THE CHROMOSOMAL THEROY OF INHERITANCE

  In the year 1903, W.S. Sutton and T. Boveri independently suggested that the cell nuclei contain specifically stained thread like chromosomes. These are visible only at specific stages of cell cycle associated with cell division (see biology Chapter 2 fig. B.2.1). each cell was found to have a characteristic number of chromosomes (diploid) with distinct size and shape, called Karyotype. Karyotype is similar for all the cells of an organism, except the gamete contain half the number of usual chromosome number (haploid)

Inheritance of coat color in rabbits. Different combinations of two alleles give different coat colors. The dominance hierarchy is C>cch>ch>ch>c.

Genotypes	CC,Ccch, CC	Cch cch	Cch ch, cch c	Chch,chc	Cc
Phenotypes	Dark gray	Chinchilla	Light gray	Point restricted	Albino

The correlation between this reduction in the chromosome number to half (haploid) in the gametes, and the Mendel’s prediction of allelic reduction in gametes led Sutton and Boveri to propose that genes are carried by chromosomes. This is the basis of the chromosomal theory of inheritance. The variable range of the chromosome numbers from I in Bacteria to about 200 in some higher eukaryotes. Over the same range of organisms, the numbers of genes vary from about 10³ to 10⁵

In 1910 T.H Morgan, studying the fruit fly Drosophila melanogaster gave the evidence that gene resides on the chromosomes. He also demonstrated that the segregation of the X chromosome, now called X-linked inheritance.

  Linkage and Crossing Over

Individual chromosomes may carry hundreds or even thousands of genes. If two genes are located relatively close together on the same pair of chromosome, they generally will not segregate independently. Such genes, which are inherited together, called Linked genes. The linkage of genes was first discovered by W. Bateson in 1905, in sweet pea (Lathyrus odoratus). However sometimes there is exchange between homologous chromosomes during crossing over. The greater the distance between two genes on a chromosome greater is the chance that a crossing over will take place between them. Chromosome maps can be constructed based on the frequency of crossing over between genes. Such genetic, or linkage maps provide approximation of the position of the genes on the chromosome.

    Cytoplasm organelles such as plastids and mitochondria contain small numbers of genes. In many organisms, cytoplasmic genes are inherited only from the mother because male gametes contribute only their nucleus (i.e. no cytoplasm) to the zygote follow Mendel’s laws in inheritance. This type of inheritance is called non-Mendelian inheritance or cytoplasmic inheritance