The idea that intrachromosomal recombinants were produced by some kind of exchange of material between homologous chromosomes was a compelling one. But experimentation was necessary to test this idea. One of the first steps was to correlate the appearance of a genetic recombinant with an exchange of parts of chromosomes. Several investigators approached this problem in the same way. In 1931, Harriet Creighton and Barbara McClintock were studying two loci of chromosome 9 of corn: one affecting seed color (C, colored; c, colorless) and the other affecting endosperm composition (Wx, waxy; wx, starchy). Furthermore, the chromosome carrying C and Wx was unusual in that it carried a large, densely staining element (called a knob) on the C end and a longer piece of chromosome on the Wx end; thus, the heterozygote was

When they compared the chromosomes of genetic recombinants with those of parental-type progeny, Creighton and McClintock found that all the parental types retained the parental chromosomal arrangements, whereas all the recombinants were

or

Thus, they correlated the genetic and cytological events of intrachromosomal recombination. The chiasmata appeared to be the sites of the exchange, but the final proof of this did not come until 1978.

But what is the mechanism of chromosome exchange in a crossover event? The short answer is that a crossover results from chromosome breakage and reunion. Two parental chromosomes break at the same position, and then join up again in two nonparental combinations. In Chapter 19 we will study models of the molecular processes that allow DNA to break and rejoin in such a precise manner.

You will notice that, in our diagrammatic representations of crossing-over in this chapter, we have shown crossovers taking place at the four-chromatid stage of meiosis. However, just from studying random recombinant products of meiosis, as in a testcross, it is not possible to distinguish this possibility from crossing-over at the twochromosome stage. This matter was settled through the genetic analysis of organisms whose four products of meiosis remain together in groups of four called tetrads. These organisms are mainly fungi and unicellular algae. The meiotic products in a single tetrad can be isolated, which is equivalent to isolating all four chromatids arising from a single meiosis. Tetrad analyses of crosses in which genes are linked clearly show that in many cases tetrads contain four different genotypes with regard to these loci; for example, from the cross

some tetrads contain four genotypes

This result can be explained only by the occurrence of a crossover at the four-chromatid stage because, if crossing over occurred at the two-chromosome stage, then there could be only two different genotypes in an individual meiosis, as shown in Figure 5-20.

Figure 5-20

Tetrad analysis provides evidence that enabled geneticists to decide whether crossing-over occurs at the two-strand (two-chromosome) or at the four-strand (four-chromatid) stage of meiosis. Because more than two different products of a single meiosis (more...)

Tetrad analysis allows the exploration of many other aspects of intrachromosomal recombination, which will be considered in detail in Chapter 6, but for the present let us use tetrads to answer two more fundamental questions about crossing-over. First, can multiple crossovers be between more than two chromatids? To answer this question, we need to look at double crossovers; and, to study double crossovers, we need three linked genes. For example, in a cross such as

there are many different tetrads possible, but some of them can be explained only by double crossovers. Consider the following tetrad as an example:

This tetrad must be explained by two crossovers involving three chromatids, as shown in Figure 5-21. Other types of tetrads show that all four of the chromatids can participate in crossing-over in the same meiosis. Therefore, two, three, or four chromatids can take part in crossing-over events in a single meiosis.

Figure 5-21

One of the several possible types of double-crossover tetrads that are regularly observed. Note that more than two chromatids exchanged parts.

If all chromatids can take part, we can ask if there is any chromatid interference; in other words, does the occurrence of a crossover between any two nonsister chromatids affect the likelihood of those two chromatids taking part in another crossover in the same meiosis? Tetrad analysis can answer this question and shows that generally the distribution of crossovers between chromatids is random; in other words, there is no chromatid interference.

Before we leave the topic of the involvement of chromatids in crossovers, it is worth raising another question: Is it possible for crossing-over to occur between sister chromatids? It has been shown in some organisms that indeed there is sister-chromatid crossing-over; but, because it produces no recombinants and, furthermore, because it is not clear whether it occurs in all organisms, it is conventional not to represent this type of exchange in crossover diagrams.

Crossing-over is a remarkably precise process. The synapsis and exchange of chromosomes is such that no segments are lost or gained, and four complete chromosomes emerge in a tetrad. A great deal has been learned about the nature of the molecular events in and around the sites of crossing-over, and these will be explored in Chapter 19.