How many gametes result from meiosis

The offspring of two different parents would be a blend of both. This was unlikely because, although it explains why individual look like their parents, it does not explain differences among individuals. After a few generations of blending every one would look the same. Mendel developed the idea of particulate inheritance. In this model the parents pass discrete, inheritable units to their offspring. These units are what we call genes.

Mendel worked with pea plants and as a monk he had a lot of time. He may not have needed to be a monk if he knew about fruit flies. Terminology: Character is the heritable feature of interest. Trait is one of the variants of the character. Mendel studied peas. They can have purple or white flowers.

So flower color is the character, purple is a trait. White is also a trait. Mendel crossed pea plants with different traits and looked at the offspring.

He chose to examine only characters that had "either-or" type variation 9. He started with "true breeding plants", for example he used purple flowered plants that, when self pollinated, gave rise to only purple flowered plants. He learned most of the important stuff by looking not at the offspring of the first generation but at their offspring.

Crossing purple flowered plants with white flowered plants was done by taking the pollen from the purple plants and putting it on the flowers of the white plants after first removing the pollen making structures of the white flowers 9. The seeds from this cross pollination were planted and the types of offspring were noted. They were all purple.

The white did not disappear, however. When this first generation F1 was mated some white flowered plants appeared in their offspring 9. Imagine trying to figure this out without the knowledge that we have today. Mendel reasoned that each plant got something from each parent during fertilization. So during the first cross each offspring plant got some white and some purple but the purple was what he called the "dominant" trait. We call the different versions of a gene in this case the flower color gene alleles.

In his first crosses he got plants with white flower alleles and purple flower alleles. We know that each of those alleles was on one half of a homologous pair of chromosomes Mendel didn't. When the purple flowers of the first generation were crossed with each other they yielded one white flowering plant for every three purple flowering plants 9.

From this evidence he developed the following four ideas:. Alternative versions of genes account for the variations in inherited characters. We call those alternative versions alleles. We now know that DNA at the same locus on each of a homologous pair of chromosomes can have different information.

For each character, an organism inherits two genes, one from each parent. Mendel didn't even know what you know about meiosis. You know that diploid organisms get one of each chromosome from the parents and that's how we get two alleles for each character. If the two alleles differ, then one, the d ominant allele , is expressed in the organisms appearance.

The recessive allele does not show up. The two alleles for each character segregate during gamete production. So if an individual has a dominant allele and a recessive allele, the gametes may get either one; they will separate. The gametes could have either the dominant or the recessive allele. This is called Mendel's law of segregation. Some more terminology: By convention, we use an upper case letter to represent the dominant allele and a lower case letter to represent the recessive allele.

An individual with two of the same alleles is called homozygous for that character. If an individual possesses two different alleles we say it is heterozygous for that character.

Mendel had purple flowering plants that were "true breeders", that is, when self pollinated they always produced purple flowered plants. The plants were homozygous for the purple allele. The purple flowering plants in the first generation were heterozygous. They had white recessive alleles. The phenotype of both purple flowered plants was the same; they're genotypes differed. Phenotype can be determined by observation, it is the appearance of an individual. The genotype is the underlying genetic makeup of an individual.

Not all alleles are completely dominant or recessive. An example from the text is the color of snapdragons. When homozygous red flowers RR are crossed with homozygous white flowers rr the F1 generation is all pink. The colors appear to have "blended", but the genetic material, the genes, have not blended. This is an example of incomplete dominance. Evidence that the genes haven't blended can be found in the f2 generation 9.

Dominant alleles don't subdue the recessive alleles. Consider Tay-Sachs disease. People with the disease can't metabolize a lipid that accumulates in the brain. These people are homozygous recessives tt. Heterozygotes and homozygous dominant individuals Tt, TT appear normal. So we say T is dominant. What's really going on is that the T allele has the genetic information to produce the enzyme necessary to metabolize the deadly lipids.

Then in anaphase II, the chromosomes separate at the centromeres. The spindle fibers pull the separated chromosomes toward each pole of the cell. Finally, during telophase II, the chromosomes are enclosed in nuclear membranes. Cytokinesis follows, dividing the cytoplasm of the two cells. At the conclusion of meiosis, there are four haploid daughter cells that go on to develop into either sperm or egg cells. Further Exploration Concept Links for further exploration cell division replication metaphase anaphase telophase linkage chromosome cytokinesis haploid prometaphase principle of segregation principle of independent assortment spindle fibers gamete DNA chromatin nucleus cytoplasm eukaryote prophase recombination principle of segregation Principles of Inheritance.

Related Concepts You have authorized LearnCasting of your reading list in Scitable. Do you want to LearnCast this session? This article has been posted to your Facebook page via Scitable LearnCast. Change LearnCast Settings. Scitable Chat. Register Sign In. This migration of the chromosomes is followed by the final and brief step of meiosis I, telophase I, which, coupled with cytokinesis physical separation of the entire mother cell , produces two daughter cells. Each of these daughter cells contains 23 dyads, which sum up to 46 monads or single-stranded chromosomes.

Meiosis II follows with no further replication of the genetic material. The chromosomes briefly unravel at the end of meiosis I, and at the beginning of meiosis II they must reform into chromosomes in their newly-created cells. This brief prophase II stage [isEmbeddedIn] is followed by metaphase II, during which the chromosomes migrate toward the metaphase plate.

During anaphase II, the spindle fibers again pull the chromosomes apart to opposite poles of the cell; however, this time it is the sister chromatids that are being split apart, instead of the pairs of homologous chromosomes as in the first meiotic step. A second round of telophase this time called telophase II and cytokinesis splits each daughter cell further into two new cells. Each of these cells has 23 single-stranded chromosomes, making each cell haploid possessing 1N chromosomes.

As mentioned, sperm and egg cells follow roughly the same pattern during meiosis , albeit a number of important differences. Spermatogenesis follows the pattern of meiosis more closely than oogenesis, primarily because once it begins human males start producing sperm at the onset of puberty in their early teens , it is a continuous process that produces four gametes per spermatocyte the male germ cell that enters meiosis.

Excluding mutation and mistakes, these sperm are identical except for their individual, unique genetic load. They each contain the same amount of cytoplasm and are propelled by whip-like flagella.

In females, oogenesis and meiosis begin while the individual is still in the womb. The primary oocytes, analogous to the spermatocyte in the male, undergo meiosis I up to diplonema in the womb , and then their progress is arrested. Once the female reaches puberty, small clutches of these arrested oocytes will proceed up to metaphase II and await fertilization so that they may complete the entire meiotic process; however, one oocyte will only produce one egg instead of four like the sperm.

This can be explained by the placement of the metaphase plate in the dividing female germ cell. Instead of lying across the middle of the cell like in spermatogenesis, the metaphase plate is tucked in the margin of the dividing cell, although equal distribution of the genetic material still occurs.

This results in a grossly unequal distribution of the cytoplasm and associated organelles once the cell undergoes cytokinesis. This first division produces a large cell and a small cell. The large cell, the secondary oocyte , contains the vast majority of the cytoplasm of the parent cell, and holds half of the genetic material of that cell as well. The small cell, called the first polar body, contains almost no cytoplasm, but still sequesters the other half of the genetic material.

This process repeats in meiosis II, giving rise to the egg and to an additional polar body. These differences in meiosis reflect the roles of each of the sex cells. Sperm must be agile and highly motile in order to have the opportunity to fertilize the egg—and this is their sole purpose. For this reason, they hardly carry any cellular organelles excluding packs of mitochondria which fuel their rapid motion , mostly just DNA.

For this reason, only a single, well-fortified egg is produced by each round of meiosis. Meiosis is a process that is conserved, in one form or another, across all sexually-reproducing organisms. This means that the process appears to drive reproductive abilities in a variety of organisms and points to the common evolutionary pathway for those organisms that reproduce sexually.