Question

Gamete

Genotype Number
Set 1   + + +   129
Set 2   a b c   135
Set 3   + + c   65
Set 4   a b +   69
Set 5   + b c   264
Set 6   a + +   272
Set 7   a + c   31
Set 8   + b +   35
   Total   1000

The order shown is not necessarily correct. Which gene locus is in the middle? Show the order.

Answer 1

a c b is the gene order, c gene locus is present in middle.

• Mendel proposed that in order to exchange information two chromosomes undergo crossing over.
• Three point cross is type of test cross which analyses inheritance patterns of 3 alleles.
• Three point cross provides basis for determination of gene order I.e the order in which genes are present on the chromosome.
• From the genotypic frequencies data sets can be classified as parental non crossovers (maximum frequencies), products of single cross over and products of double cross overs (DCO) which have least frequencies as single crossing over interferes or in a way represses second cross over.
• If parental and double crossovers differ at only a single gene loci it means that gene is present in the middle.
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