Yeast mutants that are defective in their ability to complete cell division have led to the discovery of many genes that control the cell-division cycle—the so-called Cdc genes—and have provided a detailed understanding of how these genes, and the proteins they encode, actually work.

4. When the frequency of recombinant between two genes is between 0 and 1, the relationship of these two genes is incomplete linkage.

When the frequency of recombinant between two genes is 0, the relationship of these two genes is complete non-linkage.

In positive regulation, most of the genes are usually inactive (that is, RNA polymerases do not bind to the promoters) and the cell synthesizes only the activator proteins needed to promote transcription of the subset of genes required in the cell at that time.

Most of the X-lined genes in humans are expressed in equal amounts between males and females because A: only one of the two X chromosomes is active in each cell in the females. B: X-linked genes on the X chromosome in males are expressed two-fold higher than in females. C: only one X chromosome is kept after fertilization. D: Half of the RNA transcripts from X chromosomes in female cells are degraded after transcription.