How is gene linkage mapped using crossover frequencies?

Gene linkage is mapped using crossover frequencies by calculating the percentage of recombinant offspring produced in a cross.

In more detail, gene linkage refers to the tendency of genes that are located close together on the same chromosome to be inherited together. This is because they are physically linked on the DNA molecule and do not assort independently during meiosis. However, during prophase I of meiosis, homologous chromosomes pair up and exchange segments in a process known as crossing over. This can result in the recombination of linked genes, producing new combinations of alleles.

The frequency of crossing over between two genes is proportional to the distance between them. This is because the further apart two genes are, the more likely it is that a crossover event will occur between them. Therefore, by observing the proportion of offspring that display new combinations of traits (recombinants), scientists can estimate the distance between two linked genes. This is known as a recombination frequency, and it is usually expressed as a percentage. For example, a recombination frequency of 10% suggests that the two genes are relatively far apart on the chromosome, as crossing over occurs between them in 10% of meiotic events.

To map gene linkage using crossover frequencies, scientists first perform a test cross between two individuals that are heterozygous for the genes of interest. They then analyse the phenotypes of the offspring to determine the proportion that are recombinants. This allows them to calculate the recombination frequency, which can be used to estimate the relative positions of the genes on the chromosome. This forms the basis of a genetic linkage map, which is a diagram that shows the relative locations of genes along a chromosome.

In summary, by studying the frequency of recombinant offspring produced in a cross, scientists can estimate the distance between linked genes and create a map of their relative positions on a chromosome. This is a fundamental technique in genetics, and it has been instrumental in our understanding of heredity and the structure of the genome.