Editing Mendelian inheritance (section)

==Mendel's laws of inheritance==
{| border="1" class="wikitable"
|+ Mendel's laws of inheritance
|-
! Law
! Definition
|-
| Law of dominance and uniformity
| Some alleles are dominant while others are recessive; an organism with at least one dominant allele will display the effect of the dominant allele.<ref name="Mendelian Principles">Rutgers: [http://lifesci.dls.rutgers.edu/~mcguire/Toolbox-Demo/Basic%20Genetics/Mendelian%20Principles.htm Mendelian Principles]</ref>
|-
| Law of segregation
| During gamete formation, the alleles for each gene segregate from each other so that each gamete carries only one allele for each gene.
|-
| Law of independent assortment
| Genes of different traits can segregate independently during the formation of gametes.
|}

==={{anchor|Law of Dominance and Uniformity}}Law of Dominance and Uniformity===
[[File:Dominant-recessive inheritance P - F1 - F2.png|thumb|F<sub>1</sub> generation: All individuals have the same genotype and same phenotype expressing the dominant trait (<span style="color:#990000;">red</span>).<br />F<sub>2</sub> generation: The phenotypes in the second generation show a 3 : 1 ratio.<br />In the genotype 25 % are homozygous with the dominant trait, 50 % are heterozygous  [[genetic carrier]]s of the recessive trait, 25 % are homozygous with the recessive genetic trait and [[Gene expression|expressing]] the recessive character.]]
[[File:Intermediate inheritance P - F1 - F2.png|thumb|In [[Mirabilis jalapa]] and [[Antirrhinum majus]] are examples for intermediate inheritance.<ref name="Mendelian Genetics">Biology University of Hamburg: ''[http://www1.biologie.uni-hamburg.de/b-online/e08/08a.htm Mendelian Genetics]''</ref><ref>[[Neil A. Campbell]], [[Jane B. Reece]]: Biologie. Spektrum-Verlag Heidelberg-Berlin 2003, {{ISBN|3-8274-1352-4}}, page 302–303.</ref> As seen in the F<sub>1</sub>-generation, heterozygous plants have "<span style="color:magenta;">light pink</span>" flowers—a mix of "<span style="color:#990000;">red</span>" and "white". The F<sub>2</sub>-generation shows a 1:2:1 ratio of <span style="color:#990000;">red</span>: <span style="color:magenta;">light pink</span>: <span style="color:black;">white.</span>]]
If two parents are mated with each other who differ in one [[genetic trait|genetic characteristic]] for which they are both [[homozygous]] (each pure-bred), all offspring in the first generation (F<sub>1</sub>) are equal to the examined characteristic in [[genotype]] and [[phenotype]] showing the dominant trait. This ''uniformity rule'' or ''reciprocity rule'' applies to all individuals of the F<sub>1</sub>-generation.<ref>Ulrich Weber: Biologie Gesamtband Oberstufe, 1st edition, Cornelsen Verlag Berlin 2001, {{ISBN|3-464-04279-0}}, page 170 - 171.</ref>

The principle of dominant inheritance discovered by Mendel states that in a heterozygote the dominant allele will cause the recessive allele to be "masked": that is, not expressed in the phenotype. Only if an individual is homozygous with respect to the recessive allele will the recessive trait be expressed. Therefore, a cross between a homozygous dominant and a homozygous recessive organism yields a heterozygous organism whose phenotype displays only the dominant trait.

The F<sub>1</sub> offspring of Mendel's pea crosses always looked like one of the two parental varieties. In this situation of "complete dominance", the dominant allele had the same phenotypic effect whether present in one or two copies.

But for some characteristics, the F<sub>1</sub> hybrids have an appearance ''in between'' the phenotypes of the two parental varieties. A cross between two four o'clock (''[[Mirabilis jalapa]]'') plants shows an exception to Mendel's principle, called ''incomplete dominance''. Flowers of heterozygous plants have a phenotype somewhere between the two homozygous genotypes. In cases of intermediate inheritance (incomplete dominance) in the F<sub>1</sub>-generation Mendel's principle of uniformity in genotype and phenotype applies as well. Research about intermediate inheritance was done by other scientists. The first was [[Carl Correns]] with his studies about Mirabilis jalapa.<ref name="Mendelian Genetics"/><ref>Biologie Schule - kompaktes Wissen: [http://www.biologie-schule.de/uniformitaetsregel.php Uniformitätsregel (1. Mendelsche Regel)]</ref><ref>Frustfrei Lernen: [https://www.frustfrei-lernen.de/biologie/uniformitaetsregel.html Uniformitätsregel (1. Mendelsche Regel)]</ref><ref>Spektrum Biologie: ''[http://www.spektrum.de/lexikon/biologie/unvollstaendige-dominanz/68637 Unvollständige Dominanz]''</ref><ref>Spektrum Biologie: ''[https://www.spektrum.de/lexikon/biologie/intermediaerer-erbgang/34304 Intermediärer Erbgang]''</ref>

==={{anchor|Law of Segregation}}Law of Segregation of genes===
[[File:Punnett square mendel flowers.svg|thumb|250px|left|A [[Punnett square]] for one of Mendel's pea plant experiments – [[self-fertilization]] of the F1 generation]]

The Law of Segregation of genes applies when two individuals, both heterozygous for a certain trait are crossed, for example, hybrids of the F<sub>1</sub>-generation. The offspring in the F<sub>2</sub>-generation differ in genotype and phenotype so that the characteristics of the grandparents (P-generation) regularly occur again. In a dominant-recessive inheritance, an average of 25% are homozygous with the dominant trait, 50% are heterozygous showing the dominant trait in the phenotype ([[genetic carrier]]s), 25% are homozygous with the recessive trait and therefore [[Gene expression|express]] the recessive trait in the phenotype. The genotypic ratio is 1: 2 : 1, and the phenotypic ratio is 3: 1.

In the pea plant example, the capital "B" represents the dominant allele for purple blossom and lowercase "b" represents the recessive allele for white blossom. The [[Gynoecium|pistil]] plant and the [[pollen]] plant are both F<sub>1</sub>-hybrids with genotype "B b". Each has one allele for purple and one allele for white. In the offspring, in the F<sub>2</sub>-plants in the Punnett-square, three combinations are possible. The genotypic ratio is 1 ''BB'' : 2 ''Bb'' : 1 ''bb''. But the phenotypic ratio of plants with purple blossoms to those with white blossoms is 3 : 1 due to the dominance of the allele for purple. Plants with homozygous "b b" are white flowered like one of the grandparents in the P-generation.

In cases of [[incomplete dominance]] the same segregation of alleles takes place in the F<sub>2</sub>-generation, but here also the phenotypes show a ratio of 1 : 2 : 1, as the heterozygous are different in phenotype from the homozygous because the [[Expression of genes|genetic expression]] of one allele compensates the missing expression of the other allele only partially. This results in an intermediate inheritance which was later described by other scientists.

In some literature sources, the principle of segregation is cited as the "first law". Nevertheless, Mendel did his crossing experiments with heterozygous plants after obtaining these hybrids by crossing two purebred plants, discovering the principle of dominance and uniformity first.<ref name="Neil A 2003, page 293–315">[[Neil A. Campbell]], [[Jane B. Reece]]: Biologie. Spektrum-Verlag 2003, page 293–315. {{ISBN|3-8274-1352-4}}</ref><ref name="Mendelian Principles"/>

Molecular proof of segregation of genes was subsequently found through observation of [[meiosis]] by two scientists independently, the German botanist [[Oscar Hertwig]] in 1876, and the Belgian zoologist [[Edouard Van Beneden]] in 1883. Most alleles are located in [[chromosomes]] in the [[cell nucleus]]. Paternal and maternal chromosomes get separated in meiosis because during [[spermatogenesis]] the chromosomes are segregated on the four sperm cells that arise from one mother sperm cell, and during [[oogenesis]] the chromosomes are distributed between the [[Polar body|polar bodies]] and the [[egg cell]]. Every individual organism contains two alleles for each trait. They segregate (separate) during meiosis such that each [[gamete]] contains only one of the alleles.<ref name="Bailey-2015">{{Cite web|url=http://biology.about.com/od/geneticsglossary/g/law_of_segregation.htm|title=Mendel's Law of Segregation|date=5 November 2015|access-date=2 February 2016|website=about education|publisher=About.com|last=Bailey|first=Regina}}</ref> When the gametes unite in the [[zygote]] the alleles—one from the mother one from the father—get passed on to the offspring. An offspring thus receives a pair of alleles for a trait by inheriting [[homologous chromosome]]s from the parent organisms: one allele for each trait from each parent.<ref name="Bailey-2015" /> Heterozygous individuals with the dominant trait in the phenotype are [[genetic carrier]]s of the recessive trait.

==={{anchor|Law of Independent Assortment}}Law of Independent Assortment===
[[File:Independent assortment & segregation.svg|thumb|left|Segregation and independent assortment are consistent with the [[Boveri–Sutton chromosome theory|chromosome theory of inheritance]].]]
[[File:Dog coat colour genetics - Yorkshire Terrier - third Mendelian rule 2.png|thumb|Precondition for the example: Two parent dogs (P-generation) are homozygous for two different genetic traits. In each case one parent has the dominant, one the recessive allele. Their offsprings in the F<sub>1</sub>-generation are heterozygous at both loci and show the dominant traits in their phenotypes according to the law of dominance and uniformity.<br>  
 
Now two heterozygous mature individuals of such F<sub>1</sub>-generation are bred together. The dominant allele "E" (on the extension locus) provides black eumelanin in the coat. The recessive allele "e" (on the extension locus) hinders the storage of eumelanin in the coat, so only the pigments for the "Tan" colour are in the coat. The dominant allele S (on the S-locus) provides for the pigmentation of the entire coat. The recessive allele sP (on the S-locus) causes a white [[Piebald|Piebald spotting]].<ref>Genomia.cz: [https://www.genomia.cz/en/york Yorkshire Terrier].</ref> Now in the puppies in the '''F<sub>2</sub>-generation''' all combinations are possible. The Piebald spotting and the genes for the different colour pigments are inherited independently of each other.<ref>Anna Laukner: ''Die Genetik der Fellfarben beim Hund''. Kynos 2021, ISBN 978-3954642618.</ref> Average number ratio of phenotypes 9:3:3:1.<ref>Spectrum Dictionary of Biology: ''[http://www.spektrum.de/lexikon/biologie-kompakt/mendel-regeln/7470 Mendelian Rules]''</ref>]]
[[File:Independent assortment.svg|thumb|For example 3 pairs of homologous chromosomes allow 8 possible combinations, all equally likely to move into the gamete during [[meiosis]]. This is the main reason for independent assortment. The equation to determine the number of possible combinations given the number of homologous pairs = 2<sup>x</sup> (x = number of homologous pairs)]]

The Law of Independent Assortment proposes alleles for separate traits are passed independently of one another.<ref>{{Cite news|url=http://biology.about.com/od/mendeliangenetics/ss/independent-assortment.htm#showall|title=Independent Assortment|access-date=24 February 2016|newspaper=Thoughtco|publisher=About.com|last=Bailey|first=Regina}}</ref><ref name="Neil A 2003, page 293–315"/> That is, the biological selection of an allele for one trait has nothing to do with the selection of an allele for any other trait. Mendel found support for this law in his dihybrid cross experiments. In his monohybrid crosses, an idealized 3:1 ratio between dominant and recessive phenotypes resulted. In dihybrid crosses, however, he found a 9:3:3:1 ratios. This shows that each of the two alleles is inherited independently from the other, with a 3:1 phenotypic ratio for each.

Independent assortment occurs in [[eukaryotic]] organisms during meiotic metaphase I, and produces a gamete with a mixture of the organism's chromosomes. The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent chromosome along the metaphase plate with respect to the other bivalent chromosomes. Along with [[chromosomal crossover|crossing over]], independent assortment increases genetic diversity by producing novel genetic combinations.

There are many deviations from the principle of independent assortment due to [[genetic linkage]].

Of the 46 chromosomes in a normal [[diploid]] human cell, half are maternally derived (from the mother's [[ovum|egg]]) and half are paternally derived (from the father's [[spermatozoon|sperm]]). This occurs as [[sexual reproduction]] involves the fusion of two [[haploid]] gametes (the egg and sperm) to produce a zygote and a new organism, in which every cell has two sets of chromosomes (diploid). During [[gametogenesis]] the normal complement of 46 chromosomes needs to be halved to 23 to ensure that the resulting haploid gamete can join with another haploid gamete to produce a diploid organism.

In independent assortment, the chromosomes that result are randomly sorted from all possible maternal and paternal chromosomes. Because zygotes end up with a mix instead of a pre-defined "set" from either parent, chromosomes are therefore considered assorted independently. As such, the zygote can end up with any combination of paternal or maternal chromosomes. For human gametes, with 23 chromosomes, the number of possibilities is 2<sup>23</sup> or 8,388,608 possible combinations.<ref>{{cite web | title=Meiosis | author=Perez, Nancy | url=http://www.web-books.com/MoBio/Free/Ch8C.htm | access-date=15 February 2007 }}</ref> This contributes to the genetic variability of progeny. Generally, the recombination of genes has important implications for many evolutionary processes.<ref>{{cite journal | pmc=5698631 | year=2017 | last1=Stapley | first1=J. | last2=Feulner | first2=P. G. | last3=Johnston | first3=S. E. | last4=Santure | first4=A. W. | last5=Smadja | first5=C. M. | title=Recombination: The good, the bad and the variable | journal=Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume=372 | issue=1736 | doi=10.1098/rstb.2017.0279 | pmid=29109232 }}</ref><ref>{{Cite journal |doi=10.1098/rspb.2016.1243 |doi-access=free|title=The evolution of recombination rates in finite populations during ecological speciation |year=2016 |last1=Reeve |first1=James |last2=Ortiz-Barrientos |first2=Daniel |last3=Engelstädter |first3=Jan |journal=Proceedings of the Royal Society B: Biological Sciences |volume=283 |issue=1841 |pmid=27798297 |pmc=5095376 }}</ref><ref>{{Cite journal |doi=10.1016/j.jtbi.2018.01.018 |doi-access=free|title=The advantage of recombination when selection is acting at many genetic Loci |year=2018 |last1=Hickey |first1=Donal A. |last2=Golding |first2=G. Brian |journal=Journal of Theoretical Biology |volume=442 |pages=123–128 |pmid=29355539 |bibcode=2018JThBi.442..123H }}</ref>

{{anchor|Law of Dominance}}<span id="Third Law"></span>