Physical Basis of Heredity 1.  The Nucleus and the Chromosome
The Nucleus 
Significance of nucleus : Hammerling's experiment
Number, shape and size of nucleus
Nucleus in prokaryotes and eukaryotes
Nuclear envelope
Nuclear pore complex and nucleocytoplasmic traffic
Number, size and shape of chromosomes
Morphology of chromosomes
Euchromatin and heterochromatin
Constitutive and facultative heterochromatin
Single-stranded and multi-stranded hypotheses for chromosomes
Chemical composition of chromosomes
Infrastructure of chromosomes
Function of chromosomes
Special types of chromosomes 
Lampbrush chromosomes
Salivary gland chromosomes
Prokaryotic Nucleoids

A group of plants or animals comprising a species is characterised by a set of chromosomes, which have certain, constant features. These features include chromosome number, size and shape of individual chromosomes and other attributes listed above. The term karyotype is given to the group of characteristics that identifies a particular chromosome set and is usually represented by a diagram called idiogram, where chromosomes of haploid set of an organism are ordered in a series of decreasing size. The karyotypes of different groups are sometimes compared and similarities in karyotypes are presumed to represent evolutionary relationships.

Karyotype also suggests primitive or advanced feature of an organism. A karyotype showing large differences between smallest and largest chromosome of the set and having fewer metacentric chromosomes, is called asymmetric karyotype, which is considered to be a relatively advanced feature when compared with symmetric karyotypes. A symmetric and an asymmetric karyotype are shown in Fig. 6.10.
(A) A symmetric and (B) an asymmetric karyotype (redrawn from Stebbins, Chromosomal Evolution in Higher Plants).
Fig. 6.10. (A) A symmetric and (B) an asymmetric karyotype (redrawn from Stebbins, Chromosomal Evolution in Higher Plants).

In 1931 G.A. Levitzky, a Russian scientist suggested that in flowering plants there is a predominant trend towards karyotype asymmetry. This trend has been carefully studied in the genus Crepis of the family compositae. In several cases it was shown that increased karyotype asymmetry was associated with specialized zygomorphic flowers.