A homologous chromosome is one of a pair of corresponding chromosomes that normally carry the same genes at equivalent positions. In a diploid organism produced by sexual reproduction, one member of each pair is ordinarily inherited from each parent. These chromosomes, also called homologs, are similar but need not have identical DNA sequences. Their pairing and separation during meiosis are central to the transmission of genetic information between generations. (ncbi.nlm.nih.gov)
Genetic correspondence
Homologous chromosomes carry corresponding genes at matching positions, called loci. The versions of a gene or DNA sequence at a particular locus are its alleles. Consequently, two homologs can carry different alleles while remaining homologous chromosomes: correspondence concerns their genetic organization, not complete sequence identity. (medlineplus.gov)
For example, one homolog may carry allele A at a locus, while the other carries allele a. The individual is then heterozygous at that locus. If both carry the same allele, the individual is homozygous there. These descriptions apply to particular loci, rather than implying that the entire chromosome pair is identical or different. (genome.gov)
Homologs do not have to be physically attached to qualify as homologous. In most cells, they exist as separate chromosomes; their close association during meiosis is a specialized stage of chromosome behavior. (ncbi.nlm.nih.gov)
Homologous chromosomes and sister chromatids
Homologous chromosomes must be distinguished from sister chromatids. Homologs are the corresponding chromosomes inherited from the two parents. Sister chromatids are the two copies produced when a single chromosome undergoes DNA replication. They initially remain closely associated, including around the centromere. Thus, replication of a homologous pair produces two replicated chromosomes containing four chromatids altogether. (ncbi.nlm.nih.gov)
This distinction explains the two divisions of meiosis:
- Meiosis I separates the two homologous chromosomes into different daughter cells.
- Meiosis II separates the sister chromatids of each replicated chromosome.
No additional round of DNA replication occurs between these divisions. A cell that has completed meiosis I therefore has one chromosome from each homologous pair, but each chromosome still consists of two chromatids. (ncbi.nlm.nih.gov)
In ordinary mitosis, sister chromatids separate without the characteristic pairing of homologous chromosomes seen in meiotic prophase. (ncbi.nlm.nih.gov)
Pairing, recombination, and segregation
During prophase I of meiosis, replicated homologs align and undergo close pairing, termed synapsis. The resulting association is called a bivalent, emphasizing its two chromosomes, or a tetrad, emphasizing its four chromatids. (ncbi.nlm.nih.gov)
In many organisms, a synaptonemal complex assembles between the paired homologs. This protein structure organizes their close alignment. Pairing and synapsis are related but distinguishable processes: experimental studies in the nematode Caenorhabditis elegans, for example, show that chromosome alignment can be genetically separated from assembly of the synaptonemal complex. (ncbi.nlm.nih.gov)
Crossing over exchanges corresponding DNA segments between nonsister chromatids of homologous chromosomes. It can create chromosomes containing new combinations of alleles that were previously carried on different parental homologs. This exchange is one outcome of homologous recombination. (genome.gov)
Crossovers also have a mechanical role. Together with sister-chromatid cohesion, they help hold homologs together until their separation during meiosis I. These connections promote reliable segregation in most organisms, although the detailed mechanisms differ among species. (ncbi.nlm.nih.gov)
Genes close together on a chromosome tend to remain associated because a crossover is less likely to occur between them. This relationship underlies genetic linkage and the use of recombination frequencies to map relative gene positions. Homologous chromosomes therefore provide both a mechanism for transmitting genes and an opportunity to reshuffle their combinations. (ncbi.nlm.nih.gov)
Human chromosomes and the sex-chromosome exception
Most human body cells have 46 chromosomes, organized into 22 pairs of autosomes and two sex chromosomes. The two copies of each autosome are homologs. Human egg and sperm cells normally contain a single set of 23 chromosomes rather than both members of each pair. (genome.gov)
In the usual XX chromosome complement, the two X chromosomes form a homologous pair. The X and Y chromosome in the usual XY complement are different over much of their lengths, so they are not a fully corresponding pair in the same sense as two autosomes. Nevertheless, they share homologous segments called pseudoautosomal regions. (medlineplus.gov)
The human pseudoautosomal region PAR1 supports X–Y crossing over during sperm formation. This exchange helps the sex chromosomes segregate correctly, illustrating that a limited region of homology can support meiotic pairing between otherwise substantially different chromosomes. (pubmed.ncbi.nlm.nih.gov)
Historical and practical significance
Chromosome behavior provided a physical explanation for patterns of inheritance described by Gregor Mendel. In 1902, Walter Sutton reported observations showing that gametes receive one chromosome of each type; in 1903, he developed the connection between chromosome segregation and Mendelian inheritance. Theodor Boveri’s earlier studies of chromosome numbers in reproductive cells also contributed to the chromosome theory of heredity. (genome.gov)
In cytogenetics, chromosomes are arranged into a karyotype so that their number, size, and shape can be examined. Comparing corresponding chromosomes helps identify numerical and structural differences. Such examination does not, by itself, establish that homologs have identical DNA sequences. (medlineplus.gov)
Errors in chromosome separation, called nondisjunction, can produce reproductive cells with abnormal chromosome numbers. If such a cell participates in fertilization, the resulting chromosome complement may contain an extra or missing chromosome—an aneuploidy. Trisomy 21, a common chromosomal cause of Down syndrome, is one example of an extra chromosome arising through this mechanism. (medlineplus.gov)
References
- Meiosis — Molecular Biology of the Cellncbi.nlm.nih.gov
- Diploidgenome.gov
- Genetics: MedlinePlus Medical Encyclopediamedlineplus.gov
- Allelegenome.gov
- Homozygousgenome.gov
- Meiosis — Developmental Biologyncbi.nlm.nih.gov
- Meiosis — WormBookncbi.nlm.nih.gov
- Crossing Overgenome.gov
- General Recombination — Molecular Biology of the Cellncbi.nlm.nih.gov
- Mapping Genomes — Genomesncbi.nlm.nih.gov
- How many chromosomes do people have?medlineplus.gov
- The origin and evolution of the pseudoautosomal regions of human sex chromosomespubmed.ncbi.nlm.nih.gov