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).
The expression of a small number of human genes is influenced by whether the gene has been inherited from the mother or father. This process-called genomic (or parental) imprinting-usually means that the organism expresses one of its alleles but not both.In many cases the non-expressed allele is inactivated-for example, by DNA methylation. (High levels of DNA methylation are known to inhibit gene activity.).he expression of a small number of human genes is influenced by whether the gene has been inherited from the mother or father. This process-called genomic (or parental) imprinting-usually means that the organism expresses one of its alleles but not both.