Return of the Pea Plant A. By 1882, Walther Flemming had observed threadlike chromosomes in the nuclei of dividing cells. By 1887, August Weismann had suggested that meiosis halves the number of chromosomes when gametes are made. By 1900, Mendel's work was finally appreciated, namely his view that diploid cells have two units for each trait and the units segregate during gamete formation. Genes are units of information about heritable traits. Diploid organisms possess pairs of homologous chromosomes, which are alike in length, shape, and gene sequence.
Alleles are slightly different molecular forms of the same gene, which are shuffled during meiosis.
Crossing over between homologous chromosomes results in new combinations of alleles on the chromosomes. A chromosome's structure may change due to deletion, duplication, inversion, or translocation. B. Autosomes and Sex Chromosomes 1. There are two kinds of chromosomes in human cells. Sex chromosomes determine gender.