special subjects
Factors Affecting Gene Expression
Genomic imprinting
Genomic imprinting is the differential expression of genetic material depending on whether it has been inherited from the father or mother. For most autosomes, both the parental and maternal alleles are expressed. However, in<1%of alleles, expression is possible only from the paternal or maternal allele. For example, expression of the gene forinsulin-like growth factor 2 is normally expressed only from the paternal allele. Genomic imprinting is usually determined by effects that occur normally in the development of gametes. Changes such as methylation of DNA may cause certain maternal or paternal alleles to be expressed to different degrees. A disorder may appear to skip a generation if genomic imprinting prevents the causative allele from being expressed. Defective imprinting, such as abnormal activation or silencing of alleles, can result in clinical disorders (eg, Secondary hypogonadism , Angelman syndrome). 