Chapter 5

In this chapter, we focused on linkage, chromosomal mapping, and many associated phenomena. In the process, we found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, what answers would you propose to the following fundamental questions? (a) How was it established experimentally that the frequency of recombination (crossing over) between two genes is related to the distance between them along the chromosome? (b) How do we know that specific genes are linked on a single chromosome, in contrast to being located on separate chromosomes? (c) How do we know that crossing over results from a physi- cal exchange between chromatids? (d) How do we know that sister chromatids undergo recombination during mitosis? (e) When designed matings cannot be conducted in an organism (for example, in humans), how do we learn that genes are linked, and how do we map them?

Review the Chapter Concepts list on page $138.$ Most of these center around the process of crossing over between linked genes. Write a short essay that discusses how crossing over can be detected and how the resultant data provide the basis of chromosome mapping.

Describe the cytological observation that suggests that crossing over occurs during the first meiotic prophase.

Why does more crossing over occur between two distantly linked genes than between two genes that are very close together on the same chromosome?

Why does more crossing over occur between two distantly linked genes than between two genes that are very close together on the same chromosome?

Why are double-crossover events expected less frequently than single-crossover events?

What two essential criteria must be met in order to execute a successful mapping cross?

The genes dumpy$(dp),$ clot $(cl),$ and apterous $(ap)$ are linked on chromosome II of Drosophila. In a series of two-point mapping crosses, the following genetic distances were determined. What is the sequence of the three genes?

Colored aleurone in the kernels of corn is due to the dominant allele $R$. The recessive allele $r,$ when homozygous, produces colorless aleurone. The plant color (not the kernel color) is controlled by another gene with two alleles, $Y$ and $y$. The dominant $Y$ allele results in green color, whereas the homozygous presence of the recessive $y$ allele causes the plant to appear yellow. In a testcross between a plant of unknown genotype and phenotype and a plant that is homozygous recessive for both traits, the following progeny were obtained: $$\begin{array}{lc}\text{ colored, green }& 88\\ \text{ colored, yellow }& 12\\ \text{ colorless, green }& 8\\ \text{ colorless, yellow }& 92\end{array}$$ Explain how these results were obtained by determining the exact genotype and phenotype of the unknown plant, including the precise arrangement of the alleles on the homologs.

Phenotypically wild ${\mathrm{F}}_1$ female Drosophila, whose mothers had light eyes $(lt)$ and fathers had straw $(stw)$ bristles, produced the following offspring when crossed with homozygous lightstraw males: $$\begin{array}{lc}\text{ Phenotype }& \text{ Number }\\ \text{ light-straw }& 22\\ \text{ wild }& 18\\ \text{ light }& 990\\ \text{ straw }& 970\\ \text{ Total }& 2000\end{array}$$ Compute the map distance between the light and straw loci.