Transcript Prenatal Diagnosis

Chromosomes and Human Genetics
10.4-10.5
12.2-12.3
Prenatal Diagnosis
COLLEGE PREP BIOLOGY
MR. MARTINO
Introduction
 Karyotype:
photograph of special
organization of
metaphase
chromosomes
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Paired with homologue
Autosomes arranged from
largest to smallest
Sex chromos. are last pair
Used to detect genetic
disorders
10.4 Chromosomal Basis of Inheritance
 Autosomes: the
chromsomes (numbered 1
– 22) that are determine
all traits but sex homologous
 Sex chromsomes: 23rd
pair of chromos. that
determine sex in humans
Females: XX (homo.
chromos.)
 Males: XY (non homo.
chromos.)
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10.4 Sex Determination in Humans
 Different species have different sex
determination systems
Humans have XY system –
males determine sex as do
spinach, grasshoppers, crickets,
and roaches
Females determine sex in fishes,
birds, strawberries, and
butterflies
 Most plants (monoecious) and some
animals (hermaphrodites) do not
have separate sexes.

Ex. Corn, earthworms and snails
10.5 Linked Genes
 1908 – William Bateson
and Reginald Punnett
discovered inheritance that
contradicted Mendelian
principles
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F2 generation should have been
9:3:3:1 – but it was 3:1
Discovered these genes are on
the same chromosome
 Linked Genes: genes
found on the same
chromosomes
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inherited together
 Early 1900’s, Thomas Hunt Morgan
discovered crossing over
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Used Drosophila melanogaster
Offspring should have been 1:1:1:1, but
were not
 Crossing over: is the exchange of
corresponding segments between
two homologous chromosomes
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Effects several genes at once
Genetic Recombination: the production
of gene combos different from those
carried by the original chromosomes
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Pedigrees
Pedigree: a tool used
to
assemble a family’s
history in a family
tree
Males
are squares
Females are circles
Full shade represent afflicted
Partial (if present) represent
carriers
Marriage lines join
parents
on sides
Sibling lines join
siblings
at top
10.5 Inheritance Patterns
 Autosomal Recessive:
any recessive trait carried
on a regular chromosome
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Ex. Cystic fibrosis, PKU,
albinism
 Autosomal Dominant:
any dominant trait carried
on a regular chromosome
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Ex. Polydactyly, Huntington’s,
achondroplasia
Sex-linked traits
 Sex-linked traits: are
determined by genes
located on the sex
chromosomes
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Usually the X
Ex. Fruit fly eye color,
hemophilia, some
colorblindness, red feline coat
color
 Hutchinson-Gilford progeria
syndrome
 Autosomal dominantindividual only needs one
dominant allele to show
the trait
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Result of mutation
Usually die in early teens
Both boys pictured are not yet
10…both died while in their
teens of complications with old
age
Changes in Chromosome Structure
 Occasionally chromo.
structure is altered
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1. Deletion: loss of a
chromosome segment
2. Duplication: gene
sequences that are repeated
3. Inversion: section of DNA is
reversed
4. Translocation: broken piece
of chromo. attaches to nonhomo.
chromo.
Do Changes in Chromosome Structure Ever
Evolve?
 Most chromo. changes tend to be selected against
 Duplications seem to be the exception
 Duplications may provide opportunity for beneficial mutations – still
have at least one good gene
 Several seem to be pivotal in evolution
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Duplications, inversions, & translocations helped
 Of
our 23 pairs of chromos. 18 are virtually identical to
chimps & gorillas – other 5 differ at inverted and
translocated regions
Changes in Chromosome Number
 Aneuploidy: when
there is an incorrect
number of chromos.
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Major cause of
reproductive failures
 Polyploidy: having
or more of each type
of chromo.
3
Possible in plants
 Occurs in some insects,
fishes, etc.
 Lethal in humans – only
1% survive to birth
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 Nondisjunction:
one or more pairs of
chromosomes fail to
separate during
meiosis
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Down’s syndrome
(Trisomy 21)
Turner’s syndrome –
females that have only 1 X
Klinefelter’s Syndrome –
males have 2 X’s
Prospects in Human Genetics
 Abortion: removal of a
fetus from the uterus
 Genetic Screening:
carriers are identified in order
to learn the probability of
certain disorders in offspring
 Genetic Counseling:
diagnosis, pedigree, genetic
testing for parents with
potential to pass on severe
genetic disorders
 Prenatal Diagnosis: there are
several methods available to
determine many different
genetic
disorders before birth
 Ultrasound: uses sound
waves
to produce image of fetus
No known risks
 takes measurements
 Routine in pregnancies and may detect
fetal problems
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Amniocentesis: withdrawal of
some amniotic fluid containing
fetal cells for karyotype
must be 14-16 weeks
 Results take several weeks
 1% risk of miscarriage
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 Chorionic Villi Sampling (CVS): a
tube is inserted into cervix & a small
amount of chorion is removed
This is fetal tissue and DNA
 8 – 10 weeks
 Results in hours
 2% miscarriage risk
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 Fetoscopy: a needle-thin
tube is inserted into uterus
direct veiwing
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10% miscarriage risk
 In-vitro fertilization: fertilize
egg in lab and select zygotes
with no disorder to implant into
mother
for
THE
END