Sister chromosome

A sister chromosome is one of the copies produced by replicating a particular chromosome. The term emphasizes shared replication origin, in contrast to homologous chromosomes, which may carry different parental alleles. Its exact counting convention must be stated: an ordinary replicated eukaryotic chromosome contains two sister chromatids, while chromosome copies produced by an additional genome-doubling cycle can participate as separate meiotic partners.

This distinction matters in some parthenogenesis mechanisms. A 2010 whiptail-lizard study found an extra premeiotic chromosome doubling and used fluorescence in situ hybridization probes to distinguish homologs. The paired meiotic partners were sister copies produced by the first of two premeiotic replication cycles. Observations of pairing structures and chiasmata showed that the system did not simply omit meiotic recombination.^[ref]

For a simplified one-locus example, label two homologs $A$ and $a$. An extra doubling supplies copies $A,A,a,a$. Pairing an $A$ copy with its $A$ sister and an $a$ copy with its $a$ sister permits the meiotic products to retain both allele types after reduction. Pairing $A$ with $a$ instead presents a different segregation and recombination problem. This explains why identifying which chromosomes pair is necessary; observing a doubled DNA amount alone does not establish the mechanism.

Sister copies are initially expected to carry the same inherited sequence, apart from replication changes. This does not make them two different parental homologs, nor imply that every parthenogenesis pathway preserves heterozygosity by the same process.