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You are watching: Why is the maximum recombination frequency 50

Griffiths AJF, Miller JH, Suzuki DT, et al. An Introduction to Genetic Analysis. 7th edition. New York: W. H. Freeman; 2000.


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In modern-day hereditary evaluation, the major test for determining whether 2 genes are connected isbased on the principle of recombicountry. Recombination is observed in a selection of instances but,for the present, let’s define it in relation to meiosis. Meiotic recombination isany meiotic process that generates a haploid product through a genokind that differs from bothhaploid genoforms that comprised the meiotic diploid cell. The product of meiosis so generatedis referred to as a recombinant. This meaning makes thevital allude that we detect recombicountry by comparing the output genotypesof meiosis and the parental input genoforms (Figure 5-4). The input genoforms are the 2 haploid genoforms that merged to makethe genetic constitution of the meiocyte, the diploid cell that undergoes meiosis.


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Figure 5-4

Recombinants are those products of meiosis with allelic combinations different from those ofthe haploid cells that created the meiotic diploid.


MESSAGE

In meiosis, recombicountry geneprices haploid genoforms differing from the haploid parentalgenokinds.


Meiotic recombination is a part of both haploid and also diploid life cycles; however, detectingrecombinants in haploid cycles is straightforward, whereas detecting them in diploid cycles ismore facility. The input and also output types in haploid cycles are the genoforms of people andmight therefore be inferred straight from phenoforms. Figure 5-4have the right to be perceived as summarizing the basic detection of recombinants in haploid life cycles. Theinput and also output kinds in diploid life cycles are gametes. Because we must know the inputgametes to detect recombinants in a diploid cycle, it is preferable to have pure-breedingparental fees. Furthermore, we cannot detect recombinant output gametes directly: we should testcrossthe diploid individual and also observe its progeny (Figure5-5). If a testcross offspring is presented to have actually been comprised from a recombinantproduct of meiosis, it as well is dubbed a recombinant. Notice again that thetestcross permits us to concentprice on one meiosis and also prevent ambiguity. From aself of the F1 in Figure 5-5, for instance, arecombinant A/A · B/boffspring cannot be distinguimelted fromA/A · B/B without furthercrosses. Recombinants are created by two various cellular processes: independent assortmentand crossing-over.


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Figure 5-5

The detection of recombicountry in diploid organisms. Note that Figure 5-4 is a part of this diagram. Recombinant assets of a diploidmeiosis are a lot of readily detected in a cross of a heterozygote and also a recessive tester.


Recombination by independent assortment

Mendelian independent assortment is viewed via regard to recombicountry inFigure 5-6. In a testcross, the 2 recombinant classesconstantly comprise 50 percent of the progeny; that is, tbelow is 25 percent of each recombinant typeamong the progeny.


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Figure 5-6

Independent assortment constantly produces a recombinant frequency of 50 percent. Thisdiagram shows 2 chromosome pairs of a diploid organism through A anda on one pair and B and also b on the other.Note that we might represent the haploid case by removing (even more...)


If we observe a recombinant frequency of 50 percent in a testcross, we have the right to infer that the twogenes under study astype individually. The easiest interpretation of such an outcome is thatthe two genes are on sepaprice chromosome pairs. However, genes that are far apart on thevery same chromosome pair have the right to act basically individually and develop the sameresult.


Recombicountry by crossing-over

Crossing-over also have the right to produce recombinants. Any two nonsister chromatids can cross over. (Weshall present proof of this in Chapter 6.) There isnot a crossover between 2 specific genes in all meioses, however, as soon as there is, half thecommodities of that meiosis are recombinant, as presented in Figure 5-7. Meiosis through no crossover in between the genes under research producesonly parental genotypes for these genes.


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Figure 5-7

Recombinants arise from meioses in which nonsister chromatids cross over between thegenes under study.


For genes cshed together on the very same chromosome pair, the physical link of parental allelecombinations renders independent assortment difficult and therefore produces recombinant frequenciessignificantly reduced than 50 percent (Figure 5-8). We sawan example of this case in Morgan’s data (page 142), wbelow the recombinant frequency was(151 + 154) ÷ 2839 = 10.7 percent. This is obviously much less than the 50 percent that wewould intend with independent assortment. The recombinant frequency occurring from linked genesranges from 0 to 50 percent, depending upon their closeness. What around recombinant frequenciesgreater than 50 percent? The answer is that such frequencies are neveroboffered, as we shall watch in Chapter 6.


Figure 5-8

Recombicountry from crossing-over. Notice that the frequencies of the recombinants include upto much less than 50 percent.


Note in Figure 5-7 that crossing-over generates tworeciprocal products, which describes why the reciprocal recombinant classes are generallyapproximately equal in frequency.


MESSAGE

A recombinant frequency considerably less than 50 percent shows that the genes are connected.A recombinant frequency of 50 percent generally implies that the genes are unlinked on separatechromosomes.


The remainder of this chapter focuses largely on linked genes and recombinants occurring fromcrossing-over.

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By agreement with the publisher, this book is available by the search feature, yet cannot be browsed.