Homologous Chromosomes - A Genetics Definition

Normal Human Female Karyotype
Fluorescence light micrograph of the 46 chromosomes from a normal human female. This constitutes the full karyotype, that is the total number of chromosomes found in nearly every cell of the female human body. Each cell contains 22 matched pairs of chromosomes, and one sex-determining pair (bottom right). Female and male karyotypes differ only in the sex pair: male sets would be labeled XY instead of the X pair here. DEPT. OF CLINICAL CYTOGENETICS, ADDENBROOKES HOSPITAL/Science Photo Library/Getty Images

Definition: Homologous chromosomes are chromosome pairs (one from each parent) that are similar in length, gene position, and centromere location. The position of the genes on each homologous chromosome is the same, however the genes may contain different alleles. Chromosomes are important molecules as they contain DNA and the genetic instructions for the direction of all cell activity. They also carry genes that determine individual traits.

Homologous Chromosomes: Karyotype

A human karyotype shows the complete set of human chromosomes. Human cells contain 23 pairs of chromosomes for a total of 46. Each chromosome pair represents a set of homologous chromosomes. There are 22 pairs of autosomes and one pair of sex chromosomes. In males, the X and Y sex chromosomes are homologues. In females, both X chromosomes are homologues.

Homologous Chromosomes: Mitosis

During cell division, chromosomes must replicate to ensure that each cell has the correct number of chromosomes. In mitosis, homologous chromosomes replicate forming sister chromatids (identical copies of a replicated chromosome that are attached). Each sister chromatid is separated toward the end of mitosis by spindle fibers and divided between two daughter cells.

Homologous Chromosomes: Meiosis

During meiosis, the process for gamete formation, homologous chromosomes replicate forming sister chromatids.

Sister chromatids pair up forming what is called a tetrad. While in close proximity, homologous chromosomes sometimes exchange sections of DNA. This is known as genetic recombination. Homologous chromosomes separate during the first meiotic division and sister chromatids separate during the second division.

At the end of meiosis, four daughter cells are produced. Each cell is haploid and contains half the number of chromosomes as the original cell. Each chromosome has the appropriate number of genes, however the alleles for the genes are different. The swapping of genes during homologous chromosome recombination produces genetic variation in organisms that reproduce sexually. Upon fertilization, haploid gametes become a diploid organism.

Homologous Chromosomes: Nondisjunction

Occasionally, problems arise in cell division that cause cells to divide improperly. Failure of chromosomes to separate correctly in mitosis or meiosis is called nondisjunction. Should nondisjunction occur in the first meiotic division, homologous chromosomes remain paired resulting in two daughter cells with an extra set of chromosomes and two daughter cells with no chromosomes. Nondisjunction may also occur in meiosis II when sister chromatids fail to separate prior to cell division. Fertilization of these gametes produces individuals with either too many or not enough chromosomes.

Nondisjunction is often fatal or may produce chromosomal anomalies resulting in birth defects. In trisomy nondisjunction, cells contain an extra chromosome.

In humans, this means that there are 47 total chromosomes instead of 46. Trisomy is seen in Down syndrome where chromosome 21 has an additional or partial chromosome. Nondisjunction may also produce abnormalities in sex chromosomes. Monosomy is a type of nondisjunction in which only one chromosome is present. Females with Turner syndrome have only one X sex chromosome. Males with XYY syndrome have an extra Y sex chromosome. Nondisjunction in sex chromosomes typically has less severe consequences than nondisjunction in autosomal chromosomes (non-sex chromosomes).

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Bailey, Regina. "Homologous Chromosomes - A Genetics Definition." ThoughtCo, Apr. 13, 2017, thoughtco.com/homologous-chromosomes-definition-373469. Bailey, Regina. (2017, April 13). Homologous Chromosomes - A Genetics Definition. Retrieved from https://www.thoughtco.com/homologous-chromosomes-definition-373469 Bailey, Regina. "Homologous Chromosomes - A Genetics Definition." ThoughtCo. https://www.thoughtco.com/homologous-chromosomes-definition-373469 (accessed December 13, 2017).