A meiotic study of two translocations and a tertiary trisomic in the mouse (Mus musculus)

In this section, the order of the articles has not been closely followed. Each point ends with the number(s) of the article(s) (as given in the contents), where the conclusion is based on.1) Cytological meiotic studies of T(2;8)26H and T(1;13)70H heterozygotes and Ts(1 ¹³)70H tertiary trisomics indicate, that chiasmata are more often located in the distal (translocated) segments than in the proximal (interstitial) segments containing centric heterochromatin (3 and 5).2) This study opens the possibility that the presence of centric heterochromatin decreases the probability of chiasma formation in its vicinity with a positive gradient distally (5).3) The genetic lengths of the interstitial and translocated chromosome segments coincide rather well with the physical length of these segments as estimated with the aid of Giemsa-banding. This finding does not fit the tendency expressed in the conclusions 1 and 2. The apparent exception of this rule is segment 13 _t which is overestimated when looking at genetic recombination. For cytological studies, the physical length of a segment is of a greater value (4).4) Univalence for chromosome 1 ¹³at metaphase I - anaphase I does not lead to an appreciable loss of this chromosome in the male, neither in the Ts(1 ¹³)70H tertiary trisomic karyotype nor in the T(1;13)70H heterozygote (3 and 5).5) In the T70H/+ karyotype, there is strong evidence for coorientation of the 1 ¹³univalent so that the four reciprocal translocation involved chromosomes segregate two by two. Occasionally, equational separation of the two 1 ¹³chromatids may occur at anaphase I (5).6) The segregational behavior of heterozygous translocation multivalent configurations can, within the genetic background concerned, be best explained by time differences of chiasma terminalization during metaphase I - anaphase 1 (5).7) The genetic background most likely exerts an influence on the behavior of mouse reciprocal translocations (5).8) The reliability of the formula which relates the summed frequencies of adjacent II disjunction and numerical non- disjunction and the relative viability of heterozygous translocation outcross progeny depends on the existence of selection against small litters during gestation. This is the more likely when the theoretically expected litter size decreases (5).9) A-chiasmate non-homologous chromosome association of the centric heterochromatin of chromosome 1 ¹³and the X-chromosome does occur (3 and 5).10) The majority of male Ts(1 ¹³)70H tertiary trisomics are capable of producing offspring. Thus, tertiary trisomy does not invariably lead to sterility in the male mouse (2 and 3).11) Tertiary trisomics for chromosome 1 ¹³in the mouse display a variety of phenotypes. The condition can lead to death in utero, to death before weaning, to morphologically affected but viable animals and to animals with an unaltered appearance (2 and 3).12) The ratio between morphologically affected and unaffected tertiary trisomics for chromosome 1 ¹³at birth (live or dead) amounts to between 2 and 3. This ratio might depend on the genetic background concerned (2 and 3).13) The most obvious abnormality of the morphologically affected tertiary trisomics of the Ts(1 ¹³)70H karyotype is a malformation of the bones of the skull which often leads to an abnormal growth of the upper and lower incisors (2).14) The impaired fertility of Ts(1 ¹³)70H males is most probably due to a lowered production of functional spermatozoa and the consequences this has for the continuation of pregnancy. Thus, the elimination of "unbalanced" progeny is not the first cause (3).