In field corn, kernel color is deterrmined by a single autosomal gene
with two alleles. Yellow kernel color is dominant to purple color. An
autosomal gene also determines kernel texture with two alleles, where
smooth kernel is dominant to wrinkle texture.
Iff a heterozygous yellow colored plant, also heterozygous for smooth
kernel is crossed to a plant with purple and wrinkled kernels, what
proportion of the offsprings will have yellow and smooth kernels?

OK, so let's say that 

Y is the gene for yellow kernels
p is the gene for purple kernels

S is the gene for smooth kernels, and
w is the gene for wrinkled kernels.

a heterozygous yellow colored plant, also heterozygous for smooth
kernels, would then have the genotype YpSw, and the plant with purple
and wrinkled kernels would have genotype ppww.

so the offspring would always get pw genes from the purple/wrinkled
plant, while from the other plant they'd have a 50% chance of getting
Y vs. p, and also 50% of getting S vs. w, so all together that's

one-fourth  YS
one-fourth  pS
one-fourth  Yw
one-foutrth pw

so, when you combine the genes, you'd get

          genotype        phenotype
one-fourth  YpSw         yellow, smooth
one-fourth  ppSw         purple, smooth
one-fourth  Ypww         yellow, wrinkled
one-foutrth ppww         purple, wrinkled

in other words, you would expect to have equal numbers of the four
possible genotypes; in particular, one-fourth would have yellow,
smooth kernels.

by searching for "genotype phenotype heterozygous" I found some
further discussions of these types of problems, for instance at
http://dtc.pima.edu/biology/181/A8step5/A8step5page2.html,
http://www.riverdeep.net/science/biology_gateways/bg_handouts/gen/gend3tni.pdf,
or http://ag.arizona.edu/ANS/ans102/lect13.ppt.

I hope my explanation makes sense to you, if not, feel free to ask for
a clarification.

Thanks,
David