The diploid number of chromosomes in the body cells of an animal is usually due to the fusion of sperm and egg during fertilization.
The gametes can contain the wrong number of chromosomes if the meiotic events go wrong.
The consequences of this possibility are discussed in section 10.
Meiosis provides a way to keep the number constant.
Genetic variation is ensured by the events of meiosis.
The ability to evolve and adapt in a changing environment is dependent on genetic variation.
Asexually reproducing organisms depend on the creation of variation among offspring.
They produce a lot of offspring very quickly, so this is enough for their survival.
The reshuffling of genetic material during sexual reproduction ensures that offspring will have a different combination of genes than their parents.
Genetics is brought about by crossing-over and independent assortment of chromosomes.
During meiosis, it is estimated that there are two to three crossovers between the nonsister chromatids.
At synapsis, the homologues line up side by side and there is a lattice between them.
The lattice holds the bivalent together in a way that the duplicated chromosomes of each pair are aligned.
The genes on the nonsister chromatids are directly aligned.
It is possible that crossing-over will occur.
After the exchange of genetic information between the chromatids, the homologues are distributed to different daughter cells.
There is a lattice of chromosomes between them.
The electron micrograph is a representation of the lattice.
The genes on the pairs of chromosomes are in alignment.
There are only two places where nonsister chromatids 1 and 3 have come in contact.
Where crossing-over occurred is indicated by chiasmata.
The exchange of color is related to the exchange of genetic material.
During meiosis I, there was a new combination of genetic material on the daughter's chromosomes.
The process of crossing-over plays an important role in the generation of genetic variation for sexually reproducing species.
New individuals from unfertilized eggs are called parthenogenesis.
Parthenogenesis is a form of reproduction in which only one parent contributes genetic information to the next generation.
These species don't have the same generation times as other asexual organisms.
On the surface, parthenogenesis seems to limit the amount of genetic variation in the species, and thus reduce the ability of the species to respond to changes in its environment.
In the whiptail lizard, crossing over occurs between the sisters.
The whiptail lizard has variations in the process of meiosis that allow it to increase genetic variation with each generation.
Many species that undergo parthenogenesis require diploid (2n) gametes, which can be produced by doubling the reduction division in meiosis.
The sister chromatids themselves can be crossed over by the species.
Since there are always slight differences in the sister chromatids, small amounts of variation are maintained in the genome, and this is passed on to the next generation.
The amount of genetic variation may be small, but this variation in meiosis allows for some level of genetic recombination, thus providing genetic variation to the species.
The members of a pair can carry slightly different instructions for the same genetic trait.
The chromatids held together by a centromere are no longer identical due to a swapping of genetic material.
During meiosis II, some of the daughter cells receive daughter chromosomes with recombined alleles.
The offspring have a different set of genes than their parents.
The genetic variation of the offspring is increased by this.
The Nature of Science feature, "Meiosis and the Parthenogenic Lizards," shows how a special form of crossing-over can increase diversity during asexual reproduction.
The maternal or paternal homologue may be oriented toward either pole when they align at the metaphase plate.
Once all possible alignments of independent assortment are considered for these three pairs, the result will be 23, or 8, combinations of maternal and paternal chromosomes in the resulting gametes from this cell.
The combinations are shown in each cell.
The possible combinations of the gametes in humans is over 8 million.
Fertilization enhances the variation that results from meiosis.
In humans, the parents' donated chromosomes are combined, which means there are potentially more than 70 million different combinations of the same chromosomes.
The number assumes that there was no crossing over between the nonsister chromatids.
If a single crossingover event occurs, there are genetically different zygotes for every couple.
Increasing genetic variation within a population is important to the long-term survival of a species.
The process of sexual reproduction brings about genetic recombinations.
If the environment stays the same, asexual reproduction may be beneficial.
If the environment changes, genetic variability among offspring may be beneficial.
Some offspring may have a better chance of survival under the new conditions.
The ambient temperature could rise due to climate change.
The change in the environment could have an effect on the physiology of an animal.
A reduced body fat animal could have an advantage over other individuals of its generation.
In a changing environment, sexual reproduction with its reshuffling of genes due to meiosis and fertilization, might give a few offspring a better chance to survive and reproduce, thereby increasing the possibility of passing on their genes to the next generation.