If a cell in mitosis is subjected to the action of substances, such as colchicine, called mitotic or antimitotic poisons, or statmokinetics, the mechanism of migration of the centromeres in the spindle is blocked and the chromosomes remain at the metaphase stage. they can fix, photograph and enlarge the chromosomes, then arranging them in an ordered series according to well-defined classification criteria (relative position of the centromere and size). For each cell a karyotype is thus obtained; the average value (to avoid single errors) constitutes the karyotype individual.
The most easily detectable chromosomal aberrations are those due to trisomy, monosomy or nullisomy. These correspond respectively to the presence of three, one or no chromosomes of a given type. The presence of only one or three chromosomes can be compatible with independent life, albeit with morphological and metabolic alterations that are often very serious, altering the normal balance of dosage and interaction between genes; nullisomy is to be considered lethal, as the total lack of all the genes contained within a chromosomal pair cannot be compensated.
A classic case of trisomy is "trisomy 2" or Down syndrome, commonly known as "mongolism".
A case of monosomy occurs, as we will see, in Turner syndrome, in which there is only one gonosome (X).
It is possible to distinguish aberration by translocation from that by meiotic nondisjunction. The cause, the effects, the hereditary transmission and consequently the eugenic prognosis are different, as is the cytological relief.
CYTOPLASMATIC HERITAGE
All the treatment of genetics carried out up to now is based on the study of the behavior of nuclear DNA, of its transmission, of its relationship with the cytoplasmic and external environment.
Although the role of the nucleus in heredity is absolutely paramount, there are also phenomena of hereditary transmission linked in various ways to the cytoplasm. For example, cytoplasmic organelles with their own genetic continuity (mitochondria, plastids, centrioles) can present separate hereditary transmission phenomena. from nuclear ones, albeit with complex interactions. Even the content of reserve material that the maternal organism supplies to the embryo for its initial development can determine phenomena of cytoplasmic inheritance. In general, it can be said that the expression of hereditary patrimony is variously conditioned by the interaction with factors of the cytoplasmic and extracellular environment.
