Morgan's findings led to two remarkable conclusions. First, each chromosome is actually a group of linked genes. Second, Mendel's principle of independent assortment still holds true. It is the chromosomes, however, that assort independently, not individual genes. Alleles of different genes tend to be inherited together from one generation to the next when those genes are located on the same chromosome.

How did Mendel manage to miss gene linkage? By luck, or design, several of the genes he studied are on different chromosomes. Others are so far apart that they also assort independently.

Gene Mapping In 1911, a Columbia University student was working part time in Morgan's laThis student, Alfred Sturtevant, wondered if the frequency of crossing-over between genes during meiosis might be a clue to the genes' locations. Sturtevant reasoned that the farther apart two genes were on a chromosome, the more likely it would be that crossing-over would occur between them. If two genes are close together, then crossovers between them should be rare. If two genes are far apart, then crossovers between them should be more common. By this reasoning, he could use the frequency of crossing-over between genes to determine their distances from each other.

Sturtevant gathered up several notebooks of lab data and took them back to his room. The next morning, he presented Morgan with a gene map showing the relative locations of each known gene on one of the Drosophila chromosomes. Sturtevant's method has been used to construct gene maps, like the one in Figure 11–18, ever since this discovery.

11.4 Assessment

Review Key Concepts

1. 1. Review Describe the main results of meiosis.

   2. Calculate In human cells, 2N = 46. How many chromosomes would you expect to find in a sperm cell? How many would you expect to find in an egg cell?

2. 1. Review Write a summary of each phase of meiosis.

   2. Use Analogies Compare the chromosomes of a diploid cell to a collection of shoes in a closet. How are they similar? What would make the shoe collection comparable to the chromosomes of a haploid cell?

3. 1. Review What are the principal differences between mitosis and meiosis?

   2. Apply Concepts Is there any difference between sister chromatids and homologous pairs of chromosomes? Explain.

4. 1. Review How does the principle of independent assortment apply to chromosomes?

   2. Infer If two genes are on the same chromosome but usually assort independently, what does that tell you about how close together they are?

Apply the Big idea

Information and Heredity

5. In asexual reproduction, mitosis occurs but meiosis does not occur. Which type of reproduction—sexual or asexual—results in offspring with greater genetic variation? Explain your answer.

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