1. which phenotype groups represent double crossover products? 2. Calculate the

Question

1. which phenotype groups represent double crossover products?

2. Calculate the coefficient of coincidence. For decimals, round up to the nearest 100th. For example, 0.2921 should be 0.29, and 0.3356 should be 0.34.

A heterozygous trihybrid corn plant is testcrossed and the following proge recessive to their wildtype alternative); ny are produced(all traits are booster booster, silkless booster, ligueless, silkless 293 liguleless ligueless, silkless silkless booster, liguleless wildtype Allele symbol: booster-b, silkless -sl, liguleless -I 6 142 147 32 34 339

Explanation / Answer

In the trihybrid cross a heterozygous corn plant is test crossed with homozygous recessive individual

heterzygous corn plant : BbSLslLLll ( B is the dominant allele of booster allele, SL is the dominant allele of silkless allele, and LL is the dominant allele of ligueless allele.)

homozygous recessive indiviudals with which it is crossed is bbslslllll

test cross :  BbSLslLLll X bbslslllll

Now in the table above, only the recessive traits are shown. The phenotypes which have the lowest frequency are double crossover products. This because double cross over is a rare event and occurs in low frequency. Booster (6) , ligueless and silkless (7) phenotype is a double cross over product.

Coefficient of coincidence is the ratio of observed and expected double cross over

Expected double cross over = frequency of cross over in the first region x frequency of crossover in the second region x number of progeny

For this we have to determine the order of genes to ascertain which is the first region and which is the second region.

In part 1 we said that booster and ligueless and silkless are double cross over products. In a double cross over event the middle gene is moved from one sister chromatid to another. In the trihybrid cross when  BbSLslLLll plant produces gametes there will be a cross over event between the BSLLL allele and bslll allele containing chromatids . If double cross over happens:

allele 1 __SL___B____LL

allele 2 __sl____b_____ll

The gametes will be ___SL___b___LL and __sl___B____ll   

Only the middle gene is move and other two remain at their places.

gametes of test cross plant will be __sl___b___ll

Therefore when these gametes fuse , following progeny will be produced.

Genotypes and Phenotype: SLbLL/slbll ( booster ) and slBll/slbll (ligueless and silkless)

we have now derived the gene order.

coming to coefficient of coincidence

observed double crossover = 6 ( booster) + 7 (ligueless  and  slikless) = 13

cross over in first region will produce the following genotypes and phenotypes in the progeny

Genotype and phenotype : SLbll/slbll (booster & ligueless)   and slBLL/slbll (silkless)

frequency of cross over in first region = 34 (booster and ligueless) + 32 (silkless) + 6+ 7 (double cross over progeny) divided by total progeny

= 34 + 32+ 6+7 /1000 = 0.079

frequency of cross over in second region = 142 ( booster, silkless) + 147 (liguleless) + 6 + 7 ( double crossover) divided by total number of progeny

=142+147+6+7/1000 = 0.302

expected dco = 0.079 X 0.302 X 1000 ( total number of progeny)

= 23.8

coefficient of coincidence= 13/ 23.8 = 0.5462

therefore coefficient of coincidence is 0.55

  

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