Cis and Trans | Coupling Repulsion | Gene recombination

DNA recombination involves the exchange of genetic material either between multiple chromosomes or between different regions of the same chromosome. This process is generally mediated by homology; that is, homologous regions of chromosomes line up in preparation for exchange, and some degree of sequence identity is required. Various cases of nonhomologous recombination do exist, however. One important instance of recombination in diploid eukaryotic organisms is the exchange of genetic information between newly duplicated chromosomes during the process of meiosis. In this instance, the outcome of recombination is to ensure that each gamete includes both maternally and paternally derived genetic information, such that the resulting offspring will inherit genes from all four of its grandparents, thereby acquiring a maximum amount of genetic diversity. Recombination is also used in DNA repair (particularly in the repair of double-stranded breaks), as well as during DNA replication to assist in filling gaps and preventing stalling of the replication fork. In these cases, a sister chromatid serves as the donor of missing material via recombination followed by DNA synthesis. The role of recombination during the inheritance of chromosomes was first demonstrated through experiments with maize. Specifically, in 1931, Barbara McClintock and Harriet Creighton obtained evidence for recombination by physically tracking an unusual knob structure within certain maize chromosomes through multiple genetic crosses. Using a strain of maize in which one member of a chromosome pair exhibited the knob but its homologue did not, the scientists were able to show that some alleles were physically linked to the knobbed chromosome, while other alleles were tied to the normal chromosome. McClintock and Creighton then followed these alleles through meiosis, showing that alleles for specific phenotypic traits were physically exchanged between chromosomes. Evidence for this finding came from the fact that alleles first introduced into the cross on a knobbed chromosome later appeared in offspring without the knob; similarly, alleles initially introduced on a knobless chromosome subsequently appeared in progeny with the knob. Recombination also occurs in prokaryotic cells, and it has been especially well characterized in E. coli. Although bacteria do not undergo meiosis, they do engage in a type of sexual reproduction called conjugation, during which genetic material is transferred from one bacterium to another and may be recombined in the recipient cell. As in eukaryotes, recombination also plays important roles in DNA repair and replication in prokaryotic organisms. Problem: If the alleles YyRr of a dihybrid are in coupling phase (Cis) and the distance between the genes 20 cM, identify the genotypes of the parental and recombinant progeny and its proportion from a test cross? If the alleles YyRr of a dihybrid are in repulsion phase (Trans), Identify the genotypes of the parental and recombinant progeny and its proportion from a test cross? #NikolaysGeneticsLessons #crossingOver #synapsis #linkedGenes #geneticRecombination #crossingOverInMeiosisI #crossingOverOfChromosomes #homologousChromosomes #nonsisterChromatids #sisterChromatids #MendelianGenetics #Genetics #MeiosisAndCrossingOver #crossingOverMeiosis #crossingOverBiology #meiosis #chromatids #coupling #Chromosome #gene #allele #crossingover #geneRecombination #educationalVideos #scienceVideos #highSchoolScience #Genes #Alleles #apBiology