Gene Linkage, Recombination, and Chromosome Mapping

Linked genes are genes located close together on the same chromosome that tend to be inherited together, recombination is the exchange of segments between homologous chromosomes by crossing over, and the recombination frequency between two genes is used to construct chromosome (linkage) maps. In brief: the recombination frequency is the number of recombinant offspring divided by the total offspring. A frequency of 1 percent corresponds to a map distance of 1 centimorgan (cM). Genes far apart on a chromosome have a recombination frequency approaching 50 percent, the same as for genes on different chromosomes.

Linkage breaks Mendel’s law of independent assortment for genes on the same chromosome and made it possible to map genes before DNA could be sequenced. This guide explains linkage and crossing over, how to recognize linked genes from a test cross, how to calculate recombination frequencies and build a map, with a three-point cross, discusses limits and modern uses, and corrects common mistakes.

What is gene linkage?

Mendel’s law of independent assortment applies to genes on different chromosomes (or very far apart on the same one). Genes located on the same chromosome are physically connected and are inherited together unless crossing over separates them. They form a linkage group. The number of linkage groups in a species equals the number of pairs of chromosomes (haploid number): 23 in humans, 4 in the fruit fly Drosophila, 10 in maize.

The first evidence came from William Bateson and Reginald Punnett in 1905 with sweet peas, when they found that two traits did not give the expected 9:3:3:1 ratio but were inherited together more often than chance. Thomas Hunt Morgan and his students explained this in the fruit fly.

What is genetic recombination?

In prophase I of meiosis, homologous chromosomes pair, and non-sister chromatids exchange segments at points called chiasmata (crossing over). If a crossover occurs between two linked genes, the alleles on the chromatids are combined in new ways.

Take a dihybrid individual with the alleles A B on one chromosome and a b on the other (the “coupling” arrangement). Without crossing over between them, the gametes would be AB and ab only, which are the parental types. Crossing over between the genes produces recombinant gametes, Ab and aB. The closer the genes are, the less likely it is that a crossover will happen between them, and the fewer recombinants are produced. If the genes are far apart, crossing over between them is frequent, and the four gamete types approach equal frequencies (25 percent each), as if the genes were independent.

How is recombination frequency calculated?

The most convenient method is a test cross: the dihybrid (AaBb) is crossed with a homozygous recessive (aabb) individual. Because the tester gives only ab gametes, the phenotype of each offspring directly shows the type of gamete from the dihybrid parent.

Recombination frequency (RF) = (number of recombinant offspring ÷ total number of offspring) × 100 percent

The parental types are the most frequent classes. If the genes are linked, the recombinants always make up less than 50 percent. Map distance is expressed in map units or centimorgans (cM), named after Morgan: 1 map unit = 1 percent recombination. In humans, 1 cM corresponds on average to about 1 million base pairs, and the total length of the genetic map is roughly 3 500 cM, but this varies greatly between regions and between sexes.

How are gene maps constructed?

Alfred Sturtevant, a student of Morgan, realized in 1911 that the recombination frequencies are a measure of distance along the chromosome, and drew the first genetic map, for the X chromosome of Drosophila. The method is to measure the RF for pairs of genes, and to place them in a line so that the distances are consistent:

1. Measure RF for all pairs of three genes (A, B, C).
2. The two genes with the largest distance are at the ends, and the third lies between them.
3. The distances should add up: A–C = A–B + B–C.

Three-point crosses give more information because they allow the gene order to be found in one experiment, and also detect double crossovers, the rarest class. In practice, the sum of short distances is slightly more than the measured RF for long distances, because double crossovers restore the parental combination and are not counted as recombinants. For this reason, RF underestimates the distance when genes are far apart, and the maximum observable RF is 50 percent. Mapping functions correct for this.

Dihybrid test crosses: independent or linked?

Worked example: a three-point map

Part 1: a two-gene test cross

In fruit flies, body color (gray G, black g) and wing length (normal N, vestigial n) are linked. A test cross of GgNn females (with G N on one chromosome and g n on the other) with ggnn males gives 1 000 offspring: 410 gray normal, 390 black vestigial, 105 gray vestigial, 95 black normal.

1. Parental types: gray normal (410) and black vestigial (390): 800 in total. These are the largest classes, which shows that G is linked with N and g with n.
2. Recombinant types: gray vestigial (105) and black normal (95): 200 in total.
3. RF: 200 ÷ 1 000 × 100 = 20 percent, so the genes are 20 map units (20 cM) apart.

Part 2: finding the order of three genes

For three genes, A, B and C, the RF values from separate test crosses are: A–B = 10 percent, B–C = 15 percent and A–C = 25 percent.

1. The largest value (A–C = 25) shows that A and C are the end genes.
2. B lies between them. Check the additivity: A–B + B–C = 10 + 15 = 25, which equals A–C. ✓
3. The map reads: A —10— B —15— C, over a total of 25 map units.

The additivity check is useful. If the measured A–C distance had been only 20 instead of 25, the three values could not be placed on a line without a mismatch, which would suggest an experimental error, or that double crossovers had hidden some recombinants between the distant genes A and C. Short distances are more reliable than long ones for this reason. In real studies the gene order is confirmed with a three-point cross, in which the rarest offspring class (the double crossovers) shows which gene lies in the middle: it is the one whose allele combination differs from the parental type in that class.

What are the limits of mapping?

• Crossing over is not evenly distributed along chromosomes. There are hot spots and cold spots, such as near the centromere, so genetic distance does not correspond exactly to physical distance.
• Rates of recombination differ between the sexes: in humans, females have about 1.6 times as much recombination as males, and in male fruit flies, there is no crossing over.
• Interference: one crossover reduces the chance of another close by, so double crossovers are less frequent than expected.
• Modern genetic mapping uses DNA markers (SNPs, STRs), and these are combined with genome sequences, so that genetic and physical maps can be compared.

Linkage analysis is used in humans to find the location of disease genes by tracking markers in families through generations, as it was used to find the genes for Huntington’s disease (1983, chromosome 4) and cystic fibrosis (1989, chromosome 7). Linkage disequilibrium and genome-wide association studies extend the approach to populations.

How was mapping developed?

Bateson and Punnett found linkage in 1905. Morgan’s group at Columbia University studied Drosophila from 1910, and Sturtevant drew the first map in 1911 at the age of 19 or 20, one evening in his dormitory room, according to a well-known story. Morgan received the Nobel Prize in Physiology or Medicine in 1933. Barbara McClintock and Harriet Creighton demonstrated in 1931 in maize that genetic recombination corresponds to the physical exchange of chromosome parts.

Which mistakes are common?

Mistake 1: Thinking that linked genes are never separated

Crossing over can separate them. The closer they are, the more rarely they are separated.

Mistake 2: Counting the wrong classes as recombinants

The parental types are the largest classes in the offspring. The smaller classes are recombinants.

Mistake 3: Using recombination frequencies greater than 50 percent

The maximum is 50 percent. Higher values would mean that the parental and recombinant labels were swapped.

Mistake 4: Assuming that map units are physical distances

Map units reflect the frequency of crossing over, which varies along the chromosome.