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Chapter 11
Chromosomal Basis of
Inheritance
Mendel’s inheritance with
independent assortment
Mendel had factors, we now
know these factors were
specific genes at specific loci,
(locations) on specific
chromosomes. This is the
chromosome theory of
inheritance.
Morgan’s experiments with Fruit
Flies
A.Morgan used
fruit flies to
trace a gene
to a specific
chromosome
1. Drosophila Melangaster –
wildtype phenotype = most
common and mutant
phenotypes = caused by
mutations of the wildtype
gene.
2. Sex-linked- using the
gene for white eyes Morgan
found that if a female had
the trait and mated to a
male with wildtype, only her
sons would be white-eyed.
Sex Linkage
This meant the gene
must be on the X
and not the Y.
A daughter would
receive the
wildtype X from the
father which would
override the faulty
X from mom.
Linked genes – genes close together
on the same chromosome
• Morgan found that flies
have linked genes for
body color and wing
size.
• Gray and normal wings
were wildtype, black
and vestigal wings
were mutant types.
• Gray flies with vestigal
wings and black flies
with normal wings are
much harder to breed
than the ones in b.
Independent assortment
Genes which are not linked sort
independent of one another.When they
come together in a zygote it is called
recombination.
a. A frequency of 50% or more
recombination means the genes
are not linked
b. Linked genes do not sort
independently, they travel
together, making recombination
percentages low.
• If the genes are far
apart they behave
as if they were on
different
chromosomes
• This slide shows
recombination
during meiosis.
Recombination calculations
Sex chromosomes
A. In humans XX is
female and XY is
male
1. The SRY gene has
been shown to
trigger the
development into a
male fetus at about 2
months old.
2. SRY probably
regulates other
genes
3. Some XX male and
XY females exist
with mutated SRY
genes
Sex-linked
• Genes on the X
chromosome will
only be present in
one copy in males,
as they only have
one X and the Y
does not contain
the same genes.
Males are said to be hemizygous for these
traits
a. Colorblindness is an example of
a non- lethal sex-linked traitfemales can acquire it from their
father (colorblind) and mother
(carrier).
b. Duchene muscular dystrophy –
weakening of the muscles,1/3500
males- usually die by 20s
c. Hemophilia- absence of certain
clotting factors.
• In females one X
becomes inactivated
and condenses into a
Barr body, which one
is inactive is random,
and will be different in
different
cells.Ex.tortoise shell
cats
Errors
A.
Nondisjunctio
n- when meiosis is
not accurate and
some cells are
missing a
chromosome
(anueploidy) and
one has too many
chromosomes(poly
ploidy)
Down’s Syndrome-1/700
births trisomy 21
Klinefelter – XXY, XXXY
XYY
Other nondisjunctions
• Metafemale - XXX
2. Turner – X0
Other errors
A.
Deletions – Chromosome
fragment is lost
1. Cri-du-chat- chromosome
5
B.Duplication – chromosome
fragment attached to homologue
C.Inversion – reverse orientation of
fragment on chromosome
D.Translocation – fragment on a
nonhomologue
1. Implications of cancer CML
Imprinting
• Genomic
imprinting – One
active copy either
maternal or
paternal.
• Fragile X – caused
in part by maternal
imprinting.
Extranuclear genes
A. Small circular DNA in
mitochondria and chloroplasts
B. Maternal inheritance is
the rule