and translocation are types
of damage that can occur to chromosomes during meiosis
• Genetic testing can be an important part of diagnosing
treating or preventing particular genetic conditions
Atypical Chromosome Numbers
• In most cases, a human embryo with an atypical
number of chromosomes does not survive and the
mother has a miscarriage
• Some embryos with atypical chromosome numbers
do survive depending on which chromosomes are
affected
• Trisomy: condition in which an individual has three
copies of a particular chromosome
• Down Syndrome (Trisomy 21): individual carries an
extra copy of chromosome 21
– Characterized by mild to severe developmental delays,
certain facial features, below average height
• Meiosis occurs repeatedly in a persons lifetime as
the testes produce sperm or the ovaries produce
eggs
• Almost always the meiotic spindle distributes
chromosomes to the daughter cells without error
• Occasionally mistakes can occur during meiosis that
alter chromosome number
• Non-disjunction: occurs when homologous
chromosomes do not separate during Anaphase I or
Anaphase II – page 118
• This results in gametes with either missing or extra
chromosomes
• If an abnormal gamete produced by a nondisjunction event unites with a normal gamete in
fertilization, the result is a zygote that contains an
atypical number of chromosomes
• Mitosis will then carry this number of chromosomes
to all of the embryonic cells
Other conditions caused by non-disjunction
• Trisomy 13 (Patau Syndrome): results in severe
intellectual disability and physical abnormalities (heart,
brain, spinal cord, eyes, muscle tone, extra digits, cleft lip);
affects 1 in 16 000 births, 5-10% live past year 1
• Trisomy 18 (Edwards Syndrome): results in severe
intellectual disability and low birth weight (small cranium,
heart defects); affects 1 in 5000 births, 5-10% live past year 1
• XXY (Klinefelter Syndrome): affects male sexual
development and results in testes that do not produce
enough testosterone (reduces development of secondary
sex characteristics); affects 1 in 500-100 births, testosterone
injections help treat this condition
Damage To Chromosome Structure
• Mutation: any change to the DNA of a cell
– Some mutations change only one or a few base pairs in a
DNA sequence, others occur at the level of chromosomes
• Large-scale mutations that damage chromosome
structure can occur when something unusual
happens during crossing over
• Translocation: a fragment of one chromosome
attaches to a non-homologous chromosome
• Ex: Translocation Down Syndrome
– translocation between chromosomes 21 and 14
– inheritance of two normal copies of chromosome 14 and
one that has a piece of chromosome 21 attached to it
– the extra material from chromosome 21 causes Down
Syndrome (like Trisomy 21)
• Sometimes translocations result in deletions
• Deletion: a fragment of a chromosome is lost
• Much of our DNA does not contain genes, therefore
small deletions are not usually harmful
• However, if the lost fragment contains one or more
genes then the instructions for making certain
proteins may be lost
• Those proteins have specific functions in the body
• Ex: Prader-Willi Syndrome – deletion of
chromosome 15
• Duplication: part of a chromosome is repeated
• The repeated region sometimes includes an entire
gene
• Interestingly, if a region is only duplicated once, the
duplication may have no effect
• The duplication is most likely to have an effect when
duplicated multiple times
• Ex: Three or more duplications on chromosome 15
results in developmental delays and seizures
• Inversion: reverses a fragment of the original
chromosome
• Most of the genes are still present in their normal
number and some may function
• However, inversions can cause problems with the
synapsis of chromosomes during meiosis
– Therefore someone with a chromosomal inversion may
have difficulties conceiving a baby
• Summary: figure 4.39 on page 119
Genetic Testing
• Modern technologies offer many ways to obtain
information about the genetic makeup of an
individual
• Important part of diagnosing, treating or preventing
illness
• In addition to karyotype analysis, genetic testing
can include screening for specific gene sequences
• Carrier testing: can determine if an individual
carries a copy of a mutation that his or her children
could inherit
• A person with a family history of a genetic condition
may want to undergo genetic testing to find out his
or her risk of developing the condition
• This type of testing is called preventative or
presymptomatic testing and is usually done for
disorders that appear after birth or later in a
person’s life
• Diagnostic genetic testing is used to confirm a
diagnosis when symptoms for a particular genetic
condition are present
• A human embryo becomes a fetus about seven
weeks after fertilization
• Prenatal testing is used to detect small-scale
mutations or chromosomal alterations in a fetus
– Amniocentesis: between 14 and 20 weeks of pregnancy;
extraction of some amniotic fluid that surround fetus
– Chorionic Villus Sampling: as early as week 8 of
pregnancy; extraction of some chorionic villi from the
tissue around the fetus
• Newborn screening can detect some genetic
disorders at birth through simple tests
• Ex: shortly after birth, newborns are screened for
phenylketonuria (PKU)
• PKU is an inherited disorder that results in the
inability of children to break down the amino acid
phenylalanine
• Phenylalanine accumulation can lead to severe
developmental delays
– Therefore the detection of this condition can prevent
retardation