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You are watching: Which statement explains why the recombination frequency between two genes is always less than 50%?

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


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In modern hereditary evaluation, the major test for determining whether 2 genes are linked isbased upon the idea of recombination. Recombicountry is observed in a selection of instances however,for the present, let’s specify it in relation to meiosis. Meiotic recombination isany kind of meiotic procedure that generates a haploid product with a genokind that differs from bothhaploid genokinds that constituted the meiotic diploid cell. The product of meiosis so generatedis dubbed a recombinant. This meaning renders thecrucial point that we detect recombination by comparing the output genotypesof meiosis and also the parental input genokinds (Figure 5-4). The input genoforms are the 2 haploid genokinds that linked to makethe hereditary constitution of the meiocyte, the diploid cell that undergoes meiosis.


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Figure 5-4

Recombinants are those commodities of meiosis via allelic combicountries various from those ofthe haploid cells that created the meiotic diploid.


MESSAGE

In meiosis, recombination generates haploid genotypes differing from the haploid parentalgenoforms.


Meiotic recombicountry is a part of both haploid and diploid life cycles; yet, detectingrecombinants in haploid cycles is straightforward, whereas detecting them in diploid cycles iseven more complicated. The input and output types in haploid cycles are the genokinds of people andmay therefore be inferred straight from phenotypes. Figure 5-4can be regarded as summarizing the straightforward detection of recombinants in haploid life cycles. Theinput and output kinds in diploid life cycles are gametes. Due to the fact that we need to know the inputgametes to detect recombinants in a diploid cycle, it is preferable to have pure-breedingparents. In addition, we cannot detect recombinant output gametes directly: we should testcrossthe diploid individual and also observe its progeny (Figure5-5). If a testcross offspring is displayed to have been made up from a recombinantproduct of meiosis, it also is referred to as a recombinant. Notice aacquire that thetestcross allows us to concentrate on one meiosis and also proccasion ambiguity. From aself of the F1 in Figure 5-5, for example, arecombinant A/A · B/boffspring cannot be distinguimelted fromA/A · B/B without furthercrosses. Recombinants are created by two different cellular processes: independent assortmentand also crossing-over.


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Figure 5-5

The detection of recombicountry in diploid organisms. Keep in mind that Figure 5-4 is a component of this diagram. Recombinant assets of a diploidmeiosis are most conveniently detected in a cross of a heterozygote and also a recessive tester.


Recombination by independent assortment

Mendelian independent assortment is regarded through regard to recombicountry inFigure 5-6. In a testcross, the two recombinant classesalways make up 50 percent of the progeny; that is, tright here is 25 percent of each recombinant typeamong the progeny.


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Figure 5-6

Independent assortment always produces a recombinant frequency of 50 percent. Thisdiagram mirrors two chromosome pairs of a diploid organism via A anda on one pair and B and b on the other.Keep in mind that we could recurrent 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 research askind separately. The most basic interpretation of such an outcome is thatthe two genes are on separate chromosome pairs. However, genes that are much apart on thevery same chromosome pair can act essentially individually and produce the sameresult.


Recombination by crossing-over

Crossing-over likewise can create recombinants. Any 2 nonsister chromatids have the right to cross over. (Weshall display proof of this in Chapter 6.) There isnot a crossover between 2 certain genes in all meioses, yet, as soon as tbelow is, half thecommodities of that meiosis are recombinant, as displayed in Figure 5-7. Meiosis via no crossover between the genes under research producesjust 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 close together on the very same chromosome pair, the physical link of parental allelecombinations provides independent assortment impossible and therefore produces recombinant frequenciessignificantly lower than 50 percent (Figure 5-8). We sawan example of this situation in Morgan’s data (page 142), wbelow the recombinant frequency was(151 + 154) ÷ 2839 = 10.7 percent. This is obviously a lot less than the 50 percent that wewould certainly suppose via independent assortment. The recombinant frequency arising from attached genesvarieties from 0 to 50 percent, depending upon their closeness. What around recombinant frequencieshigher than 50 percent? The answer is that such frequencies are neverobserved, as we shall check out in Chapter 6.


Figure 5-8

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


Keep in mind in Figure 5-7 that crossing-over geneprices tworeciprocal commodities, which defines why the reciprocal recombinant classes are generallyroughly equal in frequency.


MESSAGE

A recombinant frequency substantially much less than 50 percent reflects that the genes are attached.A recombinant frequency of 50 percent primarily means that the genes are unlinked on separatechromosomes.


The remainder of this chapter concentrates greatly on connected genes and also recombinants emerging fromcrossing-over.

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By agreement via the publisher, this book is available by the search function, however cannot be browsed.