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Crossing Over and Genetic Mapping During meiosis, the two chromosomes in each homologous pair exchange segments, through a process called crossing over. This process of crossing over and the resulting recombination, (exchange of gene alleles across the chromosomes in a pair) enables us to reason about genetic mapping - that is, about the order of genes on a chromosome and the distances among the genes. Crossing Over and Recombination When a chromosome replicates early in meiosis, the two duplicate copies (called chromatids) are joined at a point called the centromere. After the chromosomes duplicate but before cell division, the two homologous chromosomes in each homologous pair align. Crossovers occur when homologous chromosomes are aligned: chromatids from two different chromosomes can exchange segments chromatids from two homologous chromosomes come in contact at an equivalent point along their lengths. In Fig. 3b, the two chromatids have separated after exchanging the segments between the contact point and the tips of the chromatids. Crossover Frequency and Genetic Mapping We can use the phenomenon of crossovers described above to reason about the order of genes along a chromosome and about the distances between those genes. This reasoning depends on this basic principle: • To a first approximation, crossovers are equally likely to occur at any point along the length of a chromosome. It follows that: • the probability of a crossover between two genes is proportional to the distance between the two genes. That is, the greater the distance between the two genes, the greater the probability that a crossover will occur between them during meiosis. Problem: In a linear chromosome map distance between 4 loci is as follows: a-b is 10%, a-d is 3%, b-c is 4% and a-c is 6%. The crossover frequency between 'c' and 'd' is A) 4% B) 3% C) 3 to 6% D) 5 to 9% #educationalVideos #scienceVideos #highSchoolScience #GeneticRecombinationAwardWinningWork #GeneMappingLiteratureSubject #EducationWord #Genetics #Genes #apBiology #NikolaysGeneticsLessons #geneMapping #recombination #crossingOver #DrosophilaEyeColor #percentRecombination #RecombinationFrequency #mapUnits #recombinationUnitsExample #recombinationFrequencyExample #determiningRecombinationgFrequency #geneticRecombination
