Compaction of the duplicated chromosomes during mitosis and meiosis results in the classic four-arm structure ( pictured to the right ).

As the cell prepares to divide, i. e. enters mitosis or meiosis, the chromatin packages more tightly to facilitate segregation of the chromosomes during anaphase.

They can be hybrids produced through crossover ( a form of genetic recombination ) of chromosomes, which takes place in meiosis.

* Homologous chromosomes, chromosomes in a biological cell that pair up ( synapse ) during meiosis

During mitosis and meiosis, the condensed chromosomes are assembled through interactions between nucleosomes and other regulatory proteins.

Male progeny prove that Flora's unfertilized eggs were haploid ( n ) and doubled their chromosomes later to become diploid ( 2n ) ( by being fertilized by a polar body, or by chromosome duplication without cell division ), rather than by her laying diploid eggs by one of the meiosis reduction-divisions in her ovaries failing.

Prior to the meiosis process the cell's chromosomes are duplicated by a round of DNA replication, creating from the maternal and paternal versions of each chromosome ( homologs ) two exact copies, sister chromatids, attached at the centromere region.

The significance of meiosis for reproduction and inheritance, however, was described only in 1890 by German biologist August Weismann, who noted that two cell divisions were necessary to transform one diploid cell into four haploid cells if the number of chromosomes had to be maintained.

In 1911 the American geneticist Thomas Hunt Morgan observed crossover in Drosophila melanogaster meiosis and provided the first genetic evidence that genes are transmitted on chromosomes.

Meiosis I separates homologous chromosomes, producing two haploid cells ( N chromosomes, 23 in humans ), so meiosis I is referred to as a reductional division.

However, after meiosis I, although the cell contains 46 chromatids, it is only considered as being N, with 23 chromosomes.

In meiosis II, an equational division similar to mitosis will occur whereby the sister chromatids are finally split, creating a total of 4 haploid cells ( 23 chromosomes, N )-two from each daughter cell from the first division ..

The normal separation of chromosomes in meiosis I or sister chromatids in meiosis II is termed disjunction.

Such hybrids are generally infertile, due to the two different sets of chromosomes being unable to pair up during meiosis.

Polyploidy is important in hybrids as it allows reproduction, with the two different sets of chromosomes each being able to pair with an identical partner during meiosis.

During meiosis, sex cell precursors have their number of chromosomes halved by randomly " choosing " one homologue, resulting in haploid gametes.

Zygoidy is the state where the chromosomes are paired and can undergo meiosis.

Polyploidy is important in hybrids as it allows reproduction, with the two different sets of chromosomes each being able to pair with an identical partner during meiosis.

In such organisms, a process called meiosis creates cells called gametes ( eggs or sperm ) that have only one set of chromosomes.

The chance of this is fairly high: ( where the " two times two " comes about from two rounds of meiosis with two chromosomes ); however, this probability declines markedly with chromosome number and so the actual composition of a hybrid will be increasingly closer to the predicted composition.

Van Beneden discovered how chromosomes combined at meiosis, during the production of gametes, and discovered and named chromatin.

Additionally, base pairs in chromosomes often undergo random mutations resulting in modified DNA ( and subsequently, new proteins and phenotypes ).

A gametophyte is the haploid, multicellular phase of plants and algae that undergo alternation of generations, with each of its cells containing only a single set of chromosomes.

For example, animals undergo an " open " mitosis, where the nuclear envelope breaks down before the chromosomes separate, while fungi such as Aspergillus nidulans and Saccharomyces cerevisiae ( yeast ) undergo a " closed " mitosis, where chromosomes divide within an intact cell nucleus.

Expression trees or computer programs evolve because the chromosomes undergo mutation and recombination in a manner similar to the canonical GA.

The X and Y chromosomes do not undergo crossover events, so they are excluded from the calculation.

In the endocycle ( endomitosis or endoreduplication ) chromosomes in a ' resting ' nucleus undergo reduplication, the daughter chromosomes separating from each other inside an intact nuclear membrane.

In an alternative self assembly model, microtubules undergo acentrosomal nucleation among the condensed chromosomes.

With the activation of telomerase, some types of cells and their offspring become immortal, that is, their chromosomes will not become unstable no matter how many cell divisions they undergo ( they bypass the Hayflick limit ), thus avoiding cell death as long as the conditions for their duplication are met.

The nuclei undergo S-phase ( DNA replication ) and sister chromatids get pulled apart and re-assembled into nuclei containing full sets of homologous chromosomes, but cytokinesis does not occur.

The telomeres are long regions of repetitive noncoding DNA that cap chromosomes and undergo partial degradation each time a cell undergoes division ( see Hayflick limit ).

A feature of chromosomes that are commonly found to undergo such translocations is that they possess an acrocentric centromere, partitioning the chromosome into a large arm containing the vast majority of genes, and a short arm with a much smaller proportion of genetic content.

In Drosophila melanogaster, for example, the chromosomes of the larval salivary glands undergo many rounds of endoreplication, to produce large amounts of glue before pupation.

X chromosomes in females undergo a process known as X inactivation.

Meiosis generates genetic diversity in two ways: ( 1 ) independent alignment and subsequent separation of homologous chromosome pairs during the first meiotic division allows a random and independent selection of each chromosome segregates into each gamete ; and ( 2 ) physical exchange of homologous chromosomal regions by homologous recombination during prophase I results in new combinations of DNA within chromosomes.

During prophase I, DNA is exchanged between homologous chromosomes in a process called homologous recombination.

Kinetochore ( bipolar spindles ) microtubules shorten, severing the recombination nodules and pulling homologous chromosomes apart.

Her work was groundbreaking: she developed the technique for visualizing maize chromosomes and used microscopic analysis to demonstrate many fundamental genetic ideas, including genetic recombination by crossing-over during meiosis — a mechanism by which chromosomes exchange information.

During meiosis and gametogenesis, homologous chromosomes pair and exchange genetic material by recombination, leading to the production of sperm or eggs with chromosome haplotypes containing novel genetic variation.

In allopolyploids, the homologous chromosomes within each parental sub-genome should pair faithfully during meiosis, leading to disomic inheritance ; however in some allopolyploids, the homoeologous chromosomes of the parental genomes may be nearly as similar to one another as the homologous chromosomes, leading to tetrasomic inheritance ( four chromosomes pairing at meiosis ), intergenomic recombination, and reduced fertility.

Meiotic recombination allows a more independent selection between the two alleles that occupy the positions of single genes, as recombination shuffles the allele content between homologous chromosomes.

Genes located in the proximal areas of chromosomes may be completely linked ( very closely spaced ), thus preventing or severely hampering genetic recombination, which is necessary to incorporate such blocks.

Chromosomal crossover refers to recombination between the paired chromosomes inherited from each of one's parents, generally occurring during meiosis.

The insertion of viral DNA into chromosomes and other forms of recombination can also require the action of topoisomerases.

Today, scientists understand that independent assortment occurs when the genes affecting the phenotypes are found on different chromosomes or separated by a great enough distance on the same chromosome that recombination occurs at least half of the time.

A genetic map is a map based on the frequencies of recombination between markers during crossover of homologous chromosomes.

The reason 0. 5 is used in the denominator is that any alleles that are completely unlinked ( e. g. alleles on separate chromosomes ) have a 50 % chance of recombination, due to independent assortment.

The recombination frequency will be 50 % when two genes are located on different chromosomes or when they are widely separated on the same chromosome.

Whereas genes located on different chromosomes assort independently and have a recombination frequency of 50 %, linked genes have a recombination frequency that is less than 50 %.

In 1931, McClintock and Harriet Creighton demonstrated that cytological recombination of marked chromosomes correlated with recombination of genetic traits ( genes ).