From Wikipedia,
the free encyclopedia.
Normally, a healthy child
cannot be produced when both sets
of chromosomes come from the same
parent. Imprinting of the same
areas will occur and all these
genes will be suppressed. No known
cases of
parthenogenesis exist in
mammals because of imprinted
genes. Experimental manipulation
of a paternal methylation imprint
controlling the Igf2 gene has,
however, recently allowed the
creation of rare individual mice
with two sexual sets of
chromosomes - but this is not a
true parthenogenote.
A process known as
reprogramming occurs in the
parent female or male when the egg
or sperm is maturing. In many
instances this is achieved through
methylation of the DNA of
genes or
regulatory sequences, which
results in the gene not being
expressed. In other instances,
phosphorylation or other
chemical modification of
histone proteins appears to
lead to silencing.
Imprinting is known to cause
problems in
cloning, with clones having
DNA that is not
methylated in the right
places. Some scientists think this
is due to there not being enough
time for reprogramming to be
properly achieved. When a
nucleus is added to an egg
during
somatic cell nuclear transfer,
the egg starts dividing in
minutes, as compared to the days
or months it takes for
reprogramming during
embryonic development. If time
is the responsible factor, it may
be possible to delay cell division
in clones, giving time for proper
reprogramming to occur.
Several
genetic diseases that map to
15q13 (locus 3 of section 1 of the
long arm of chromosome 15) in
humans are due to abnormal
imprinting. The
Prader-Willi syndrome is due
to 2 copies of the chromosome 15
being inherited from the mother,
and the locus is imprinted; the
Angelman syndrome gene is due
to 2 copies of the chromosome 15
inherited from the father and the
locus is similarly imprinted.
Thus, someone who inherits both
chromosomes 15 from one parent
(called
uniparental disomy) has
Prader-Willi or Angelman syndrome,
depending on which parent they
come from.
A gene in
sheep called "callipyge"
produces large buttocks consisting
of muscle with very little fat.
This gene is expressed only if it
is on chromosome 18 inherited from
the father and is not on the
chromosome from the mother.
