I. Introduction

• Discusses the assumption in Mendelian genetics and the chromosomal basis of inheritance that a given allele will have the same effect whether it was inherited from the mother or the father.
• Mentions that geneticists have identified a number of traits in placental mammals and some flowering plants that depend on which parent passed along the alleles for those traits, which is called genomic imprinting.

II. Definition and Occurrence of Genomic Imprinting

• Defines genomic imprinting as the variation in phenotype depending on whether an allele is inherited from the male or female parent.
• States that about 100 imprinted genes have been identified in humans and 120-180 in mice, and that genomic imprinting occurs during gamete formation and results in the silencing of a particular allele of certain genes.

III. Imprinting During Development

• Explains that imprints are transmitted to all body cells during early development and that in each generation, the old imprints are erased in gamete-producing cells.
• Points out that in a given species, the imprinted genes are always imprinted in the same way and that a gene imprinted for maternal allele expression is always imprinted this way, generation after generation.

IV. Example: The Mouse Gene for Insulin-Like Growth Factor 2 (Igf2)

• Uses the mouse gene for insulin-like growth factor 2 (Igf2) as an example of an imprinted gene.
• Explains that the paternal allele is expressed and mentions evidence that came from crosses between normal-sized (wild-type) mice and dwarf (mutant) mice homozygous for a recessive mutation in the Igf2 gene.

V. What is a Genomic Imprint?

• Describes a genomic imprint as either silencing an allele in one type of gamete (egg or sperm) or activating it in the other.
• Mentions that in many cases, the imprint seems to consist of methyl (¬CH3) groups added to cytosine nucleotides of one of the alleles, and that heavily methylated genes are usually inactive.
• Explains that for a few genes, methylation has been shown to activate expression of the allele, and gives the example of the Igf2 gene.

VI. Importance of Genomic Imprinting

• States that imprinted genes are involved in regulation of body temperature, sleep, and other metabolic functions.
• Discusses the proposal by some evolutionary biologists that genomic imprinting represents a kind of evolved “competition” between males and female mammals.
• Mention that normal development seems to require that embryonic cells have exactly one active copy of certain genes, and that improper imprinting is associated with abnormal development and certain cancers.

VII. Conclusion

• Summarizes the importance of genomic imprinting in embryonic development and ongoing studies in the field.