Thomas Hunt Morgan, Genetic Recombination, and Gene Mapping

In 1911, while studying the chromosome theory of heredity, biologist Thomas Hunt Morgan had a major breakthrough. Morgan occasionally noticed that "linked" traits would separate. Meanwhile, other traits on the same chromosome showed little detectable linkage. Morgan considered the evidence and proposed that a process of crossing over, or recombination, might explain his results. Specifically, he proposed that the two paired chromosomes could "cross over" to exchange information. Today, we know that recombination does indeed occur during prophase of meiosis (Figure 1), and it creates different combinations of alleles in the gametes that result (i.e., the F₁ generation; Figure 2).

Figure 1: Recombination and gamete production.

A comparison of nonrecombination (left) with recombination (right), shows how recombination affects the way chromosomes are passed into gametes in Meiosis II. On the right, a single crossover event produces half nonrecombinant gametes and half recombinant gametes.

© 2014 Nature Education Adapted from Pierce, Benjamin. Genetics: A Conceptual Approach, 2nd ed. All rights reserved.

Figure Detail

When proposing the idea of crossing over, Morgan also hypothesized that the frequency of recombination was related to the distance between the genes on a chromosome, and that the interchange of genetic information broke the linkage between genes. Morgan imagined that genes on chromosomes were similar to pearls on a string (Weiner, 1999); in other words, they were physical objects. The closer two genes were to one another on a chromosome, the greater their chance of being inherited together. In contrast, genes located farther away from one another on the same chromosome were more likely to be separated during recombination. Therefore, Morgan correctly proposed that the strength of linkage between two genes depends upon the distance between the genes on the chromosome. This proposition became the basis for construction of the earliest maps of the human genome.

Figure 2: Allele recombination.

Recombination is the sorting of alleles into new combinations. Following the formation of gametes over two generations shows how recombination can produce new allelic combinations (lower right) or stay the same (lower left).

© 2014 Nature Education Adapted from Pierce, Benjamin. Genetics: A Conceptual Approach, 2nd ed. All rights reserved.

Figure Detail

Soon after Morgan presented his hypothesis, Alfred Henry Sturtevant, a 19-year-old Columbia University undergraduate who was working with Morgan, realized that if the frequency of crossing over was related to distance, one could use this information to map out the genes on a chromosome. After all, the farther apart two genes were on a chromosome, the more likely it was that these genes would separate during recombination. Therefore, as Sturtevant explained it, the "proportion of crossovers could be used as an index of the distance between any two factors" (Sturtevant, 1913). Collecting a stack of laboratory data, Sturtevant went home and spent most of the night drawing the first chromosomal linkage map for the genes located on the X chromosome of fruit flies (Weiner, 1999).

Figure 3: Sturtevant's Drosophila gene map.

In Sturtevant's gene map, six traits are arranged along a linear chromosome according to the relative distance of each from trait B. Traits include yellow body (B), white eyes (C, O), Vermillion eyes (P), miniature wings (R), and rudimentary wings (M).

© 2013 Nature Education Adapted from Pierce, Benjamin. Genetics: A Conceptual Approach, 2nd ed. All rights reserved.

When creating his map, Sturtevant started by placing six X-linked genes in order. B was a gene for black body color. C was a gene that allowed color to appear in the eyes. Flies with the P gene had vermilion eyes instead of the ordinary red, and flies with two copies of the recessive O gene had eyes that appeared a shade known as eosin. The R and M factors both affected the wings. Sturtevant placed C and O at the same point because they were completely linked and were always inherited together — in other words, he never saw any evidence for recombination between C and O. Sturtevant then placed the remainder of the genes in the order shown in Figure 3 (Sturtevant, 1913). Crossover events were tracked by examining the F₂ progeny in crosses for "new" phenotypes.

Figure 4: Phenotypes used in Sturtevant's cross.

Sturtevant crossed flies with long wings (M) and vermillion eyes (p) with flies with rudimentary wings (m) and red eyes (P). These traits are X-linked.

© 2008 Nature Education All rights reserved.

For example, to find the distance between P (vermilion eyes) and M (long wings), Sturtevant performed crosses between flies that had long wings and vermilion eyes and flies that had small wings and red eyes. These crosses resulted in F₁ flies that either had long wings and red eyes or long wings and vermilion eyes. Sturtevant then crossed these two types of F₁ flies and analyzed the offspring for evidence of recombination. Unexpected phenotypes observed in the male F₂ progeny from this cross were then examined. (Because very little recombination occurs in the male germ line of Drosophila, only the female F₁ chromosomes are considered for predicting phenotypes [Figure 4].) Sturtevant noted four classes of male flies in this F₂ generation, as shown in Table 1.

The two additional classes of flies that appeared in this generation (long wings with red eyes and rudimentary wings with vermilion eyes) could only be explained by recombination occurring in the female germ line.

Sturtevant then worked out the order and the linear distances between these linked genes, thus forming a linkage map. In doing so, he computed the distance in an arbitrary unit he called the "map unit," which represented a recombination frequency of 0.01, or 1%. Later, the map unit was renamed the centimorgan (cM), in honor of Thomas Hunt Morgan, and it is still used today as the unit of measurement of distances along chromosomes.

In addition to describing the order of the genes on the X chromosome of fruit flies, Sturtevant's 1913 paper elucidated a number of other interesting points, including the following:

• The relationship between crossing over and genetic map distance
• The effects of multiple crossover events
• The fact that a first crossover can inhibit a second crossover (a phenomenon called interference, which is described later in this article)

When Sturtevant drew his chromosomal map, he placed the C and O genes at the same location because they were always inherited together (Figure 3; Sturtevant, 1913). Genes that are so close together on a chromosome that they are always inherited as a single unit show a relationship referred to as complete linkage. In fact, two genes that are completely linked can only be differentiated as separate genes when a mutation occurs in one of them. There is no other way to identify genes with complete linkage from single genes that show multiple phenotypes.

On the other hand, the phenomenon known as incomplete linkage occurs when two genes show linkage with a recombination level greater than 0% and less than 50%. In incomplete linkage, all expected types of gametes are formed, but the recombinant gametes occur less often than the parental gametes.

In addition, if two genes are on the same chromosome and are far enough apart that they undergo recombination at least 50% of the time, the genes are independently assorting and do not show linkage. Genes independently assort at a distance of 50 cM or more apart. This means that no statistical test would allow researchers to measure linkage.

Finally, linked genes that do not independently assort show statistical linkage. Statistical linkage is detected as deviation from independent assortment that favors the parental gametes. Syntenic genes are genes that are physically located on the same chromosome, whether or not the genes themselves exhibit linkage (Passarge et al., 1999). Therefore, all linked genes are syntenic, but not all syntenic genes show genetic linkage.