Chromosome Theory Of Inheritance

Mendel completed his results in 1866, American genetist Karl Correns rediscovered Mendel's work in 1900 and suggested the central role for chromosome in heredity. In 1902 Walters S. Sutton noticed the similarities between the behavior of chromosomes during the formation of gametes and fertilization of Mendel's hereditary factor. W.S. Sutton of USA and Theodor Boveri of Germany suggested independently that the chromosomes were the carrier of Mendel's hereditary factors, the chromosome theory of inheritance.

The chromosome theory states that genes are located on chromosomes and that the behavior of chromosomes during meiosis and fertilization accounts for inheritance patterns. The chromosomes undergo segregation and independent assortment during meiosis and thus, account for Mendel's principles.

We can see the chromosomal basis of Mendel's principles by following the fate of two genes during meiosis and fertilization in plants. In the figure N0.20.6 pea the genes for seed shape (alleles R and r) and seed color (alleles Y and y) are shown as black bars on different chromosomes. We start with the F1 generation, in which all individuals have the RrYy genotype, to simplify the diagram, we show only two of the seven pairs of pea chromosomes and three of the stages of meiosis: metaphase I, anaphase I, and metaphase II.

Chromosome Theory of Inheritance

To see the chromosomal basis of the principle of segregation, let's follow just the pair of long chromosomes, the ones carrying R and r, taking either the left or the right branch from the F1 cell. Whichever arrangement the chromosomes assume at metaphase I, the two alleles segregate as the homologous chromosomes, separate in anaphase I. And at the end of meiosis II, a single long chromosome ends up in eaGh of the gametes. Random fertilization then leads to F2 offspring with the 3:1 (12 round to 4 wrinkled) ratio of phenotypes that Mendel observed.

To see the chromosomal basis of the principle of independent assortment, follow both the long and the short chromosomes from metaphase I in the F1 generation. Two alternatives, equally likely arrangements of tetrad can occur at this stage of meiosis. The non-homologous chromosomes and the genes they carry assort independently, forming gametes of four genotypes. Random fertilization leads to the 9:3:3:1 phenotypic ratio in the F2 generation.

Note: The genes are on the chromosomes; therefore, they behave similarly during meiosis and fertilization. All the genes on one chromosomes form a linkage group that tends to stay together, except when crossing-over occurs.

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