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yucheng9255木虫 (著名写手)
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[求助]
国外第一次上遗传课就被课后作业难倒，大家帮我看看，急！！
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Graduate student Evert Explorer found an old ear of corn in Mexico that contained both plump kernels and shrunken kernels. Shrunken seed accounted for 25% of the total seed (a 3 to 1 segregation ratio). Curious about the genetic cause of the shrunken seed, he brought the seed back to the UF, and planted these seeds as well as mutant seeds of three homozygous recessive mutants, termed shrunken-2, brittle-1 and brittle-2. These mutants are phenotypically indistinguishable from the mutant seed found in Mexico. Phenotypically mutant seed from Mexico did not germinate whereas non-mutant, or plump, seed from the ear did. Six plants derived from the plump seed were crossed onto each of the three mutants. Whereas all crosses onto plants from shrunken-2 and brittle-1 mutants yielded all plump seed, crosses of four of the six plants onto plants from the brittle-2 seeds gave rise to ears that were 50% plump and 50% shrunken. What is the cause of the mutant seed found in Mexico? Why didn’t all crosses onto brittle-2 yield mutant seed? What is the origin of the 50:50 ratios in the brittle-2 crosses that segregated mutant seed? |
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2楼2012-08-24 11:20:38
yucheng9255
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3楼2012-08-24 20:40:51
irenescbg
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yucheng9255: 金币+10 2012-08-28 07:28:58
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That's very easy..... 1. brittle-2 mutation in the seeds found in mexico caused the shrunken phenotype. 2. brittle-2 is a RECESSIVE mutant. When plump seeds carried WT genes, the F1 progeny of the cross has heterozygous genotype. These seeds are not shrunky. 3. If the plump seeds are heterozygous (bB), when these plants were crossed to brittle-2 (bb), half of the progeny will be bB, and half will be bb. bB genotype will result in a plump seed while bb results in shrunky seeds. |
4楼2012-08-27 16:18:23
irenescbg
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Sorry I made some mistakes and I didn't make some part clear. Here are the corrections. brittle-2 is allelic to the mutation that caused shrunken phenotype in the mexican line. The other two were not allelic. shrunken mutation is recessive. Same with brittle-2. Among the 6 plump plants used for cross, two have willd type allele of brittle gene while the rest four are heterozygous. |
5楼2012-08-28 08:39:25