Homologous chromosome

Crossing over takes place between the chromatids of the two homologous parent chromosomes. Maternal chromatids are red; paternal chromatids are blue. Lines point to the chiasmata (cross overs)

Homologous chromosomes are the pairs of chromosomes in a diploid organism. They are the chromosomes which pair during meiosis.^[1]

With the exception of the sex chromosomes, each pair has the gene loci in the same positions on each chromosome, and the centromere in the same position. They are not genetically identical because they will usually have different alleles (versions of a gene) at some of the loci.

The most important fact is that homologous chromosomes physically pair for meiosis, the cell division which produces the gametes. During this pairing, crossing over takes place between chromatids of each pair. This exchange of alleles means the offspring are not identical to each other, or to either parent. In other words, the process increases the genetic variation in the population.

Sex chromosomes only pair along those parts where there is similar genetic material (loci) on both members of the pair.

Homologous Chromosome Media

As this karyotype displays, a diploid human cell contains 22 pairs of homologous chromosomes and 2 sex chromosomes. The cell has two sets of each chromosome; one of the pair is derived from the mother and the other from the father. The maternal and paternal chromosomes in a homologous pair have the same genes at the same locus, but possibly different alleles.
During the process of meiosis, homologous chromosomes can recombine and produce new combinations of genes in the daughter cells.
Sorting of homologous chromosomes during meiosis.
1. Meiosis I 2. Meiosis II 3. Fertilization 4. Zygote*Nondisjunction is when chromosomes fail to separate normally resulting in a gain or loss of chromosomes. In the left image the blue arrow indicates nondisjunction taking place during meiosis II. In the right image the green arrow is indicating nondisjunction taking place during meiosis I.
Diagram of the general process for double-stranded break repair as well as synthesis-dependent strand annealing.

References

↑ King, Robert C; Stansfield, William D. & Mulligan, Pamela K. 2006. A dictionary of genetics. 7th ed, Oxford University Press, 211. ISBN 0-19-530761-5

[1] King, Robert C; Stansfield, William D. & Mulligan, Pamela K. 2006. A dictionary of genetics. 7th ed, Oxford University Press, 211. ISBN 0-19-530761-5

[1]