Principles of Genetics and Breeding

7.1.4 Chromosome mapping

In 1913, Sturtevant,constructed first chromosome map showing the position of 5 genes on the X chromosome of Drosophila.

• Genetic maps of chromosomes are also known as chromosome maps. The percentage of crossing-over is directly proportional to the distance between the genes concerned.

Gene Mapping

The development of somatic cell hybridization methods provided a major impetus for determining linkage relationships of loci phenotypically expressed by somatic cells in culture.

• Comparative mapping within the primates has revealed an extremely strong homology between ape and human gene maps.

Development of gene mapping methods and their application to fish species

• Gene mapping refers to the analysis of loci on the genome revealing the linear order of different genes on the chromosomes. There are two types of gene maps.

The vast majority of mapped genes can be assigned to one of four classes:

Genes producing inherited diseases or defects.

Genes producing “mutant” phenotypes, though not necessarily affecting viability.

Genes producing distinctive colour patterns, and

Biochemical loci, coding for enzyme and other proteins.

In the first three classes, establishment of homology between species is usually difficult. Even within a single genus of fishes, Xiphophorus (platy fishes and sword tails of the family Poecilidae), very similar patterns of lateral black spots comprised of macromelanophore cells are coded by quite different, almost certainly non-homologous genes.

In poecilid fishes a few species characterized by unusual diversity of pigment pattern polymorphisms received early genetic study resulting in considerable linkage data.

• In the guppy, Poecilia reticulata at least eight autosomal loci have been identified as well as 10 sex-linked loci expressed either as colour patterns or variations in fin shape. An even large number of colour pattern loci, both sex-linked and autosomal, have been described, and in many cases carefully studied, in species of the genus Xiphophorus. These loci control black pigment patterns comprised of macromelanophore or micromelanophore pigment cells and red and yellow patterns comprised of xanthoerythropores.

Methods available for assigning genes to chromosomes or linkage groups
1.Genetic crosses are utilized to generate an individual that is heterozygous for two or more loci of interest. This is the classical method of detecting linkage between two loci.

• The most efficient cross for detection of linkage is the backcross, where a doubly heterozygous individual is crossed to a double homozygote for two loci (a double recessive where dominance is involved).
• A significant excess of parental-type individuals in the backcross progeny constitutes evidence for linkage;
• the frequency of nonparental types provides an estimate of recombination between the loci that can often be related to the actual separation distance on a chromosome.
• The strengths of the classical method are that it can be used for any gene resulting in relatively unambiguous phenotypes, and that the method yields estimates of recombination, thus allowing detection of sex or population differences reflecting either physically different gene locations or differences in regulation.
• The major weakness of the classical method is that genes can be assigned only to linkage groups in the absence of other data concerning chromosomal location.

2. Somatic cell hybrids

3. In situ hybridization method

• The last and most recently developed method of gene mapping with general utility is the mapping of genes to chromosomes by hybridizing DNA. Through this method the chromosomal location of the structural gene for any protein could be determined.

Genetic maps of protein coding loci in fishes

Only few families of fishes have been utilized for genetic mapping studies: Salmonidae (trouts and salmon), Centrarchidae (freshwater sunfishes), and Poecilidae (platyfishes and swordtails, Xiphophorus sp. and Poecilia reticulata, the guppy).

The vast majority of linkage data in Xiphophorus are derived from backcrosses produced using an interspecific F₁ hybrid, Xiphophorus helleri x X. maculatus.

Additionally, some intraspecific matings between various populations of X. maculatus, X. helleri have been produced and yield linkage data for a substantial number of locus pairs.

Gene mapping studies in fishes have yielded respectable bodies of data only in systems utilizing interspecific hybrids-Xiphophorus, Poecilopsis, Lepomis, Salmo, Salvelinus and Oncorhynchus.

The breeding of hybrids has been extremely important both in aquarium trade and in commercial fisheries research and development. Thus, it appears that the major reason for the paucity of linkage data in fishes is not the lack of producible hybrids, but rather the lack of use of such hybrids for genetic analysis, particularly with respect to mapping of enzyme loci.

1. Physical maps

These are based on the assignment of loci to chromosomes and can be accomplished mainly by the methods such as somatic cell hybrid panels, in situ hybridization and comparative mapping.

In physical maps the coordinates are the chromosome regions or bands, and the distance between two loci are measured in kilobases.

2. Genetic maps

These are constructed by studying the meiotic recombination between two or more loci through linkage analysis.

In a linkage analysis, a new locus are assigned to a chromosome following the inheritance of the new locus and of an already mapped locus. Such maps do not provide an absolute location of loci but they reveal the genetic distance of the loci as a function of the frequency of crossing overs occurring during recombination.

This is the second major gene mapping method, which has produced a quantum expansion in gene map assignments. The power of this method lies in the use of cells from distantly related species. The disadvantage of somatic cell method is that, in the absence of detected deletions, insertions, or translocations, no information is obtained concerning the positions of loci relative to each other on the chromosome.