Editing Mendelian inheritance (section)

==Mendel's genetic discoveries==
Five parts of Mendel's discoveries were an important divergence from the common theories at the time and were the prerequisite for the establishment of his rules.

# Characters are unitary, that is, they are discrete e.g.: purple ''vs''. white, tall ''vs''. dwarf. There is no medium-sized plant or light purple flower.
# Genetic characteristics have alternate forms, each inherited from one of two parents. Today these are called [[allele]]s.
# One allele is dominant over the other.  The phenotype reflects the dominant allele.
# Gametes are created by random segregation. Heterozygotic individuals produce gametes with an equal frequency of the two alleles.
# Different traits have independent assortment. In modern terms, genes are unlinked.

According to customary terminology, the principles of inheritance discovered by Gregor Mendel are here referred to as Mendelian laws, although today's geneticists also speak of ''Mendelian rules'' or ''Mendelian principles'',<ref>Science Learning Hub: ''[https://www.sciencelearn.org.nz/resources/2000-mendel-s-principles-of-inheritance Mendel's principles of inheritance]''</ref><ref>Noel Clarke: ''[https://slideplayer.com/slide/4659559/ Mendelian Genetics - An overview]''</ref> as there are many exceptions summarized under the collective term [[Non-Mendelian inheritance]]. The laws were initially formulated by the geneticist [[Thomas Hunt Morgan]] in 1916.<ref>{{cite journal |url=https://onlinelibrary.wiley.com/doi/pdf/10.1002/evan.20192 |last=Marks |first=Jonarhan |title=The Construction of Mendel's Laws |journal=Evolutionary Anthropology |volume=17 |issue=6 |doi=10.1002/evan.20192 |date=22 December 2008|pages=250–253 }}</ref>
[[File:Gregor Mendel - characteristics of pea plants - english.png|thumb|Characteristics Mendel used in his experiments<ref>Gregor Mendel: ''[http://vlp.mpiwg-berlin.mpg.de/references?id=lit26745 Versuche über Pflanzenhybriden]'' Verhandlungen des Naturforschenden Vereines in Brünn. Bd. IV. 1866, page 8</ref>]]
[[File:Dominant-recessive inheritance - flowers of pea plants.png|thumb|P-Generation and F<sub>1</sub>-Generation: The dominant allele for purple-red flower hides the phenotypic effect of the recessive allele for white flowers. F<sub>2</sub>-Generation: The recessive trait from the P-Generation phenotypically reappears in the individuals that are homozygous with the recessive genetic trait.]]
[[File:Mendel-flowers.jpg|thumb|[[Myosotis]]: Colour and distribution of colours are inherited independently.<ref>Write Work: ''[https://www.writework.com/essay/mendel-s-impact Mendel's Impact]''</ref>]]

Mendel selected for the experiment the following characters of pea plants:
* Form of the ripe seeds (round or roundish, surface shallow or wrinkled)
* Colour of the [[Seed coat|seed–coat]] (white, gray, or brown, with or without violet spotting)
* Colour of the [[seed]]s and [[cotyledon]]s (yellow or green)
* Flower colour (white or violet-red)
* Form of the ripe pods (simply inflated, not contracted, or constricted between the seeds and wrinkled)
* Colour of the unripe pods (yellow or green)
* Position of the flowers (axial or terminal)
* Length of the stem <ref>[[Gregor Mendel]]: ''[http://www.esp.org/foundations/genetics/classical/gm-65.pdf Experiments in Plant Hybridization]'' 1965, page 5</ref>

When he crossed purebred white flower and purple flower pea plants (the parental or P generation) by [[Artificiality|artificial]] pollination, the resulting flower colour was not a blend. Rather than being a mix of the two, the offspring in the first generation ([[F1 generation|F<sub>1</sub>-generation]]) were all purple-flowered. Therefore, he called this [[trait (biology)|biological trait]] dominant. When he allowed [[self-fertilization]] in the uniform looking F<sub>1</sub>-generation, he obtained both colours in the F<sub>2</sub> generation with a purple flower to white flower ratio of 3 : 1. In some of the other characters also one of the traits was dominant.

He then conceived the idea of heredity units, which he called hereditary "factors". Mendel found that there are alternative forms of factors—now called [[gene]]s—that account for variations in inherited characteristics. For example, the gene for flower color in pea plants exists in two forms, one for purple and the other for white. The alternative "forms" are now called [[allele]]s. For each trait, an organism inherits two alleles, one from each parent. These alleles may be the same or different. An organism that has two identical alleles for a gene is said to be [[homozygous]] for that gene (and is called a homozygote). An organism that has two different alleles for a gene is said to be [[heterozygous]] for that gene (and is called a heterozygote).

Mendel hypothesized that allele pairs separate randomly, or segregate, from each other during the production of the [[gametes]] in the seed plant ([[egg cell]]) and the pollen plant ([[sperm]]). Because allele pairs separate during gamete production, a [[sperm]] or [[egg]] carries only one allele for each inherited trait. When sperm and egg unite at [[fertilization]], each contributes its allele, restoring the paired condition in the offspring. Mendel also found that each pair of alleles segregates independently of the other pairs of alleles during gamete formation.

The [[genotype]] of an individual is made up of the many alleles it possesses. The [[phenotype]] is the result of the [[Genetic expression|expression]] of all characteristics that are genetically determined by its alleles as well as by its environment. The presence of an allele does not mean that the trait will be expressed in the individual that possesses it. If the two alleles of an inherited pair differ (the heterozygous condition), then one determines the organism's appearance and is called the [[Dominance (genetics)|dominant allele]]; the other has no noticeable effect on the organism's appearance and is called the [[Dominance (genetics)|recessive allele]].