Example Question 8 : Understanding Crossing Over. Possible Answers: Tetrads. Correct answer: Homologous chromosomes. Explanation : Crossing over occurs when chromosomal homologs exchange information during metaphase of Meiosis I. Example Question 9 : Understanding Crossing Over. Possible Answers: Interphase. Explanation : Crossing over occurs during prophase I when parts of the homologous chromosomes overlap and switch their genes.
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Find the Best Tutors Do not fill in this field. Your Full Name. Phone Number. Zip Code. Track your scores, create tests, and take your learning to the next level! Gillies, C. Last reviewed: November The process whereby one or more gene alleles present in one chromosome may be exchanged with their alternative alleles on a homologous chromosome to produce a recombinant crossover chromosome which contains a combination of the alleles originally present on the two parental chromosomes.
Genes which occur on the same chromosome are said to be linked, and together they are said to compose a linkage group. In eukaryotes, crossing-over may occur during both meiosis and mitosis, but the frequency of meiotic crossing-over is much higher. This article is concerned primarily with meiotic crossing-over. See also: Allele ; Chromosome ; Gene ; Linkage genetics. Crossing-over is a reciprocal recombination event which involves breakage and exchange between two nonsister chromatids of the four homologous chromatids present at prophase I of meiosis; that is, crossing-over occurs after the replication of chromosomes which has occurred in premeiotic interphase.
The result is that half of the meiotic products will be recombinants, and half will have the parental gene combinations Fig. Using maize chromosomes which carried both cytological and genetical markers, H. Creighton and B. McClintock showed in that genetic crossing-over between linked genes was accompanied by exchange of microscopically visible chromosome markers. See also: Recombination genetics. During meiosis, crossing-over occurs at the pachytene stage, when homologous chromosomes are completely paired.
At diplotene, when homologs separate, the sites of crossing-over become visible as chiasmata, which hold the two homologs of a bivalent together until segregation at anaphase I. Each metaphase I bivalent will necessarily have at least one chiasma. In favorable material, such as grasshopper spermatocytes, it is possible to observe that each diplotene chiasma involves a crossover of two of the four chromatids at one site.
Where two or more crossovers occur in one bivalent, they usually do not cluster together but are widely separated; this is known as chiasma interference. The occurrence of one crossover event appears to preclude the occurrence of a second crossover in the immediate vicinity.
This outcome is the normal way for crossover to occur. This is known as an unequal crossover. Related Stories. In order to mediate this exchange of genetic material, The discovery could speed up plant breeding and Now, an international collaborative work has analyzed all together The breakdown of this Since the chromosomes are homologous, breaks at corresponding points mean that the segments that are broken off contain corresponding genes , i. The broken sections are then exchanged between the chromosomes to form complete new units, and each new recombined chromosome of the pair can go to a different daughter sex cell.
Crossing over results in recombination of genes found on the same chromosome, called linked genes, that would otherwise always be transmitted together.
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