<P> Humans have a total of 46 chromosomes, but there are only 22 pairs of homologous autosomal chromosomes . The additional 23rd pair is the sex chromosomes, X and Y . If this pair is made up of an X and Y chromosome, then the pair of chromosomes is not homologous because their size and gene content differ greatly . The 22 pairs of homologous chromosomes contain the same genes but code for different traits in their allelic forms since one was inherited from the mother and one from the father . So humans have two homologous chromosome sets in each cell, meaning humans are diploid organisms . </P> <P> Homologous chromosomes are important in the processes of meiosis and mitosis . They allow for the recombination and random segregation of genetic material from the mother and father into new cells . </P> <P> Meiosis is a round of two cell divisions that results in four haploid daughter cells that each contain half the number of chromosomes as the parent cell . It reduces the chromosome number in a germ cell by half by first separating the homologous chromosomes in meiosis I and then the sister chromatids in meiosis II . The process of meiosis I is generally longer than meiosis II because it takes more time for the chromatin to replicate and for the homologous chromosomes to be properly oriented and segregated by the processes of pairing and synapsis in meiosis I. During meiosis, genetic recombination (by random segregation) and crossing over produces daughter cells that each contain different combinations of maternally and paternally coded genes . This recombination of genes allows for the introduction of new allele pairings and genetic variation . Genetic variation among organisms helps make a population more stable by providing a wider range of genetic traits for natural selection to act on . </P> <P> In prophase I of meiosis I, each chromosome is aligned with its homologous partner and pairs completely . In prophase I, the DNA has already undergone replication so each chromosome consists of two identical chromatids connected by a common centromere . During the zygotene stage of prophase I, the homologous chromosomes pair up with each other . This pairing occurs by a synapsis process where the synaptonemal complex - a protein scaffold - is assembled and joins the homologous chromosomes along their lengths . Cohesin crosslinking occurs between the homologous chromosomes and helps them resist being pulled apart until anaphase . Genetic crossing - over, a type of recombination, occurs during the pachytene stage of prophase I. In addition, another type of recombination referred to as synthesis - dependent strand annealing (SDSA) frequently occurs . SDSA recombination involves information exchange between paired homologous chromatids, but not physical exchange . SDSA recombination does not cause crossing - over . </P>

When are homologous chromosomes for the same trait separated from each other during meiosis