Question

A wild-type trihybrid soybean plant is crossed to a pure-breeding soybean plant with the recessive phenotypes pale leaf l, oval seed (r), and short height (t). the results of the three-point test cross are shown in the table. traits not listed are wild type.

Answer 1

Pale - 648 - Parental

Pale, oval - 64 - Single recombinant (recombination between T and R)

Pale, short - 10 - Double recombinant

Pale, oval, short - 102 - Single recombinant (recombination between T and L )

Oval - 6 - Double recombinant

Oval, Short - 618 - Parental

Short - 84 - Single recombinant (recombination between T and R)

Wild type - 98 - Single recombinant (recombination between T and L )

Total 1630

A). From the given table, the genotype of the parents is lTR and Ltr. Double cross over gave rise to the offspring with the phenotypes oval and plale, short. Thus, the gene order is L-T-R.

B). The recombination fraction between T and L pair of genes. = number of gametes produced by single crossover of T and L + number of double crossover gametes*100/ total number of gametes.

= [(102+98) + (10+6)]* 100/ 1630 = 13.25 map units.

The genetic distance between genes T and L = 13.25 map units.

C). The recombination fraction between T and R pair of genes. = number of gametes produced by single crossover of T and R + number of double crossover gametes*100/ total number of gametes.

= [(64+84) + (10+6)]* 100/ 1630 = 10.06 map units.

The genetic distance between genes T and R = 10.06 map units.