Editing Genetics (section)

== Features of inheritance ==

=== Discrete inheritance and Mendel's laws ===
{{Main|Mendelian inheritance}}
[[File:Punnett square mendel flowers.svg|right|thumb|A [[Punnett square]] depicting a cross between two pea plants heterozygous for purple (B) and white (b) blossoms]]
At its most fundamental level, inheritance in organisms occurs by passing discrete heritable units, called [[gene]]s, from parents to offspring.<ref name=griffiths2000sect199>{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W.H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.199 |chapter=Patterns of Inheritance: Introduction}}</ref> This property was first observed by Gregor Mendel, who studied the segregation of heritable traits in [[pea]] plants, showing for example that flowers on a single plant were either purple or white—but never an intermediate between the two colors. The discrete versions of the same gene controlling the inherited appearance (phenotypes) are called [[allele]]s.<ref name="mendel" /><ref name=griffiths2000sect200>{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W.H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.200 |chapter=Mendel's experiments}}</ref>

In the case of the pea, which is a [[diploid]] species, each individual plant has two copies of each gene, one copy inherited from each parent.<ref name=griffiths2000sect484 /> Many species, including humans, have this pattern of inheritance. Diploid organisms with two copies of the same allele of a given gene are called [[homozygous]] at that [[Locus (genetics)|gene locus]], while organisms with two different alleles of a given gene are called [[heterozygous]]. The set of alleles for a given organism is called its [[genotype]], while the observable traits of the organism are called its [[phenotype]]. When organisms are heterozygous at a gene, often one allele is called [[Dominant allele|dominant]] as its qualities dominate the phenotype of the organism, while the other allele is called [[Recessive allele|recessive]] as its qualities recede and are not observed. Some alleles do not have complete dominance and instead have [[Dominance relationship#Incomplete dominance|incomplete dominance]] by expressing an intermediate phenotype, or [[Dominance relationship#Co-dominance|codominance]] by expressing both alleles at once.<ref name=griffiths2000sect630>{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W.H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.630 |chapter=Interactions between the alleles of one gene}}</ref>

When a pair of organisms [[Sexual reproduction|reproduce sexually]], their offspring randomly inherit one of the two alleles from each parent. These observations of discrete inheritance and the segregation of alleles are collectively known as [[Mendelian inheritance#Law of Segregation|Mendel's first law]] or the Law of Segregation. However, the probability of getting one gene over the other can change due to dominant, recessive, homozygous, or heterozygous genes. For example, Mendel found that if you cross heterozygous organisms your odds of getting the dominant trait is 3:1. Real geneticist study and calculate probabilities by using theoretical probabilities, empirical probabilities, the product rule, the sum rule, and more.<ref>{{Cite web |title=Probabilities in genetics (article) |url=https://www.khanacademy.org/science/ap-biology/heredity/mendelian-genetics-ap/a/probabilities-in-genetics |access-date=2022-09-28 |website=Khan Academy}}</ref>

=== Notation and diagrams ===
[[File:Pedigree-chart-example.svg|thumb|left|upright=1.1|Genetic pedigree charts help track the inheritance patterns of traits.]]

Geneticists use diagrams and symbols to describe inheritance. A gene is represented by one or a few letters. Often a "+" symbol is used to mark the usual, [[Wild type|non-mutant allele]] for a gene.<ref>{{cite web|url=http://faculty.users.cnu.edu/rcheney/Genetic%20Notation.htm|title=Genetic Notation| vauthors = Cheney RW |publisher=Christopher Newport University|access-date=18 March 2008|archive-url=https://web.archive.org/web/20080103021518/http://faculty.users.cnu.edu/rcheney/Genetic%20Notation.htm|archive-date=3 January 2008}}</ref>

In fertilization and breeding experiments (and especially when discussing Mendel's laws) the parents are referred to as the "P" generation and the offspring as the "F1" (first filial) generation. When the F1 offspring mate with each other, the offspring are called the "F2" (second filial) generation. One of the common diagrams used to predict the result of cross-breeding is the [[Punnett square]].<ref name="Müller-Wille-Parolini_2020">{{cite book |chapter=Punnett squares and hybrid crosses: how Mendelians learned their trade by the book | vauthors = Müller-Wille S, Parolini G |publisher=[[British Society for the History of Science]] / [[Cambridge University Press]] |date=2020-12-09 |series=BJHS Themes |volume=5 |title=Learning by the Book: Manuals and Handbooks in the History of Science |pages=149–165 |doi=10.1017/bjt.2020.12 |s2cid=229344415 |url=https://www.cambridge.org/core/journals/bjhs-themes/article/punnett-squares-and-hybrid-crosses-how-mendelians-learned-their-trade-by-the-book/18A1CE37A6EE536CC1CE1D4FF6FF3174 |access-date=2021-03-29 |url-status=live |archive-url=https://web.archive.org/web/20210329111650/https://www.cambridge.org/core/journals/bjhs-themes/article/punnett-squares-and-hybrid-crosses-how-mendelians-learned-their-trade-by-the-book/18A1CE37A6EE536CC1CE1D4FF6FF3174 |archive-date=2021-03-29}}</ref>

When studying human genetic diseases, geneticists often use [[pedigree chart]]s to represent the inheritance of traits.<ref name=griffiths2000sect229>{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W.H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.229 |chapter=Human Genetics}}</ref> These charts map the inheritance of a trait in a family tree.

=== Multiple gene interactions ===
[[File:Galton-height-regress.png|thumb|right|Human height is a trait with complex genetic causes. [[Francis Galton]]'s data from 1889 shows the relationship between offspring height as a function of mean parent height.]]

Organisms have thousands of genes, and in sexually reproducing organisms these genes generally assort independently of each other. This means that the inheritance of an allele for yellow or green pea color is unrelated to the inheritance of alleles for white or purple flowers. This phenomenon, known as "[[Mendelian inheritance#Law of Independent Assortment (The "Second Law")|Mendel's second law]]" or the "law of independent assortment," means that the alleles of different genes get shuffled between parents to form offspring with many different combinations. Different genes often interact to influence the same trait. In the [[Omphalodes verna|Blue-eyed Mary]] (''Omphalodes verna''), for example, there exists a gene with alleles that determine the color of flowers: blue or magenta. Another gene, however, controls whether the flowers have color at all or are white. When a plant has two copies of this white allele, its flowers are white—regardless of whether the first gene has blue or magenta alleles. This interaction between genes is called [[epistasis]], with the second gene epistatic to the first.<ref name=griffiths2000sect644>{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W.H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.644 |chapter=Gene interaction and modified dihybrid ratios}}</ref>

Many traits are not discrete features (e.g. purple or white flowers) but are instead continuous features (e.g. human height and [[Human skin color|skin color]]). These [[complex traits]] are products of many genes.<ref>{{cite journal | vauthors = Mayeux R | title = Mapping the new frontier: complex genetic disorders | journal = The Journal of Clinical Investigation | volume = 115 | issue = 6 | pages = 1404–1407 | date = June 2005 | pmid = 15931374 | pmc = 1137013 | doi = 10.1172/JCI25421 }}</ref> The influence of these genes is mediated, to varying degrees, by the environment an organism has experienced. The degree to which an organism's genes contribute to a complex trait is called [[heritability]].<ref name=griffiths2000sect4009>{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W. H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.4009 |chapter=Quantifying heritability}}</ref> Measurement of the heritability of a trait is relative—in a more variable environment, the environment has a bigger influence on the total variation of the trait. For example, human height is a trait with complex causes. It has a heritability of 89% in the United States. In Nigeria, however, where people experience a more variable access to good nutrition and [[health care]], height has a heritability of only 62%.<ref>{{cite journal | vauthors = Luke A, Guo X, Adeyemo AA, Wilks R, Forrester T, Lowe W, Comuzzie AG, Martin LJ, Zhu X, Rotimi CN, Cooper RS | title = Heritability of obesity-related traits among Nigerians, Jamaicans and US black people | journal = International Journal of Obesity and Related Metabolic Disorders | volume = 25 | issue = 7 | pages = 1034–1041 | date = July 2001 | pmid = 11443503 | doi = 10.1038/sj.ijo.0801650 | doi-access = free }}</ref>