Non-Mendelian Inheritance (Easy)

Genomic Imprinting

• Genomic imprinting occurs in several species, including insects, mammals, and flowering plants.
• It can involve a single gene, a part of a chromosome, an entire chromosome, or even all the chromosomes from one parent.
• Genomic imprinting can be used for X inactivation in some species.

Imprinting and DNA Methylation

• Genomic imprinting involves a marking process at the molecular level.
• The imprinting of several genes involves an imprinting control region (ICR) located near the imprinted gene.
• The ICR is methylated either in the oocyte or sperm, but not both.
• The ICR contains binding sites for one or more transcription factors that regulate the imprinted gene.
• For most genes, methylation causes inhibition of transcription.

Pattern of Methylation

• Each parent inherits one methylated and one unmethylated gene, which is maintained in somatic cells.
• Methylation is removed in gamete-forming cells.

X-Chromosome Inactivation

• The inactivated X chromosome is maintained as such during subsequent cell divisions.
• Some genes on the inactivated X chromosome are expressed in the somatic cells of adult female mammals.
• Dosage compensation is unnecessary for these genes because they are located on both the X and Y chromosomes.
• Up to a quarter of X-linked genes in humans may escape full inactivation.

Genomic Imprinting

• Genomic imprinting is a phenomenon in which a segment of DNA is marked, and the effect is maintained throughout the life of the organism.
• The offspring expresses either the maternally-inherited or the paternally-inherited allele, but not both (monoallelic expression).

Mouse Igf2 Gene as an Example of Genomic Imprinting

• The Igf2 gene codes for a growth hormone called insulin-like growth factor 2.
• A functional Igf2 gene is necessary for normal size.
• Imprinting results in the expression of the paternal but not the maternal allele.
• The paternal allele is transcribed into RNA, while the maternal allele is not transcribed.

Stages of Imprinting

• Imprinting can be divided into three stages:
  • Establishment of the imprint during gametogenesis
  • Maintenance of the imprint during embryogenesis and in adult somatic cells
  • Erasure and reestablishment of the imprint in the germ cells

This quiz covers the concept of genomic imprinting in various species, including its involvement in DNA methylation and X inactivation. Learn about the different levels of genomic imprinting, from single genes to entire chromosomes.