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Advanced Genetics
Hypertrichosis
• Women have twice the number of X
chromosomes as men do – how can this be?
• This means that they have twice the “gene
dosage” for genes on the X chromosome
as men.
• In order to correct this, one X chromosome is
“turned off” and turned into a “Barr Body.”
• The Barr Body shows up as a dark spot within
the nucleus during interphase.
• Cells which have the same X
chromosome inactivated tend
to group together in adult
females, forming patches.
• This means that different
patches of the body will
express different phenotypes.
• For example, hair
colour in cats is
determined by a gene
on the X chromosome.
• The gene has two
O
alleles: X (orange) and
B
X (black).
• For males:
–XOY will be orange.
–XBY will be black.
• For females:
–XOXO will be orange.
–XBXB will be black.
• What about females with the
genotype XOXB?
• Since different patches of skin will
have different X chromosomes
inactivated
– some patches will express the orange
phenotype
– some will express the black phenotype
• The result will be
calico:
• Do male calico cats exist?
• In order to be calico, a cat must have an XOXB
genotype.
• Only way a male can have this is if they have the
genotype XOXBY
• This means they have
–Klinefelter Syndrome
– polysomy X
– 2 copies of the X chromosome (remember XXY)
• The same thing happens in women
who have anhydrotic ectodermal
dysplasia… patches of their bodies
expressing the affected X
chromosome have no sweat
glands, hair, or teeth. For example:
Barbara
McClintock
• Discovered the method by
which “crossing over” of
chromosomes or genetic
recombination occurs.
• She did this despite facing
considerable sexism, in a
field where women were
almost universally
excluded.
• She also discovered that some
elements of the chromosome
(called “transposons”) can
“jump” from one chromosome
to another.
• For this work, Dr. McClintock
won the 1983 Nobel Prize in
Medicine.
• These transposons exist all over
the place in nature, especially in
simple genomes like those of
bacteria.
• Additionally, bacteria sometimes
have circular segments of DNA
called “plasmids” which they can
“inject” into other bacteria to
transmit genetic information.
• For example, look at the
animation on this web page.
• This mechanism allows
bacteria to transmit antibiotic
resistance or other dangerous
traits to each other – this can
often occur in areas with a
large variety of bacteria, such
as hospitals.
• We can also make use of similar
elements of DNA to “repair”
genetic abnormalities before the
fetus has developed a disorder.
• This is done by replacing the
defective gene with a working copy
of the gene
– in other words, an undesirable allele
is taken out and a desirable allele is
put in
Controlled Breeding!!
•
For thousands of years before DNA was even
known to exist, human beings engaged in a form
of “genetic engineering”.
1. Selective breeding:
– Animals which possess desirable traits are encouraged
to breed, while those which do not are prohibited from
breeding….examples????
2. Inbreeding:
– Animals from the same “family” are interbred to
strengthen desirable characteristics.
– Gene pool becomes limited….examples????
•
Both forms of breeding can “limit the gene pool”
• Using just these two methods, humans have
made some amazing changes.
• This:
• Became this:
• And this:
• Became this: