Only diploid have dominant and recessive traits?

In a sense, you're right. However, nondisjunction can occur during meiosis (mitosis too) whereby the daughter cell can contain too many (trisomy) or too few alleles (monosomy). For example, in Turner Syndrome the female genotype is XO, as the sperm cell lost the Y or X allele during formation due to nondisjunction. In this syndrome the phenotype is female (though shorter than average) and they are infertile. In Klinefelter syndrome the males are genotypically XXY (trisomy). They have male characteristics yet tend to have enlarged , little facial hair and low sperm count. So, as you see, even a haploid cell (such as occurs in Turner's syndrome) can produce traits. According to the site below, XO is the only case where monosomy results in a viable fetus. To be more exact then, it is better to say that diploid cells contain genes capable of producing a dominant or recessive trait (phenotype) while haploid cells contribute either a dominant or recessive allele to the genotype upon fertilization.