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.
The figure below shows this concept. 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. The figure below depicts the gamete composition for linked genes from coupling and repulsion crosses.
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. However, in cases such as desynaptic mutants, some hybrids, and the female silkworm, complete pachytene synaptonemal complexes have been observed, but no crossing-over occurs, showing that the synaptonemal complex alone is not sufficient to cause crossing-over.
In Drosophila melanogaster oocytes, the occurrence at pachytene of dense spherical bodies bridging the central region of the synaptonemal complex has been described. These bodies coincided in number and position with expected crossover events, and therefore were named recombination nodules.
A variety of oval and bar-shaped recombination nodules Fig. In many cases their number correlates with crossover frequency.
It has been suggested that recombination nodules are prerequisites for crossing-over. If this is so, the recombination nodule may represent a complex of enzymes involved in the early events of recombination nicking, strand separation, repair synthesis. DNA repair synthesis has been observed during pachytene in lily microsporocytes, and has been shown to be reduced in an achiasmatic mutant. Prophase I of lilies is characterized by the presence of several proteins which could have a role in crossing-over, for example, DNA binding protein, endonucleases, ligases, and kinase.
Inhibition of protein synthesis at zygotene-pachytene results in failure of crossing-over. Thus both DNA synthesis and protein synthesis appear necessary for meiotic crossing-over in lily. The differentiated X and Y sex chromosomes in human males and many animals Z and W chromosomes in female birds have small regions near one tip which undergo pairing and crossing-over at meiotic prophase I. Electron microscopy of the pachytene XY reveals the formation of a short synaptonemal complex segment with a recombination nodule in the majority of cases; the presence of a chiasma between the X and Y at metaphase I indicates the occurrence of crossing-over.
An obligatory crossover in the XY bivalent is necessary to ensure regular segregation of X and Y to opposite poles at anaphase I. The pairing region contains a few gene loci on both X and Y chromosomes which exhibit an autosomallike inheritance pattern. Recombination between genes and DNA sequences in this pseudoautosomal region confirms the occurrence of obligatory crossing-over.
The rare occurrence of XX males in some cases is accounted for by abnormal recombination events outside the pseudoautosomal region which have transferred the male sex-determining gene from the Y to the X chromosome. See also: Sex determination ; Sex-linked inheritance.
To learn more about subscribing to AccessScience, or to request a no-risk trial of this award-winning scientific reference for your institution, fill in your information and a member of our Sales Team will contact you as soon as possible.
Recognized as an award-winning gateway to scientific knowledge, AccessScience is an amazing online resource that contains high-quality reference material written specifically for students. Contributors include more than 10, highly qualified scientists and 46 Nobel Prize winners. Crossing-over genetics Article by: Gillies, C. 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.
See also: Recombination genetics Fig. In general, centromeres and loci proximal to the chiasma crossover segregate at first division, while loci distal to the chiasma segregate at second division.
Example Question 7 : Understanding Crossing Over. Chromosomal crossover occurs in which phase of meiosis? Possible Answers: Anaphase II. Explanation : During prophase I, homologous chromosomes pair with each other and exchange genetic material in a process called chromosomal crossover.
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.
Copyright Notice. View Tutors. Erin Certified Tutor. Kimberly Certified Tutor. Michael Certified Tutor. Report an issue with this question If you've found an issue with this question, please let us know. Do not fill in this field. Louis, MO Or fill out the form below:. Company name. Copyright holder you represent if other than yourself. I am the owner, or an agent authorized to act on behalf of the owner of an exclusive right that is allegedly infringed.
I have a good faith belief that the use of the material in the manner complained of is not authorized by the copyright owner, its agent, or the law.
This notification is accurate. I acknowledge that there may be adverse legal consequences for making false or bad faith allegations of copyright infringement by using this process. 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! Top Subjects. Our Company. Varsity Tutors. Privacy Policy. Terms of Use. Sign In.
0コメント