Question

618 Short A wild-type trihybrid soybean plant Phenotype Number is crossed to a pure-breeding Pale 648 soybean plant with the recessive Pale, oval 64 phenotypes pale leaf (!), oval seed Pale, short 10 (r), and short height (t). The results Pale, oval, short 102 of the three-point test cross are Oval shown in the table. Traits not listed Oval, short are wild-type. Wild type 98 1. True / False The phenotypes 1630 listed in the table represent the trihybrid's progeny 2. True / False Given the cross design the table reflects the trihybrid's gamete genotypes and frequencies/count 3. If each of the genes considered were on different chromosome types, how many possible phenotypes would this cross produce? 4. If the genes considered are not linked at all, how many possible phenotypes would this cross produce? 5. If the genes considered are not linked at all, at what proportion would wild-type (for all 3 traits) progeny be observed? 6. If the genes considered are not linked at all, at what proportion would the wild-type (for all 3 traits) gamete in the trihybrid be observed? 7. True / False if the genes considered are all incompletely linked, the number of phenotypes produced by this cross would be less than calculated under the no linkage model.

Answer 1

1. False, because if it would be a trihybrid progeny all the phonotypes would show all the three distinct characters like color of leave, shape of seeds and height of plant. But here some phonotypes are having either color of leaf of shape of seed. Only one is showing all the 3 characters.

2. False, gamete genotype would also show all the characters but its not shown in the tabe.

3. This cross would produce 8 possible phenotyps because it has 3 types of variables.

GgRrTt × ggrrtt

Gametes	GRT	GRt	GrT	Grt	gRT	gRt	grT	grt
grt	GgRrTt	GgRrtt	GgrrTt	Ggrrtt	ggRrTt	ggRrtt	ggrrTt	ggrrtt

4. If the genes are bot linked 8 type of phenotypes of progeny would be seen because the genes will be independently assorted.

5. For a progeny of wild type for all the 3 traits, we saw only one out of 8 is having all the 3 wild type character. So 1/8th population will be having wild type character.

6. Wild type gametes will also be 1/8th of the total progeny.

7. True. If the genes are incompletely linked then they will not be independently assorted and the linked will always come together. Eg. If green leave if linked with oval seed then green leave with round seed will not come, thereby decreasing the number of phenotypes observed.