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You are watching: Why is recombination frequency maximum at 50

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


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In modern hereditary evaluation, the primary test for determining whether two genes are attached isbased on the concept of recombicountry. Recombicountry is oboffered in a variety of situations but,for the existing, let’s define it in relation to meiosis. Meiotic recombination isany meiotic procedure that geneprices a haploid product through a genokind that differs from bothhaploid genoforms that made up the meiotic diploid cell. The product of meiosis so generatedis referred to as a recombinant. This definition makes theimportant point that we detect recombicountry by comparing the output genotypesof meiosis and the parental input genokinds (Figure 5-4). The input genotypes are the two haploid genoforms that combined to makethe hereditary 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 various from those ofthe haploid cells that formed the meiotic diploid.


MESSAGE

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


Meiotic recombicountry is a part of both haploid and diploid life cycles; however, detectingrecombinants in haploid cycles is straightforward, whereas detecting them in diploid cycles iseven more complex. The input and also output kinds in haploid cycles are the genotypes of people andmight hence be inferred directly from phenoforms. Figure 5-4have the right to be perceived as summarizing the basic detection of recombinants in haploid life cycles. Theinput and output forms in diploid life cycles are gametes. Since we have to know the inputgametes to detect recombinants in a diploid cycle, it is preferable to have pure-breedingparents. Additionally, 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 been made up from a recombinantproduct of meiosis, it too is referred to as a recombinant. Notice aacquire that thetestcross allows us to concentprice on one meiosis and prevent ambiguity. From aself of the F1 in Figure 5-5, for example, arecombinant A/A · B/boffspring cannot be distinguished fromA/A · B/B without furthercrosses. Recombinants are created by two different cellular processes: independent assortmentand crossing-over.


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Figure 5-5

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


Recombicountry by independent assortment

Mendelian independent assortment is perceived through regard to recombination inFigure 5-6. In a testcross, the two recombinant classesalways comprise 50 percent of the progeny; that is, there 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 shows 2 chromosome pairs of a diploid organism with A anda on one pair and also B and b on the other.Keep in mind that we can represent the haploid instance by rerelocating (more...)


If we observe a recombinant frequency of 50 percent in a testcross, we can infer that the twogenes under examine askind independently. The most basic interpretation of such an outcome is thatthe two genes are on sepaprice chromosome pairs. However before, genes that are much apart on theexact same chromosome pair can act basically individually and create the sameoutcome.


Recombicountry by crossing-over

Crossing-over likewise deserve to create recombinants. Any 2 nonsister chromatids deserve to cross over. (Weshall display proof of this in Chapter 6.) Tright here isnot a crossover between two certain genes in all meioses, yet, when there is, half theproducts of that meiosis are recombinant, as displayed 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 in between thegenes under examine.


For genes cshed together on the very same chromosome pair, the physical link of parental allelecombicountries makes independent assortment impossible and thus produces recombinant frequenciessignificantly lower than 50 percent (Figure 5-8). We sawan example of this case in Morgan’s data (web 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 intend through independent assortment. The recombinant frequency arising from attached genesarrays from 0 to 50 percent, depending on their closeness. What about recombinant frequenciesbetter than 50 percent? The answer is that such frequencies are neverobserved, 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.


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


MESSAGE

A recombinant frequency significantly much less than 50 percent mirrors that the genes are connected.A recombinant frequency of 50 percent mostly implies that the genes are unconnected on separatechromosomes.


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

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