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Nondisjunction Mutations
Chapter 12
Nondisjunction ("not coming apart") is the failure of
chromosome pairs to separate properly during cell
division.
• This could arise from a failure of homologous
chromosomes to separate in meiosis I, or the failure of
sister chromatids to separate during meiosis II or
mitosis.
• The result of this error is a cell with an imbalance of
chromosomes. Such a cell is said to be aneuploid.
• Loss of a single chromosome (2n-1), in which the
daughter cell(s) with the defect will have one
chromosome missing from one of its pairs, is referred
to as a monosomy.
• Gaining a single chromosome, in which the daughter
cell(s) with the defect will have one chromosome in
addition to its pairs is referred to as a trisomy.
• If these gametes are fertilized, it will result in an embryo
in which all the cells have an abnormal chromosome #
• Great video demo here
• Narrated video here
Trisomy 21: Down syndrome
Down syndrome
• The condition is characterized by a combination of major and minor
differences in structure.
• Often Down syndrome is associated with some impairment of
cognitive ability and physical growth, and a particular set of facial
characteristics.
• Down syndrome in a fetus can be identified with amniocentesis
during pregnancy, or in a baby at birth.
• Individuals with Down syndrome tend to have a lower than average
cognitive ability, often ranging from mild to moderate
developmental disabilities.
• A small number have severe to profound mental disability.
• The incidence of Down syndrome is estimated at 1 per 800 to
1,000 births, although it is statistically much more common with
older mothers.
• http://en.wikipedia.org/wiki/Down_syndrome
Trisomy 18: Edwards syndrome
Edwards syndrome
• The incidence of the syndrome is estimated as
one in 3,000 live births[2].
• The incidence increases as the mother's age
increases.
• The syndrome has a very low rate of survival,
resulting from heart abnormalities, kidney
malformations, and other internal organ
disorders.
• Only 50% of liveborn infants live to 2 months, and
only 5–10% survive their first year of life.
• http://en.wikipedia.org/wiki/Edwards_syndrome
Trisomy 13: Patau syndrome
Patau syndrome
• The risk of this syndrome in the offspring increases
with maternal age at pregnancy, with about 31 years
being the average.[1]
• Patau syndrome affects approximately one in 10,000
live births.
• A few symptoms are:
–
–
–
–
mental & motor challenged
polydactyly (extra digits)
microcephaly low-set ears
holoprosencephaly (failure of the forebrain to divide
properly).
– heart and kidney defects
XXY - Klinefelter’s syndrome
Klinefelter’s syndrome
• The condition exists in roughly 1 out of every 1,000
males.
• The principal effects are development of small testicles
and reduced fertility.
• Some degree of language learning impairment may be
present,[7] and neuropsychological testing often reveals
deficits in executive functions.[8]
• In adults, possible characteristics vary widely and
include little to no signs of affectedness, a lanky,
youthful build and facial appearance, or a rounded
body type with some degree of gynecomastia
(increased breast tissue).
• http://en.wikipedia.org/wiki/Klinefelter%27s_syndrom
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Monosomy X: Turner’s syndrome
Turner’s syndrome
• Occurring in 1 out of every 2500 girls, the syndrome manifests itself
in a number of ways.
• There are characteristic physical abnormalities, such as short
stature, swelling, broad chest, low hairline, low-set ears, and
webbed necks.[3]
• Girls with Turner syndrome typically experience gonadal
dysfunction (non-working ovaries), which results in amenorrhea
(absence of menstrual cycle) and sterility.
• Concurrent health concerns are also frequently present, including
congenital heart disease, hypothyroidism (reduced hormone
secretion by the thyroid), diabetes, vision problems, hearing
concerns, and many autoimmune diseases.[4]
• Finally, a specific pattern of cognitive deficits is often observed,
with particular difficulties in visuospatial, mathematical, and
memory areas.[5]
• http://en.wikipedia.org/wiki/Turner%27s_syndrome
XYY – Jacobs syndrome
XYY
• 47, XYY boys have an increased growth velocity during
earliest childhood, with an average final height
approximately 7 cm above expected final height.[3]
• Severe acne was noted in a very few early case reports, but
dermatologists specializing in acne now doubt the
existence of a relationship with 47,XYY.[4]
• Testosterone levels (prenatally and postnatally) are normal
in 47,XYY males.[5] Most 47,XYY males have normal sexual
development and usually have normal fertility.
• Since XYY is not characterized by distinct physical features,
the condition is usually detected only during genetic
analysis for another reason.
• XYY boys have an increased risk of learning…and delayed
speech and language skills.
• http://en.wikipedia.org/wiki/XYY
Prenatal detection
• Amniocentesis: a small
amount of amniotic fluid,
which contains fetal tissues,
is extracted from the
amniotic sac surrounding a
developing fetus, and the
fetal cells (DNA) is
examined for genetic
abnormalities.
• Can be performed between
the 16th-20th week of
pregnancy.
Prenatal detection
• Chorionic Villus
Sampling (CVS): It
entails getting a
sample of the
chorionic villus
(placental tissue)
and testing it. CVS
usually takes place
10-12 weeks of
pregnancy (earlier
than amniocentesis)