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Physical crossing over during meiosis I is a normal event. The effect of this event is to rearrange heterozygous homologous chromsomes into new combinations. The term used for crossing over is recombination. Recombination can occur between any two genes on a chromosome, the amount of crossing over is a function of how close the genes are to each other on the chromosome. If two genes are far apart, for example at opposite ends of the chromosome, crossover and non-crossover events will occur in equal frequency. Genes that are closer together undergo fewer crossing over events and non-crossover gametes will exceed than the number of crossover gametes. Finally, for two genes are right next to each other on the chromosome crossing over will be a very rare event. Two types of gametes are possible when following genes on the same chromosomes. If crossing over does not occur, the products are parental gametes. If crossing over occurs, the products are recombinant gametes. The allelic composition of parental and recombinant gametes depends upon whether the original cross involved genes in coupling or repulsion phase. It is usually a simple matter to determine which of the gametes are recombinant. These are the gametes that are found in the lowest frequency. This is the direct result of the reduced recombination that occurs between two genes that are located close to each other on the same chromosome. Also by looking at the gametes that are most abundant you will be able to determine if the original cross was a coupling or repulsion phase cross. For a coupling phase cross, the most prevalent gametes will be those with two dominant alleles or those with two recessive alleles. For repulsion phase crosses, gametes containing one dominant and one recessive allele will be most abundant. Understanding this fact will be important when you actually calculate a linkage distance estimate from your data. The important question is how many recombinant chromosomes will be produced. If the genes are far apart on the chromosome a cross over will occur every time that pairing occurs and an equal number of parental and recombinant chromosomes will be produced. Test cross data will then generate a 1:1:1:1 ratio. But as two genes are closer and closer on the chromosome, fewer cross over events will occur between them and thus fewer recombinant chromosomes will be derived. We then see a deviation from the expected 1:1:1:1 ratio. How can we decide how close two genes are on a chromosome? Because fewer crossover events are seen between two genes physically close togehter on a chromosome, the lower the percentage of recombinant phenotypes will be seen in the testcross data. By definition, one map unit (m.u.) is equal to one percent recombinant phenotypes. In honor of the work performed by Morgan, one m.u. is also called one centimorgan (cM). #Genetics #alleles #genes #CentiMorgans #mapUnits #crossingover #geneMapping
