Introduction

• The discovery of linked genes and recombination due to crossing over motivated the study of genetic mapping.

Alfred H. Sturtevant’s Method of Constructing a Genetic Map

• Sturtevant, a student of Morgan, worked out a method for constructing a genetic map by assigning relative positions to genes on the same chromosomes.
• He hypothesized that the percentage of recombinant offspring (recombination frequency) depends on the distance between genes on a chromosome.
• He assumed that crossing over is a random event with equal chance of occurring at all points along a chromosome.
• Based on these assumptions, the farther apart two genes are, the higher the recombination frequency and the greater the points for crossing over.
• Using recombination data from fruit fly crosses, Sturtevant mapped genes and expressed distances between them in map units (equivalent to 1% recombination frequency).

Challenges in Interpreting Recombination Data

• The interpretation of recombination data is complicated by genes that are so far apart that a crossover between them is virtually certain, leading to a maximum frequency of 50% recombination, indistinguishable from genes on different chromosomes.
• These genes, although physically connected on the same chromosome, behave as if they were on different chromosomes.
• Mapping genes located far apart on a chromosome involves adding recombination frequencies from closer pairs of genes lying between the two distant genes.

Success of Sturtevant’s Method

• Sturtevant and his colleagues were able to map numerous Drosophila genes in linear arrays, clustering into four groups of linked genes (linkage groups).
• The linkage map provided evidence that genes are located on chromosomes.
• Each chromosome has a linear array of specific genes, each gene with its own locus.

Limitations of a Linkage Map

• A linkage map is based strictly on recombination frequencies and gives only an approximate picture of a chromosome.
• The frequency of crossing over is not uniform over the length of a chromosome and map units do not correspond to physical distances.
• A linkage map portrays the order of genes along a chromosome but not the precise locations of genes.

Advances in Mapping Gene Locations

• Cytogenetic maps of chromosomes locate genes with respect to chromosomal features visible in a microscope.
• DNA sequencing has advanced and is now used to map the locations of genes in a species.
• The entire nucleotide sequence is the ultimate physical map of a chromosome, revealing physical distances in DNA nucleotides between gene loci.