Glossary
Anaphase I
The phase in meiosis I where homologous chromosomes separate and move to opposite poles of the cell.
Example:
If nondisjunction happens during Anaphase I, both homologous chromosomes might move to the same pole, leading to gametes with an incorrect chromosome count.
Anaphase II
The phase in meiosis II where sister chromatids separate and move to opposite poles of the cell.
Example:
A failure of sister chromatids to separate during Anaphase II can also lead to nondisjunction, affecting only two of the four resulting gametes.
Crossing Over
The exchange of genetic material between non-sister chromatids of homologous chromosomes during prophase I of meiosis.
Example:
During crossing over, a segment of DNA from your mother's chromosome might swap places with the corresponding segment on your father's chromosome, creating a unique combination of alleles.
Down syndrome (Trisomy 21)
A genetic disorder caused by the presence of an extra copy of chromosome 21, typically resulting from nondisjunction during meiosis.
Example:
The characteristic features and developmental differences associated with Down syndrome are a direct result of having an extra chromosome 21.
Gamete
A haploid reproductive cell (sperm or egg) that contains a single set of chromosomes and is involved in sexual reproduction.
Example:
In humans, each gamete carries 23 chromosomes, half the number found in a typical body cell.
Genetic Diversity
The variation in the genetic makeup of individuals within a population or species.
Example:
The wide range of coat colors and patterns seen in dog breeds is a testament to the genetic diversity within the canine species.
Homologous Chromosomes
A pair of chromosomes, one inherited from each parent, that are similar in length, gene position, and centromere location.
Example:
You have 23 pairs of homologous chromosomes; for example, both chromosomes in pair 1 carry genes for the same traits, though they may have different versions (alleles).
Independent Assortment
The random orientation and separation of homologous chromosome pairs during metaphase I of meiosis, leading to different combinations of chromosomes in gametes.
Example:
Due to independent assortment, a gamete might receive the chromosome 7 from your mother and chromosome 12 from your father, or vice versa, in a completely random fashion.
Meiosis
A specialized type of cell division that reduces the chromosome number by half, creating four haploid cells from one diploid cell.
Example:
In humans, meiosis is the process that produces sperm and egg cells, each containing 23 chromosomes.
Metaphase I
The phase in meiosis I where homologous chromosome pairs align randomly at the metaphase plate.
Example:
The random lineup of chromosome pairs at the cell's equator during Metaphase I is the physical basis for independent assortment.
Metaphase II
The phase in meiosis II where sister chromatids align at the metaphase plate in each of the two haploid cells.
Example:
Although less impactful for genetic diversity than Metaphase I, the alignment of sister chromatids in Metaphase II ensures their proper separation into individual gametes.
Monosomy
A chromosomal condition characterized by the absence of one copy of a particular chromosome in an otherwise diploid cell.
Example:
Turner syndrome, where a female has only one X chromosome (XO), is an example of monosomy.
Nondisjunction
An error during meiosis where chromosomes or sister chromatids fail to separate properly, resulting in gametes with an abnormal number of chromosomes.
Example:
If nondisjunction occurs, a gamete might end up with an extra chromosome, leading to a genetic disorder like Down syndrome if fertilized.
Prophase I
The first and longest phase of meiosis I, characterized by chromosome condensation, pairing of homologous chromosomes, and the occurrence of crossing over.
Example:
The crucial event of crossing over, which shuffles genetic material, takes place during prophase I.
Random Fertilization
The unpredictable union of any single sperm with any single egg, dramatically increasing the genetic variation in offspring.
Example:
Even if two parents produce millions of unique gametes, random fertilization means that any one of those sperm can fertilize any one of those eggs, leading to immense diversity among siblings.
Trisomy
A chromosomal condition characterized by the presence of an extra copy of a particular chromosome in an otherwise diploid cell.
Example:
Individuals with Down syndrome exhibit trisomy 21, meaning they have three copies of chromosome 21 instead of the usual two.