Question

Researchers cross a corn plant that is pure-breeding for the dominant traits colored aleurone (C1), full kernel (Sh), and waxy endosperm (Wx) to a pure-breeding plantwith the recessive traits colorless aleurone (c1), shrunken kernel (sh), and starchy (wx). The resulting {rm F}_{1} plants were crossed to pure-breeding colorless,shrunken, starchy plants. Counting kernels from about 30 ears of corn, the results are reported in the table.

Kernel Phenotype Number
Colored, shrunken, starchy 116
Colored, full, starchy 601
Colored, full, waxy 2538
Colored, shrunken, waxy 4
Colorless, shrunken starchy 2708
Colorless, full, starchy 2
Colorless, full, waxy 113
Colorless, shrunken, waxy 626
6708

Part C

Calculate the recombination fraction between the Cl-Sh gene pair.

Part D

Calculate the recombination fraction between the Sh-Wx gene pair.

Part E

Calculate the recombination fraction between the Cl-Wx gene pair

Part F

What is the interference value for this data set?

Answer 1

C and S: 100 * [(113+116+4+2)/6708] = 3.5 cM

S and W: 100 * [(626+601+4+2)/6708] = 18.4 cM

C and W: 100 * [(113+116+626+601)/6708] = 21.7 cM

Determine gene order. Since 21.7 cM is the largest distance, we conclude that C and W are the

outside genes, with S being in the middle:

____|______|___________________|_____

| 3.5 | 18.4 |

C SW

Note that 3.5 + 18.4 = 21.9 cM is slightly larger than the 21.7 cM distance calculated above. The

combined distance between C and S and S and W is a more accurate measure of distance

between C and W because it compensates for undetectable double-crossovers in each interval.

Finally, it is also possible to determine gene order without calculating map distances. The two

least frequent phenotype classes (Colored, plump, starchy and colorless, shrunken, waxy) had to

have been produced by two crossovers: one between C and S and another between S and W.

Because we know the parental types are C s W and c S w, a crossover in each interval (between

C and S and S and W) results in CSw and csw, which are the phenotypes of the two least

frequent progeny classes. Thus, S must be in the middle