Mendel and Meiosis Meiosis Genes, Chromosomes, and Numbers Organisms have tens of thousands of genes that determine their traits These genes are lined up on chromosomes (one can contain 1,000+ genes) In body cells of most plants and animals, chromosomes occur in pairs (one from mom, one from dad) Diploid: a cell with two of each kind of chromosome (2n) Organisms have two factors, called alleles, for each trait Organisms produce gametes that contain one of each kind of chromosome Haploid: a cell containing one of each kind of chromosome (n) Homologous chromosomes: the two chromosomes of each pair in a diploid cell Each member of a pair has genes for the same traits, arranged in the same order, but not necessarily identical