One lab in the genetics course I teach is devoted to epistasis, with the exercise centering on kernel color in maize. Over the years, I grew dissatisfied with the quality of the ears I acquired from commercial sources. The colors were often indistinct, and in some cases the phenotype ratios were not as advertised. I therefore decided to do my own crosses to produce ears that illustrated four epistatic interactions: (1) dominant, (2) recessive, (3) duplicate recessive, and (4) dominant and recessive.

In planning these crosses I was assisted by Phil Stinard of the Maize Genetics Cooperation Stock Center. Of the 12 crosses that I completed, only four will be described here. These involved the parental lines listed in Table 1, all of which were acquired from the Maize Genetics Cooperation Stock Center.

The parental lines were crossed in 1999, and the F1 plants were self-fertilized in 2000 to give F2 ears. Owing to the small size of these ears, new F1 crosses were re-done in 2001; these are the F2 ears that are reported here. Table 2 shows the results of the four crosses, and Figure 1 shows representative F2 ears from each cross. In each cross (except for cross 4; see below), there were two genes segregating that affect kernel color.

In cross 4, only white and purple kernels were expected in the F2 generation, but a small number (69 = 1.6%) of red kernels appeared. These red kernels are the result of the segregation of Pr1 alleles. The red kernels were unexpected because parental line 507A(y1) was believed to be Pr1, not pr1. There were thus three genes segregating in cross 4. When the red kernels are combined with the purple kernels, however, the expected F2 phenotype ratio for dominant and recessive epistasis is approximated.

As shown in Table 3, in all four crosses the observed ratio of F2 kernel phenotypes was very close to the expected ratio based on an assumption of recessive epistasis, dominant epistasis, duplicate recessive epistasis, and dominant and recessive epistasis between the two segregating genes (ignoring the third segregating gene in cross 4). These F2 ears are superior to those commercially available, and I have begun using them in my genetics labs. Future crosses will be aimed at getting a better example of dominant and recessive epistasis (the 13:3 ratio). In the meantime, I will make the F1 seeds available to anyone who would like them.

Table 1. Parental lines used in crosses to illustrate epistasis.
 

Stock No.	Genotype	Phenotype
ACRY	A1 A2 C1 R1 Pr1 Y1	purple
506B	A1 A2 C1 R1 pr1 y1	red
507A(Y1)	A1 a2 C1 R1 Pr1 Y1	yellow
507A(y1)	A1 a2 C1 R1 pr1 y1	white
906D	A1 A2 CI-1 R1 Pr1 y1	white
910D	A1 A2 c1 R1 Pr1 Y1	yellow

Table 2. Phenotypes in the F1 and F2 generations of crosses to illustrate epistasis.
 

Cross no.	Parental lines	F1 phenotype	F2 phenotypes*
1	906D x 506B	white	white	purple	red
			1809	448	166
2	906D x ACRY	yellow	yellow	white	purple
			1836	574	772
3	507A(Y1) x 910D	purple	purple	yellow
			2901	2195
4	906D x 507A(y1)	white	white	purple	red
			3530	657	69

*7, 6, 12, and 9 F2 ears were examined for crosses 1, 2, 3, and 4 respectively.

Table 3. Ratio of phenotypes in the F2 generation of crosses to illustrate epistasis.
 

Cross no.	Epistatic interaction	Expected F2 ratio	Observed F2 ratio
1	Dominant	12 white 3 purple 1 red	11.95 white 2.96 purple 1.10 red
2	Recessive	9 yellow 3 white 4 purple	9.23 yellow 2.89 white 3.88 purple
3	Duplicate recessive	9 purple 7 yellow	9.11 purple 6.89 yellow
4	Dominant and recessive	13 white 3 purple	13.27 white 2.73 purple+red

 
 

Cross 1 906D x 506B	Cross 2 906D x ACRY	Cross 3 507A(Y1) x 910D	Cross 4 906D x 507A(y1)

Figure 1. Representative F2 ears from crosses to illustrate epistasis.
 
 

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