Homologous chromosome

Homologous chromosomes are chromosome pairs of the same length, centromere position, and staining pattern, with genes for the same characteristics at corresponding loci. One homologous chromosome is inherited from the organism's mother; the other from the organism's father.

Homologous chromosomes pair (synapse) during meiosis - the cell division that occurs as part of the creation of gametes.

Each chromosome pair contains genes for the same biological features, such as eye color, at the same locations (loci) on the chromosome. Each pair, however, can contain the same allele (both alleles for blue eyes) or different alleles (one allele for blue eyes and one allele for brown eyes) for each feature.

Homologous chromosomes are similar in length, except for sex chromosomes in several taxa, where the X chromosome is considerably larger than the Y chromosome. These chromosomes share only small regions of homology.

Humans have 22 pairs of homologous non-sex chromosomes (called autosomes), and one pair of sex chromosomes, making a total of 46 chromosomes in a genetically normal human. Each member of a pair is inherited from one of the two parents. In addition to the 22 pairs of homologous autosomes, female humans have a homologous pair of sex chromosomes (two Xs), while males have an X and a Y chromosome.

Non-homologous chromosomes
Non-homologous chromosomes representing all the biological features of an organism form a set, and the number of sets in a cell is called ploidy. In diploid organisms (most plants and animals), each member of a pair of homologous chromosomes is inherited from a different parent. But polyploid organisms have more than two homologous chromosomes.

Sister chromatids
In meiosis sister chromatids are each strand of the conjoined chromosomes which result from the replication and crossover of homologous chromosomes during that process, prior to separation during each of the two anaphases. This causes each of the four resultant gametes to have a unique genetic combination from both parents, providing the genetic variation upon which natural selection (the driver of the process of evolution) depends.

(Sister chromatids in mitosis are each strand of the two identical conjoined chromosomes (the product of chromosome replication) prior to separation during the anaphase.)