BIOLOGY Volume 3 - A Guide to General Biology - 2004

24. VARIATION AND GENETICS

24.4. Genetic Maps

Data on recombination frequency are primarily important because they enable geneticists to map the relative positions of genes on Chromosomes. Chromosome maps are constructed by directly converting crossover frequencies into estimated map distances along a chromosome. A crossover frequency of 4% between genes A and B means that these genes are located on the same chromosome at a distance of 4 genetic units (map units or centiMorgans). If the crossover frequency between genes A and C is 9%, they are separated by a distance of 9 map units. However, these data tell us nothing about the linear sequence of genes A, B, and C (Fig. 24.12).

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Fig. 24.12. Determining the relative positions of Gene loci A, B, and C based on the data presented in the text.

In practice, crossover frequencies are usually determined for at least three genes simultaneously; this method, known as triangulation, makes it possible to determine not only the distance between genes but also their sequence. Consider, for example, the crossover frequencies established through a series of experimental crosses involving four genes P, Q, R, and S:

P — Q = 24%

R — P = 14%

R — S = 8%

S — P = 6%

To establish the gene sequence and the distances between them, a line representing the chromosome is drawn and the following steps are performed.

1. Place the genes with the lowest recombination frequency, i.e., S - P = 6%, in the middle of the chromosome (Fig. 24.13, I).

2. Select the next largest recombination frequency, i.e., R - S = 8%, and indicate the two possible positions of R on the chromosome relative to S (Fig. 24.13, II).

3. Do the same with the next recombination frequency, i.e., R - P = 14% (Fig. 24.13, III). This reveals that R cannot be located to the right of P.

4. Do the same with P - Q = 24% (Fig. 24.13, IV). THE POSITION OF Q cannot be established without additional information. If, for example, it turns out that the recombination frequency for Q - R = 10%, this confirms the Location of gene Q at the left end of the chromosome.

Fig. 24.13. Using the triangulation method to map the positions of genes P, Q, R, and S on a chromosome.

Difficulties can arise in chromosome mapping due to double crossovers; this is especially true when the genes under study are separated by large distances, since the number of detected recombinants in such cases is lower than the actual number. If, for example, Crossing-over occurs at two points — between A and B and between B and C (Fig. 24.14) — A and C will phenotypically appear as linked genes, but the chromosome will now carry the recessive allele b.

Fig. 24.14. A. A pair of homologous chromatids, one carrying dominant alleles A, B, and C, and the other carrying recessive alleles a, b, and c. Crossing-over occurs at two points — *1 and *2. B. The result of chromatid Separation: their allele sequences are altered, although The sequence of gene loci and the distances between them remain the same.

24.9. In maize, the genes for colored seed and smooth endosperm are dominant over those for colorless seed and wrinkled endosperm. A pure-breeding line homozygous for both dominant traits was crossed with a line homozygous for both recessive traits. When F₁ plants were test-crossed, the following results were obtained:

colored, smooth seeds 380

colorless, wrinkled seeds 396

colored, wrinkled seeds 14

uncolored, smooth seeds 10

Calculate the map distance in centimorgans on the chromosome between the genes that determine seed color and endosperm Morphology.