Editing Genetics (section)

=== 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>