B

produces only the nonrecombinant gametes containing alleles AB or ab. The alleles do not assort into new combinations such as Ab or aB.

The third situation, incomplete linkage, is intermediate between the two extremes of independent assortment and complete linkage. Here, the genes are physically linked on the same chromosome, which prevents independent assortment. However, occasional crossovers break up the linkage and allow them to recombine. With incomplete linkage, an individual heterozygous at two loci produces four types of gametes—two types of recombinants and two types ofnonrecombinants—but the nonrecombinants are produced more frequently than the recombinants because crossing over does not take place in every meiosis. Linkage and crossing over are two opposing forces: linkage binds alleles at different loci together, restricting their ability to associate freely, whereas crossing over breaks the linkage and allows alleles to assort into new combinations.

Earlier in the chapter, the term recombination was defined as the sorting of alleles into new combinations. We can now distinguish between two types of recombination that differ in the mechanism that generates these new combinations of alleles.

Interchromosomal recombination is between genes on different chromosomes. It arises from independent assortment—the random segregation of chromosomes in anaphase I of meiosis. Intrachromosomal recombination is between genes located on the same chromosome. It arises from crossing over—the exchange of genetic material in prophase I of meiosis. Both types of recombination produce new allele combinations in the gametes; so they cannot be distinguished by examining the types of gametes produced. Nevertheless, they can often be distinguished by the frequencies of types of gametes: interchromosomal recombination produces 50% nonrecombinant gametes and 50% recombinant gametes, whereas intrachromosomal recombination frequently produces less than 50% recombinant gametes. However, when the genes are very far apart on the same chromosome, intrachromosomal recombination also produces 50% recombinant gametes. The two mechanisms are then genetically indistinguishable.

Concepts

Recombination is the sorting of alleles into new combinations. Interchromosomal recombination, produced by independent assortment, is the sorting of alleles on different chromosomes into new combinations. Intrachromosomal recombination, produced by crossing over, is the sorting of alleles on the same chromosome into new combinations.

The Physical Basis of Recombination

William Sutton's chromosome theory of inheritance, which stated that genes are physically located on chromosomes, was supported by Nettie Stevens and Edmund Wilson's discovery that sex was associated with a specific chromosome in insects (p. 000 in Chapter 4) and Calvin Bridges' demonstration that nondisjunction of X chromosomes was related to the inheritance of eye color in Drosophila (p. 000 in Chapter 4). Further evidence for the chromosome theory of heredity came in 1931, when Harriet Creighton and Barbara McClintock ( FIGURE 7.9) obtained evidence that intrachro-mosomal recombination was the result of physical exchange between chromosomes. Creighton and McClintock discovered a strain of corn that had an abnormal chromosome 9, containing a densely staining knob at one end and a small piece of another chromosome attached to the other end. This aberrant chromosome allowed them to visually distinguish the two members of a homologous pair.

They studied the inheritance of two traits in corn determined by genes on chromosome 9: at one locus, a dom

I 7.9 Barbara McClintock (left) and Harriet Creighton (right) provided evidence that genes are located on chromosomes. (Karl Maramorosch/Cold Spring Harbor Laboratory Archives.)

inant allele (C) produced colored kernels, whereas a recessive allele (c) produced colorless kernels; at another, linked locus, a dominant allele (Wx) produced starchy kernels, whereas a recessive allele (wx) produced waxy kernels. Creighton and McClintock obtained a plant that was heterozygous at both loci in repulsion, with the alleles for colored and waxy on the aberrant chromosome and the alleles for colorless and starchy on a normal chromosome:

Knob Extra piece

They crossed this heterozygous plant with a plant that was homozygous for colorless and heterozygous for waxy:

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