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The sequence of the human genome integrates the keystone of our life. Part of it is transcribed to RNA, which in turn provides the information required by our cells to produce proteins. Discoveries in the genetics field have been essential to medicine and have been used to develop strategies to modify, prevent and propose new therapeutic approaches for human diseases. In the 19th Century, Gregor Johann Mendel developed a theoretical model which was able to predict in an accurate way hereditary mechanisms; indeed, his laws still explain the basis of human inheritance. Almost ninety years later, James Watson and Francis Crick announced their double-helix model of the DNA molecule. Then, positional cloning and the polymerase chain reaction (PCR) were introduced; more recently, almost 99% of the sequence of our genome was made public. The current period of time is known as the post-genomic era, due to the fact that researchers are not only obtaining the complete sequences of thousands of genomes, but are also searching for clues that may help understand the mechanisms that affect gene activation and deactivation, in which epigenetic factors are also involved. In medical domains, twins constitute a suitable group to study inherited disorders. Dizygotic or fraternal twins are produced by different egg and sperm cells, and even when these two fertilization events occur simultaneously, dizygotic twins share approximately the same percentage of genetic material than any pair of siblings, that is, around 50%. Some authors have suggested that the tendency for spontaneous dizygotic twinning could be attributed to a double ovulation which is genetically determined in an autosomal dominant manner. Monozygotic, as opposed to dizygotic twins, are produced by a single zygote whose cells are dissociated and originate two independent organisms; approximately a third of monozygotic twins are separated before the 5th day after fertilization, and the rest between the 5th and the 15th day. Most monozygotic twins are very similar; nevertheless, some few exceptions prove that in fact they actually do not have to be identical. Relatives of a person with a mental disorder tend to share traits associated with this disease, especially if the patient and the relative are monozygotic twins. However, important differences may be detected even between each pair of identical twins. Parameters such as concordance and heritability have shown that a monozygotic twin can develop an inherited disorder while his or her co-twin will always be disease-free. In addition to differences in susceptibility to inherited diseases, this kind of twins can display dissimilarities in somatic cell mutations (more overtly noticeable when ageing), their set of antibodies and T cell receptors, their number of mitochondrial DNA molecules, and chromosome X inactivation patterns in women, all of which are the main subject of many ongoing studies. A recent report shows that from 160 monozygotic twin pairs who were 3 to 74 years old, epigenetic patterns were identical early in life, but differences were more obvious at older ages, especially if twins were raised apart or if they had different medical history.… [ABSTRACT FROM AUTHOR]

A diferencia de los varones, las mujeres disfrutan de un sistema de relaciones sociales más extenso y más cercano y presentan índices más altos de depresión mayor. Estudios anteriores indicaron que las mujeres eran más vulnerables a los efectos depresogénicos de la falta de apoyo social, pero la mayoría de estos estudios presentan importantes limitaciones metodológicas. Método: En dos tandas de entrevistas separadas por al menos un año se evaluaron 1. 057 parejas de gemelos dicigóticos de sexo opuesto procedentes de un registro poblacional. Los autores pronosticaron el riesgo de depresión mayor durante el ano que precedió a la segunda tanda de entrevistas a partir de los niveles de apoyo social evaluados en la primera tanda de entrevistas. Resultados: Las mujeres refirieron un nivel más alto de apoyo social global que sus gemelos de sexo masculino. El apoyo social global determinado en la primera evaluación pronóstico en mayor medida el riesgo de depresión mayor, determinado en la segunda tanda de entrevistas, en las mujeres que en los varones de las parejas de gemelos, observándose el mismo efecta cuando el análisis controlo los antecedentes de depresión mayor correspondiente al año que precedio a la primera tanda de entrevistas. Las mujeres fueran más sensibles que los varones a los efectos depresogénicos de un bajo nivel de apoyo social, especialmente del gemelo, de otros familiares, de los padres y de los cónyuges. Los niveles de apoyo social no explicaban las diferencias de sexo en cuanto al riesgo de depresión mayor. Conclusiones: El apoyo emocional que proporcionan las relaciones sociales protege en mayor medida a las mujeres que a los varones de la depresión mayor. Aunque estos efectos no explican las diferencias de sexo en la prevalencia de depresión mayor, sí sugieren importantes diferencias de sexo en cuanto al desarrollo del riesgo, se debe esclarecer el origen de las relaciones causales entre el escaso soporte social y la depresión en las mujeres. [ABSTRACT FROM PUBLISHER]