Theory of Inheritance

Waldeyer is credited with the discovery of chromosomes in the year 1888 within the nucleus. He observed them as darkly stained bodies. However, it was the American William Sutton who noticed the similarities between the behaviour of the mendelian factors or genes and that of the chromosomes during meiosis. He noticed the following similarities:

• Both occurred in pairs - genes as alleles and chromosomes as homologous chromosomes.

• Both separated from the pairs and entered the gametes independently - each gamete received only one of the alleles of a pair and also received only one of the homologous chromosomes during meiosis.

• The pairs of both are restored during fertilization - the zygotes receive one allele each from the two parents through the gametes and the zygotes also receive the two homologous chromosomes, one from each parent, during fertilization.

• The above observations led Sutton and Boveri to formulate the Chromosomal theory of inheritance, independently in 1902. According to this theory,

• Each adult organism grows from a zygote. The zygote is a diploid cell having two sets of chromosomes - maternal and paternal. The zygote by mitotic divisions results in the formation of an adult and thus, all the vegetative or somatic cells of an organism are diploid with two sets of chromosomes.

• The chromosomes maintain their individuality through out the life cycle of the organism.

• The chromosomes contain the Mendelian factors or genes, which determine the various characteristics of an individual.

• It was later discovered that the genes are arranged in a linear manner on the chromosomes.

The position of a gene on the chromosome is called the locus. It always remains the same for a gene. The alleles of a gene are present on the homologous chromosomes at the same loci.