MCAT Content / Analytic Methods / Gene Mapping Crossover Frequencies

Gene mapping: crossover frequencies

Topic: Analytic Methods

Genetic mapping provides information about the location of a specific gene along a chromosome; crossing over occurs more frequently when genes are farther apart.

The study of genetic maps begins with linkage analysis, a procedure that analyzes the recombination frequency between any two genes to determine if they are linked together. Gene mapping, often called linkage mapping, provides information about which chromosomes contain specific genes and precisely where the genes lie on that chromosome. The exchange of DNA between homologous pairs of chromosomes is called genetic recombination, which occurs by the crossing over of DNA between homologous strands of DNA, such as non-sister chromatids. 

Gene linkage describes the phenomenon that specific genes are physically linked by being located on the same chromosome and tend to be inherited together. The greater the distance between two genes, the higher the chance that a recombination event will occur between them, and the higher the recombination frequency between them. If the recombination frequency between the two genes is less than 50 per cent, they are said to be more linked. Crossing over occurs more frequently when genes are farther apart.

Crossovers and Recombination. Crossover may occur at different locations on the chromosome. Recombination between genes A and B is more frequent than recombination between genes B and C because genes A and B are farther apart; a crossover is, therefore, more likely to occur between them.

Key Points

• Gene mapping, often called linkage mapping, provides information about the location of a specific gene along a chromosome.

• Genetic recombination is the exchange of DNA between homologous pairs of chromosomes that occurs by the crossing over of DNA between homologous strands of DNA, such as non-sister chromatids. 

• The greater the distance between two genes, the higher the chance that a recombination event will occur between them, and the higher the recombination frequency between them. 

• Crossing over occurs more frequently when genes are farther apart.


Key Terms

recombination: a process by which pieces of DNA are broken and recombined to produce new combinations of alleles

crossing over: the exchange of chromosome segments between non-sister chromatids of homologous chromosomes

linkage analysis: studies the linkage between genes, where linkage is the tendency for genes to be inherited together because of their location

recombination frequency: the frequency with which a single chromosomal crossover will take place between two genes during meiosis

gene mapping: the methods used to identify the locus of a gene and the distances between genes

homologous chromosomes: chromosome pairs inherited from the organism’s mother; the other is inherited from the organism’s father



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