Chromosomes and Inheritance
Using Fruit Flies to Study
Genetics
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Thomas Hunt Morgan, 1908
Among normal red-eyed Drosophila he found a
mutated white-eyed fly (male).
Since it was a recessive gene that was expressed
only in male flies, Morgan suggested that the
chromosome was located on the X chromosome.
Miniature winged mutation was transmitted on the
same gene. These two characteristics did not
always travel together.
Morgan suggested that the chromosomes could
swap parts and received the Nobel prize in 1934.
Drosophila
melanogaster
X-linked Inheritance
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A situation where the genes that cause
a disorder are located on the X
chromosome.
More male victims than female
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Hemophilia
Color-blindness
Duchenne muscular dystrophy
Color Blindness
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The inability to distinguish among certain
shades of green and red.
These genes are found on the X
chromosome.
A recessive disorder where the genes do not
code for the proper pigment protein.
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Passed from a mother who is heterozygous for the
trait, she will see correctly
Since the son only gets one X chromosome, he
has no other allele to prevent the condition.
The Ishihara color blind test.
Autosomal Genetic Disorders
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Autosomal recessive disorders – a genetic
disorder related to a recessive trait found on
an autosomal chromosome.
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Sickle-cell anemia
Albinism
Cystic fibrosis – impaired lung function
Phenylketonuria – mental retardation
Tay-Sachs disease – nervous system degeneration
Werner syndrome – premature aging
Sickle-cell Anemia
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Called sickle cell because of the shape that
the red blood cell takes in the victim.
These cells clog capillaries, resulting in
decreased oxygen supply to brain, tissues.
Lower life expectancy (42, men 48, women)
Normal hemoglobin is type A
Sickle-cell anemia hemoglobin is type S
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Causes cell shape distortion
Result of one single amino acid substitution:
Sickle cell anemia
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Because it is a recessive condition, the
child must inherit two recessive genes,
one from each parent.
The parent who carries the recessive
allele is called a carrier.
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The individuals with hemoglobin S are
resistant to Malaria.
Dominant Autosomal
Disorders
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A single faulty allele can cause damage, even
when a second functional allele exists.
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Huntington’s Chorea – brain tissue degeneration
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Mental impairment, uncontrollable spastic movements
Polydactyly – extra fingers or toes
Campodactyly – inability to straighten little finger
Huntington’s Chorea
Polydactyly, an autosomal dominant trait
Pedigrees
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Can be used to determine if a condition
is dominant or recessive, X-linked or
autosomal.
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Circle = female
Square = male
Filled = has the condition
Half filled = carrier
Pedigree of a family with polydactyly
Polyploidy
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A condition in which one or more entire
sets of chromosomes has been added
to the genome of a diploid organism.
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When two sperm fertilize one egg.
During a meiosis malfunction.
Happens frequently in plants, not in
humans.
Incorrect Chromosome
Number
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Aneuploidy is when an organism has
more or fewer than the normal number
of chromosomes.
Occurs commonly, often unnoticed.
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Responsible for 30% of miscarriages.
Down Syndrome
Turner syndrome
Klinefelter syndrome
Nondisjunction
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A failure of homologous chromosomes or sister
chromatids to separate during meiosis.
 Meiosis I or meiosis II
 This produces an egg or sperm with a chromosome
count of 22 or 24.
 The embryo formed has a chromosome count of 45 or
47.
Presence of the extra chromosome causes an unbalance
in the proportion of biological ingredients.
All cause miscarriage except in the cases of
chromosomes 13, 18, 21
Normal homologous chromosome separation at anaphase I
Nondisjunction : Incomplete separation
of homologous chromosomes at anaphase I
Normal human karyotype, male
Normal Human karyotype, female
Human karyotype indicating Trisomy 21
Down Syndrome
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90% from egg nondisjunction
10% from sperm nondisjunction
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Smallish, oval heads
IQ well below normal
Reduced life span
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Risk increases with mothers over 35.
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Turner’s Syndrome
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Only one X chromosome
Called XO
Ovaries don’t develop properly
Short in stature
Brown spots on their bodies
Klinefelter Syndrome
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Phenotypically male
XXY
Some breast development
Lack of facial hair
More feminine figure
Tall stature
Non-functional testis
Deletions
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When a chromosome fragment breaks
off and does not rejoin any
chromosome.
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Cri-du-chat (cry of the cat) Syndrome
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Mental retardation
Improperly constructed larynx
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A deletion of the short arm of
chromosome #5.
Inversions and Translocations
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When a fragment of a chromosome rejoins
the chromosome it came from it may do so in
a flipped manner, this is an inversion
A translocation is an abnormality where two
chromosomes that are not homologous
exchange pieces, leaving both with improper
gene sequences.
Inversion
Translocation, called the Philadelphia
Chromosome is found in tumor cells.
Duplication
Duplications
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When two homologous chromosomes
exchange unequal pieces, one loses genetic
material and the other gains.
When duplicated, the chromosome with the
extra material will function normally.
In evolution, the extra gene could be altered
through mutation and eventually produce a
new protein.
Summary of Chromosomal
Structural Changes