Editing Mendelian inheritance (section)

==={{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>