Are all animals produced by pronuclear injections of the same transgene guaranteed to have the same phenotype and genotype?

• A. True
• B. False
• C. They have the same genotype and phenotype
• D. They have no genotype differences

Answer: (B)

The variation comes from how transgenes are integrated into the genome. When a transgene is introduced by pronuclear injection, its insertion is random, so different founders can have different insertion sites and copy numbers. The site where the gene lands affects how strongly it is expressed (position effects) and how many copies are present can amplify or dampen expression. Epigenetic silencing can further reduce or alter expression over time. Additionally, founders can be mosaic, meaning not all cells carry the transgene, which can lead to differing genotypes and phenotypes within the same animal or among offspring. Because of these factors, animals produced with the same transgene are not guaranteed to share the same genotype or phenotype. To work with reliable lines, researchers genotype and phenotype each founder and establish multiple lines with characterizable expression patterns.