My boy is half brahma and he has a straight comb. It just depends on which genes are passed to each chick.
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Your brahma must carry one copy of the pea comb gene and one copy of the single comb gene, then
Pea comb is dominant, so in a brahma that has two copies (most should, hatchery stock notwithstanding since hatcheries don't exactly breed to the standard, lol) all its offspring, when crossed to a bird with a single comb (which is a recessive gene) should have pea combs.