Chromosomal Theory of Inheritance

Chromosomal basis of inheritance

The chromosome basis of inheritance was put forth by Sutton and Boveri independently in the year 1902. W.S. Sutton and Theodor Boveri faced and solved the problem of drawing a parallel between chromosomes and genes. Both had concluded that the genes are contained in chromosomes. Allelic genes present in a heterozygote segregate independently because the chromosomes carrying these genes segregate when the sex cells are formed. This conclusion of Sutton and Boveri was verified extensively by further studies conducted by various geneticists and cytologists
In order to accept this conclusion we must be able to understand the behaviour of chromosomes in the light of Mendel's assumption.
i) Individuality of Chromosomes :Every organisms has a fixed number of chromosomes. The nuclei of gametes contain haploid (n) and those of zygotes have double the number or diploid (2n) number of chromosomes.
ii) Meiosis : At the time of meiosis, for the formation of gametes, the pairs of chromosomes of the diploid sets undergo pairing.
iii) The chromosomes of each pair segregate independently of every other pair during their distribution into gametes. This is similar to Mendel's law of independent assortment in the segregation of factors.
iv) During the fusion of haploid gametes, the homologous chromosomes from two parents are brought together to form the diploid zygote. Accordingly Mendel had maintained that maternal and paternal characters mix up in the progeny.
v) The chromosomes maintain the structure and uniqueness during the life time of the individual whether observable or not. Mendel had also demonstrated that the characters are never lost though they are not expressed in a particular generation.
From these points it is evident that a clear parallelism exists between Mendel's factors and chromosomes and so there is a firm basis for Mendel's Laws of heredity in the behaviour of chromosomes during meiosis and fertilisation and therefore the Chromosomal Theory of Inheritance has been proposed.

Postulates of the Chromosomal Theory of Inheritance

i. The factors described by Mendel are the genes which are the actual physical units of heredity.
ii. The genes are present on chromosomes in a linear order.
iii. Each organism has a fixed number of chromosomes which occur in two sets referred to as diploid (2n). A pair of similar chromosomes constitute the homologous pair.
iv. Of this, one set is received from the male parent (paternal) and the other from the female parent (maternal).
v. The maternal and paternal chromosomes are contributed by the egg and sperm respectively during zygote formation. But only sperm nucleus is involved proving that chromosomes are present within the nucleus.
vi. The chromosomes and therefore the genes segregate and assort independently at the time of gamete formation as explained in Mendel's law of segregation and Law of Independent Assortment.