Sex determination.

  • Bacci, Guido.
Date:
[1965]
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Sex determination. Source: Wellcome Collection.

    23/328 (page 7)
    SEXUAL AND ASEXUAL REPRODUCTION 7 and is sometimes observed in animal groups where amphigonic alternate with parthenogenetic phases (see Chapter 11). Elimination processes in mitosis have thus become a feature of the normal life cycle in some Metazoa but disturbances occurring in the course of mitotic divisions at more or less early life stages produce the exceptional phenomenon of gynandromorphism (see p. 80) which has had much importance in determining the process of sex determination. It was noticed that mitosis insures that every kind of self reproducing elements of the nucleus become identically distributed in the daughter cells. For this reason alterations in this process often cause important disturbances in the inheritance of sex, 3. Meiosis, Segregation and Recombination Meiosis halves the number of chromosomes and it takes place during gamete formation in most animals and during spore formation in most plants. It essentially consists of two divisions of the nuclei that follow each other rapidly while a single chromosome division takes place. Two meiotic or maturation divisions must therefore be considered. The first division has been designated the reductional division because it reduces the chromosome number and the second division is called equational and is apparently mitotic. The prophase of the first division is very long and involves very significant modifications. It is simply called the meiotic prophase and it has been divided into five stages. Leptotene stage (Fig. 1.3a). In this early prophase stage the filaments of the diploid set of chromosomes appear very long and slender and show a series of chromomeres which look like the beads of a rosary. In this stage the chromosomes may be arranged at random or they may lie with a definite orientation and be polarized with their ends at a side of the nucleus and the remaining portions spraying out in the interior of the nuclear membrane. This arrangement is called synizesis and it may be observed also in the fol¬ lowing stages. Zygotene stage. The pairing of the homologous chromosomes, which are of paternal and maternal origin, takes place at this stage. This process of union, which is named synapsis, starts at different places along the chromo¬ some filaments and it is completed at the pachytene stage. Pachytene stage (Fig. 1.3b). The pairs of chromosomes, which form the so- called bivalent chromosomes, have become shorter and thicker and are present in half the initial number of chromosome filaments. Each homologous chromosome of the new units that have thus developed, splits at this stage into two daughter chromatids and bivalents are said to form tetrads, being apparently composed of four chromatids.