Well,
I have discussed about sex influences on heredity on the last posts. I could
not forget this amazing topic in my literature reviews here. Genomic imprinting
is a phenomenon whereby the expression of the genotype is different when
inherited from the male or female parent. Some genes have their expression
significantly differentiated according to their parental origin (Pierce, 2008).
The mechanisms of control of this expression are still not totally clear, but
the methylation of DNA is undoubtedly fundamental to the process, being a way
of epigenetic phenomenon.
 |
| Fig. 1: Genomic imprinting in fetal growth of mice. (Pierce, 2008) |
The
gene Igf2 of mice and humans express
under genomic imprinting process, encoding the protein insulin-like growth
factor II, which determines the size of the fetus and placenta. Both alleles of
the gene are inherited from the father and the mother, but only the paternal
one is expressed, while maternal copy is completely silent. Demonstrations of deleted
paternal copy of Igf2 in mice show
small placenta and low-birth-weight in the offspring as result. Like in the
placenta in mammals, plants have also exhibited differential expression of
maternal and paternal genes in the endosperm, which provides nutrients for the
growth of the embryo (Pierce, 2008).
 |
| Fig. 2: Genomic imprinting in deletion of region in chromosome 15 causing Prader-Willi and Angelman Syndrome. |
Genomic
imprinting has also received importance in human syndromes. It has been
observed this event in the Prader-Willi and Angelman syndromes. Many
individuals with the Prader-Willi syndrome do not have a small region on the
long arm of chromosome 15. This deletion is always inherited from the father.
When the same deletion is maternal, the expression results in a totally
different set of symptoms, originating another syndrome: the Angelman syndrome
(Pierce, 2008). Children with Prader-Willi syndrome present small hands and
feet, mental retardation, poor sexual development and short stature. Symptoms
of Angelman syndrome include frequent uncontrolled muscle movement, a large
mouth and unusual seizures. A normal development requires the presence of both
paternal and maternal regions of the chromosome 15(Pierce, 2008).
REFERENCE:
Pierce B. A. (2008). Genetic
Maternal Effect. In: Genetics: A
Conceptual Approach. (third edition) W. H. Freeman and Company, New York. p
119-120.
Figure 2: http://atlasgeneticsoncology.org/Deep/GenomImprintID20032.html
(accessed 10/05/2014)
This topic is a fascinating one and amazing for its influence on the phenotype! Do you know how often genomic imprinting influences the phenotype of an organism? That is, are there innumerable genes that can be imprinted, or are there only a specific subset of genes that will be genomically imprinted? Particularly with the syndromes you present, why has genomic imprinting on chromosome 15 not been slowly removed over time, as surely these individuals (in terms of natural selection theory) show decreased fitness? Interesting information!
ReplyDeleteThe genomic imprinting affects only a subset of genes. I'm not sure about the syndromes, but I believe they are not removed because the methylation (present in the process of genomic imprinting) is erased in the germ cells each generation and then reestablished in the course of gamete formation, with sperm and eggs undergoing different levels of methylation. This causes differential expression of male and female alleles in the offspring. Also, methylation can be reversed when the DNA passes through a gamete. So, normal humans can get affected individuals in their offspring. Therefore, it is expected that these syndromes will continue over the time.
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