During the preparation for cell division, for example, before the formation of the gametes or reproductive cells, the chromosomes may “cross over” so that material which originated from the male parent interchanges with that from the female parent. To quote from a standard work on evolutionary theory and genetics, Ernst Mayr’s Evolution and the Diversity of Life:
At some time prior to the formation of the gametes, the two homologous chromosomes exchange equivalent pieces with each other by a process called “crossing over.” By and large (there are many exceptions) no laws seem to determine where the chromosomes will break or how large the pieces will be that are exchanged. Which particular combination of pieces of maternal and paternal chromosomes making up the new chromosome will enter a given egg or spermatozoon is largely a matter of chance, at least in most chromosomes and most species. Likewise, it is largely a matter of accident which chromosomes will go into which germ cell, provided only that each cell receives its full set of chromosomes.
It is easy to attribute events to chance, but this only expresses scientific ignorance as to the real cause.
Another phenomenon is mutation: sometimes genetic codes suddenly change. This, too, is usually attributed to accident or chance, but then it is difficult to explain why any progression is made at all once biological systems have evolved to a certain level of complexity and perfection. The chance of a mutation leading to fatal, or at least less fit, properties is far greater than of a mutation making the individual fitter. Besides “crossing over” and mutation at one or several places on the chromosomes, another uncertain factor that Mayr and others mention is the distribution of chromosomes during reduction division (meiosis) to form reproductive cells. This process affects which side in the gamete the originally paternal or maternal genetic material goes to. Moreover, there are other opportunities for the course of events to be influenced from within: only part of the cell’s genetic code is active at certain times and under certain circumstances. Other parts are not active and may never become so during the present life. Thus, there are many secrets not yet unveiled by science, but nevertheless attributed to “chance.”
Chance, however, has no place in the theosophical view. Whatever happens is karmic, that is, it can be attributed to a cause, and this cause comes from within. In theosophy the different combinations of hereditary qualities in individuals are governed by psychomagnetic attractions inherent in the skandhas (स्कन्ध) of the reincarnating entity. Skandhas are the individual’s aggregates of properties — such as higher and lower mental consciousness, feelings, attractions, and physical characteristics — carried over from former lives. Thus, the specific magnetism of the soul, formed by its store of properties, determines which combinations of hereditary qualities will manifest in a particular incarnation. “It is . . . unquestionable that in the case of human incarnations the law of Karma, racial or individual, overrides the subordinate tendencies of ‘Heredity,’ its servant” (The Secret Doctrine).