An Introduction to the Invertebrates, Second Edition - tiera.ru

WHAT IS THE CELLULAR BASIS OF HEREDITY?3Fig.1.1 The Mendelian ratio:showing that, if ‘T’ is tall and ‘t’ isshort, the first generationwill all appear tall and that(after self-pollinating) the secondgeneration will have the ratioof three tall to one short.the ‘recessive’ and has no apparent effect on the organism but ismaintained when it reproduces (Figure 1.1). The organism containsa very large number of such pairs (some 50 000 pairs in humans)most of which segregate and recombine independently at everysexual reproduction. Mendel’s analysis explained both the basicresemblance between parents and offspring and the introduction ofvariation between them.1.3 What is the cellular basis of heredity?Early in the twentieth century, T. H. Morgan’s studies of cell structureidentified Mendel’s factors as ‘genes’ borne on the elongated bodies,‘chromosomes’, contained in the nucleus of almost every cell in thebody (see Chapter 15, where the contributions of studies of the fruitfly Drosophila melanogaster are discussed). All organisms develop fromthe division of cells which previously formed part of one or (wherereproduction is sexual) two parent organisms. August Weismann firstrecognised that the ‘germ-plasm’ that gives rise to gametes is distinctfrom the rest of the body, the ‘soma’. Somatic cells divide by ‘mitosis’,the longitudinal splitting of each chromosome with self-replicationof each gene so that each half chromosome has exactly the samegenes as its parent (Figure 1.2a): all the somatic cells in an individualare genetically identical. Gamete-forming cells first multiply by the