<P> Homologous chromosomes do not function the same in mitosis as they do in meiosis . Prior to every single mitotic division a cell undergoes, the chromosomes in the parent cell replicate themselves . The homologous chromosomes within the cell will ordinarily not pair up and undergo genetic recombination with each other . Instead, the replicants, or sister chromatids, will line up along the metaphase plate and then separate in the same way as meiosis II - by being pulled apart at their centromeres by nuclear mitotic spindles . If any crossing over does occur between sister chromatids during mitosis, it does not produce any new recombinant genotypes . </P> <P> Homologous pairing in most contexts will refer to germline cells, however also takes place in somatic cells . For example, in humans, somatic cells have very tightly regulated homologous pairing (separated into chromosomal territories, and pairing at specific loci under control of developmental signalling). Other species however (notably Drosophila) exhibit homologous pairing much more frequently . Various functions of homologous pairing in somatic cells have been elucidated through high - throughput screens in the early 21st century . </P> <P> There are severe repercussions when chromosomes do not segregate properly . Faulty segregation can lead to fertility problems, embryo death, birth defects, and cancer . Though the mechanisms for pairing and adhering homologous chromosomes vary among organisms, proper functioning of those mechanisms is imperative in order for the final genetic material to be sorted correctly . </P> <P> Proper homologous chromosome separation in meiosis I is crucial for sister chromatid separation in meiosis II . A failure to separate properly is known as nondisjunction . There are two main types of nondisjunction that occur: trisomy and monosomy . Trisomy is caused by the presence of one additional chromosomes in the zygote as compared to the normal number, and monosomy is characterized by the presence of one fewer chromosome in the zygote as compared to the normal number . If this uneven division occurs in meiosis I, then none of the daughter cells will have proper chromosomal distribution and severe effects can ensue, including Down's syndrome . Unequal division can also occur during the second meiotic division . Nondisjunction which occurs at this stage can result in normal daughter cells and deformed cells . </P>

A duplicated homologous chromosome pair contains how many possible alleles