Parallel Behavior and the Chromosome Theory

The successful completion of mitosis or meiosis requires the cell to move large objects with precision and control many detailed events. The process surpasses any engineering accomplishments of NASA. The importance of mitosis and meiosis to an organism is obvious when we consider that genes are a part of the chromosome and the genes must be copied and distributed properly to produce viable daughter cells. The mechanisms of these events are far from being completely understood. From our current understanding we can appreciate how the principles of segregation and independent assortment are controlled by the mechanics of meiosis.

When cell division was first observed and described by cytogeneticists, biologists were just beginning to accept the idea that genes were tiny objects that controlled traits and existed in the cells of living things. Two biologists, a German named Boveri and an American graduate student named Sutton, recognized that chromosome behavior during meiosis matched Mendel’s principles of gene behavior. Both scientists proposed the idea that while genes had not yet been directly observed, they must be a part of the chromosome. Sutton and Boveri are both given credit for proposing this chromosome theory; genes are a part of chromosomes.

Segregation predicts gene behavior that matches the chromosome behavior observed by cytogeneticists. Genes are in pairs because chromosomes are in pairs. The gene pairs associate during gamete formation when the homologous chromosomes pair in prophase I. The gene pairs separate when the homologous pairs separate in anaphase I followed by chromatid separation in anaphase II. Thus chromosome behavior dictates the gene’s segregation behavior.

Independent assortment of gene pairs also correlates with chromosome behavior. Lets consider our dihybrid individual with the genotype BbEe (Fig. 15). How many kinds of gametes can it make? The four possible combinations (BE, bE, Be and be) are made at equal frequencies. We can understand why this occurs if we think about what happens to chromosomes at metaphase I of meiosis. The chromosome pair that carries the ’E’ and ’e’ genes will be moved independently from the chromosome pair with the ’B’ and ’b’ chromosomes. When the chromosomes arrive at the center of the cell at metaphase I, the two tetrads may be aligned so that the ’E’ and ’B’ genes move to one cell and the ’e’ and ’b’ genes move to the other during the first division. In the other half of the cells that go through meiosis, the tetrads will line up so that the ’E’ genes will be passed on with the ’b’ genes and the ’e’ genes will go with the ’B’ genes. Keep in mind that organisms that make gametes make thousands of them. The chance alignment of the tetrads at metaphase I therefore dictates the overall frequency of gametes with different combinations of genes when the genes are on separate chromosomes.

Fig. 15: Alignment of chromosomes during metaphase I will determine the gene combinations that segregate into gametes. (Image by D. Lee)

When more gene pairs are considered, the same scenarios described above will be true as long as the genes are on separate chromosomes. When genes are on the same chromosome, the role of crossing over on gene inheritance needs to be considered in more detail. We will cover the inheritance of genes on the same chromosome in a later lesson.