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The linked frequency of crossing over between two gene loci is the crossing-over value. For fixed set of genetic and environmental conditions, recombination in a particular region of a linkage structure ( chromosome ) tends to be constant and the same is then true for the crossing-over value which is used in the production of genetic maps .
The NCO/SDSA pathway contributes little to genetic variation, since the arms of the chromosomes flanking the recombination event remain in the parental configuration. Thus, explanations for the adaptive function of meiosis that focus exclusively on crossing-over are inadequate to explain the majority of recombination events.
Chromosomal reciprocal translocation of the 4th and 20th chromosome. In genetics, chromosome translocation is a phenomenon that results in unusual rearrangement of chromosomes. This includes balanced and unbalanced translocation, with two main types: reciprocal, and Robertsonian translocation.
Unequal crossing over is the process most responsible for creating regional gene duplications in the genome. [1] Repeated rounds of unequal crossing over cause the homogenization of the two sequences. With the increase in the duplicates, unequal crossing over can lead to dosage imbalance in the genome and can be highly deleterious. [1] [2]
Points of crossing over become visible as chiasma after the synaptonemal complex dissembles and the homologous chromosomes slightly apart from each other. The phenomenon of genetic chiasmata (chiasmatypie) was discovered and described in 1909 by Frans Alfons Janssens, a Professor at the University of Leuven in Belgium. [3] [4]
A Robertsonian translocation. The short arms of the chromosomes (shown on right) are often lost . Robertsonian translocation (ROB) is a chromosomal abnormality where the entire long arms of two different chromosomes become fused to each other. It is the most common form of chromosomal translocation in humans, affecting 1 out of every 1,000 ...
A crossing-over within the loop can produce unbalanced gametes. In a paracentric inversion, recombination results in one dicentric chromatid and one acentric chromatid. During Anaphase, both recombinants are faced with problems. The acentric chromatid is pulled to one pole or the other, and the dicentric recombinant generates dicentric bridges ...
Crossing over between the X and Y chromosomes is normally restricted to the pseudoautosomal regions; thus, pseudoautosomal genes exhibit an autosomal, rather than sex-linked, pattern of inheritance. So, females can inherit an allele originally present on the Y chromosome of their father.