Water-siphon test. Dr Henry Knipe â and Dr Marcin Czarniecki â et al. The water siphon test may be performed as part of a barium swallow to assess for gastro-oesophageal reflux in a hiatal hernia. It is performed in the supine RPO position with the patient drinking water continuously. The test is said to be positive if there is visible barium reflux in the oesophagus.
In some instances, offspring can demonstrate a phenotype that is outside the range defined by both parents. In particular, the phenomenon known as overdominance occurs when a heterozygote has a more extreme phenotype than that of either of its parents. Indeed, in a few examples, a trait that shows overdominance sometimes confers a survival advantage in the heterozygote (Parsons & Bodmer, 1961).
Genomic imprinting is an example of epigenetic inheritance in which differences in gene function depend on whether the allele was inherited from the mother or father. Many disorders appear to involve epigenetic alterations to imprinted genes. Genes involved in imprinting play important roles in both preâ and postânatal growth and neurodevelopment. Imprinting disorders include several childhood genetic disorders such as PraderâWilli, Angelman and BeckwithâWiedemann syndromes, as well as several types of cancer including Wilms tumour. Recent evidence suggests a correlation between assisted reproductive technologies (ART) and imprinting disorders, a phenomenon requiring more inâdepth studies as the prevalence of ART is increasing.