Chromosomal Theory of Inheritance
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Chromosomal Theory of Inheritance Proposed by Sutton and Boveri in 1902. It states that Mendelian principles (segregation and independent assortment) can be explained by the behavior of chromosomes during meiosis. Rediscovery of Mendel's Work Mendel published his work in 1865, but it remained unrecognized until 1900 due to: Lack of communication and widespread acceptance. His concept of 'factors' (genes) as discrete units, not blending, was not accepted. Mathematical approach was new and difficult to understand for biologists. He couldn't provide physical proof for the existence of factors. In 1900, De Vries, Correns, and von Tschermak independently rediscovered Mendel's results. Key Developments Leading to the Theory Advancements in microscopy allowed scientists to observe cell division. Observation of the nucleus and structures within it that appeared to double and divide just before cell division. These structures were called chromosomes . Walter Sutton and Theodore Boveri noted that the behavior of chromosomes paralleled the behavior of genes. Parallels between Genes and Chromosomes Genes (Mendel's Factors) Chromosomes Occur in pairs. Occur in pairs. Segregate at the time of gamete formation such that only one of each pair is transmitted to a gamete. Segregate at gamete formation such that only one of each pair is transmitted to a gamete. Independent pairs segregate independently of each other. One pair segregates independently of another pair. The two alleles of a gene pair are located on homologous sites on homologous chromosomes. Validation of Chromosomal Theory T.H. Morgan and his colleagues provided experimental verification using Drosophila melanogaster (fruit fly) . Why Drosophila ? Can be grown on simple synthetic medium in the laboratory. Short life cycle (about two weeks). Single mating produces a large number of progeny flies. Clear differentiation of sexes (male and female). Many types of hereditary variations visible with low power microscopes. Meiosis and Germ Cell Formation Chromosomes segregate during meiosis to form germ cells (gametes). Meiosis I: Homologous chromosomes separate. Anaphase I: Bivalents separate. Meiosis II: Sister chromatids separate (similar to mitosis). Anaphase II: Sister chromatids separate. Result: Four haploid germ cells, each with half the number of chromosomes of the parent cell. This is a reductional division . Independent Assortment of Chromosomes During Anaphase I, the two chromosome pairs can align independently at the metaphase plate. Possibility I: Long orange and short green chromosomes go to one pole; long yellow and short red chromosomes go to the other. Possibility II: Long orange and short red chromosomes go to one pole; long yellow and short green chromosomes go to the other. This independent assortment of chromosomes explains Mendel's Law of Independent Assortment for genes.
