Answers to Review Questions

1. Studies in cytogenetics suggested that Mendel's "factors", or as we now know them, the genes, were located on the chromosomes. The chromosomes occur in pairs as do the genes. The existence of the sex chromosomes also provided some evidence that the genes were located on the chromosomes. Males presumably had to have some genes different from females to explain the physical differences between the sexes. This presumed difference in genes correlated with an observed difference in chromosomes while no other cell feature differed in any obviously observable way. The behavior of chromosomes during mitosis and especially meiosis mirrored the predicted behavior of the genes during those processes. For example, the equal contribution of chromosomes by sperm and egg to the fertilized egg while the amount of cytoplasm differed greatly suggested that the genes resided on the chromosomes. Genetic crosses that displayed linkage phenomena, an exception to Mendel's Law of Independent Assortment, convinced some investigators that different genes could travel together on the same chromosome. Analysis of these crosses allowed the relative positions of genes on chromosomes to be mapped. The linkage groups that were thus defined soon were correlated with particular chromosomes. The strange results (sex linkage) obtained with such traits as eye color in Drosophila and hemophilia in humans could be explained by locating them on the X chromosome. Crossovers of chromosomes during meiosis were eventually correlated with the unexpected inheritance patterns that were being discovered. Exchanges of pieces of chromosomes could be correlated with changes in inheritance. A more recent piece of evidence was the discovery of moving pieces of DNA called transposons whose movement had an effect on the expression of genetic traits.

3. In animal cells, a cleavage furrow forms around the periphery of the dividing cell. The furrow becomes progressively deeper until it pinches off the cell in the middle and divides its contents into two cells. Plant cells undergo cytokinesis by the gathering of membranous vesicles at the cell equator. These vesicles contain the materials that will form the cell wall. They fuse to form cell plate vesicles, which then fuse with the cell membrane dividing the cell into two separate cells. The presence of the rigid cell wall in plants would not allow the type of cytokinesis seen in animal cells.

5. A gene's locus is the specific position that gene occupies on the chromosomes. The gene's locus will not normally change after mitosis or meiosis. If a piece of a chromosome is moved by accident to another chromosome, its locus will change. Such a change in position is called a translocation. Genes may also be deleted if a piece of a chromosome is removed or separated from the rest of the chromosome.

7. A diploid cell is a cell that possesses both members of each homologous pair. A haploid cell possesses only one member of each homologous pair. All human germ cells (sperm and egg) are haploid. All other human cells (somatic cells) are diploid.

9. There are 22 pairs of autosomal (non-sex) chromosomes in humans plus the X and Y chromosomes. Each pair of autosomal chromosomes would represent a linkage group since each pair carries a different complement of genes. For the same reason, the X and Y chromosomes would each be considered to be a separate linkage group. Thus, there are 24 human linkage groups.

11. Initially, protein seemed to be the best candidate for the genetic material. It is the most complex, varied, and ubiquitous substance in living cells. It also plays many roles in cells and is present in chromosomes. All of these things seemed to suggest that it might be the genetic material. Griffith's experiments proved that one could alter the genetic constitution of bacteria with some component extracted from other bacteria, a process he called transformation. He called this component the transforming principle. He did not, however, prove what that transforming principle actually does. That was accomplished some years later by Avery, MacLeod, and McCarty.

13. Hershey and Chase used radioactive isotopes to label DNA and proteins. 32P was used to label DNA since proteins do not normally contain phosphorus; 35S was used to label proteins because DNA does not normally contain sulfur.