Name: punnett squares for predicting traits
Uploaded: 2019-03-11T09:21:16.000Z
Duration: 1 min
Channel: JoVE

K12 Standards

Virginia  Standards of Learning

HIgh School

Life Science

LS.10

LS.10.c

Punnett Squares

- [Instructeur] Het Punnet vierkant, een eenvoudig diagram vernoemd naar de uitvinder, is een grafische manier om de waarschijnlijkheid te voorspellen dat nakomelingen een enkel-gen eigenschap kunnen erven, zoals lactose-intolerantie, gezien de ouderlijke genotypen. Met behulp van dit rooster worden de genetische bijdragen van één ouder die in dit geval homozygoot recessief is, met allelen die als twee kleine letters l's worden aangeduid, bovenaan geplaatst. Die van de andere, heterozygoot met gemengde allelen, vertegenwoordigd door een dominante hoofdletter L en een recessieve kleine letter l staan op de linkerzijde geschreven.

Het combineren van deze rij- en kolomkoppen in de dozen resulteert in twee mogelijke genotypen in beide fenotypen voor een kind. Een 50% kans op heterozygoot zijn, en in staat om lactose te verteren, en een 50% kans om homozygoot recessief en lactose-intolerant te zijn.

punnett-vierkanten

<h4>Overzicht</h4>

Een Punnett-vierkant geeft de mogelijke genotypen weer die nakomelingen kunnen erven van twee ouderlijke genotypen. Als het overervingspatroon van een eigenschap (bijvoorbeeld dominant of recessief) bekend is, kunnen Punnett-vierkanten ook worden gebruikt om de waarschijnlijkheid van het erven van een fenotype te bepalen. Punnett-vierkanten zijn toepasbaar in situaties waarin de overerving van eigenschappen wordt bepaald door een enkele genlocus en eigenschappen onafhankelijk worden geërfd, maar ze kunnen geen eigenschapskansen voorspellen voor complexere genetische overervingsscenario's.

<h4>Punnett-vierkanten geven de waarschijnlijkheid weer van het erven van een of meer eigenschappen</h4>

Punnett-vierkanten zijn visuele representaties die mogelijke genotypen van nakomelingen weergeven als resultaat van een kruising tussen twee ouderlijke genotypen. Ze kunnen de overerving van een of meerdere fenotypes of eigenschappen weergeven, hoewel andere hulpmiddelen geschikter zijn voor het onderzoeken van de overerving van meer dan twee eigenschappen.

Punnett-vierkanten kunnen worden gebruikt om de waarschijnlijkheid te bepalen dat nakomelingen een specifiek genotype erven, mits het fenotype wordt veroorzaakt door een enkele genlocus en onafhankelijk wordt geassorteerd tijdens de meiose. Dit betekent que Punnett-vierkanten nuttig zijn voor het bepalen van de overervingskansen in scenario's waarin de waarschijnlijkheid van het erven van één eigenschap geen invloed heeft op de waarschijnlijkheid van het erven van een andere eigenschap. Hoewel er veel uitzonderingen op deze aannames bestaan (bijvoorbeeld eigenschappen die worden gecodeerd door naburige genen op hetzelfde chromosoom), gelden deze kwalificaties voor veel planten- en diereneigenschappen.

<h4>De experimenten van Mendel inspireerden de creatie van het Punnett-vierkant</h4>

Punnett-vierkanten werden begin 20e eeuw gecreëerd door Reginald Punnett, enkele decennia nadat de baanbrekende experimenten met erwtenplanten van Gregor Mendel de fundamentele wetten van overerving aan het licht brachten. Tegenwoordig worden Punnett-vierkanten vaak gebruikt om de principes te illustreren die ten grondslag liggen aan de experimenten van Mendel.

Mendel bestudeerde de overerving van verschillende kenmerken van erwtenplanten, waaronder de vorm en kleur van de erwt en de peul, de bloemkleur en -positie en de plantgrootte. Voor elk kenmerk onderzocht Mendel ook erfelijke eigenschappen. Paars en wit zijn bijvoorbeeld mogelijke kenmerken voor de bloemkleureigenschap. Bij erwtenplanten worden paarse en witte bloemen bepaald door verschillende genvarianten, of allelen, op de locus van het bloemkleurgen.

<h4>Punnett-vierkanten zijn rasters die genetische informatie organiseren</h4>

Elk vak in een Punnett-vierkant vertegenwoordigt een mogelijke bevruchtingsgebeurtenis, of nakomelingengenotype, voortkomend uit twee ouderlijke gameten. Punnett-vierkanten zijn doorgaans gerangschikt in 2×2- of 4×4-configuraties om respectievelijk de overerving van één of twee eigenschappen te visualiseren.

Wat de nomenclatuur betreft, worden allelen aangegeven door de eerste letter van de eigenschap die wordt veroorzaakt door het dominante allel. Omdat geel bijvoorbeeld de dominante kleureigenschap van erwten is, worden allelen die voor de erwtenkleur coderen aangeduid met de cursieve letter y. Hoofdletters en kleine letters vertegenwoordigen respectievelijk dominante en recessieve allelen. Y vertegenwoordigt dus het dominante gele allel en y het recessieve groene allel.

Om een Punnett-vierkant van 2×2 te maken dat één eigenschap onderzoekt, wordt één ouderlijk genotype boven het diagram weergegeven, met één allel boven elke kolom. Het andere ouderlijke genotype wordt verticaal links van het diagram weergegeven, met één allel naast elke rij. Elke vierkante Punnett-box bevat de twee ouderlijke allelen (één van elke ouder) die overeenkomen met de rij en kolom van de box, wat één mogelijk bevruchtingsresultaat vertegenwoordigt. De volledige inhoud van het Punnett-vierkant kan worden gebruikt om de waarschijnlijkheid te bepalen dat nakomelingen een bepaalde eigenschap erven.

<h4>Punnett-vierkanten zijn informatieve hulpmiddelen voor genetische adviseurs en fokkers</h4>

Ondanks dat ze meer dan 100 jaar geleden zijn gemaakt, hebben Punnett-vierkanten nog steeds verschillende relevante toepassingen. Voor een echtpaar dat genetisch advies krijgt, kunnen Punnett-vierkanten helpen bij het bepalen van het risico van hun kind op een erfelijke ziekte. Als één ouder bijvoorbeeld cystische fibrose heeft (twee recessieve, causale allelen) en de andere ouder dit niet heeft of draagt, zal het kind drager zijn (d.w.z. één causaal allel hebben) maar geen risico lopen op cystische fibrose. Punnett-vierkanten kunnen ook dieren- en plantenveredelaars helpen bij het selecteren van organismen met specifieke eigenschappen voor verdere veredeling.

Hoewel Punnett-vierkanten in veel contexten nuttig zijn, kunnen ze de complexe genetische overerving niet accuraat weergeven. Eigenschappen die door naburige genen op hetzelfde chromosoom worden gecodeerd, worden bijvoorbeeld vaak samen van één ouder geërfd, een fenomeen dat koppeling wordt genoemd. Deze eigenschappen zijn niet onafhankelijk geassorteerd, dus een Punnett-vierkant kan hun overervingspatronen niet nauwkeurig voorspellen. Sommige eigenschappen, zoals lengte, zijn onmogelijk om met Punnett-vierkanten te voorspellen, omdat ze worden bepaald door meerdere genen y worden beïnvloed door omgevingsomstandigheden zoals voeding.

Punnett-vierkanten

Overview

A Punnett square displays the possible genotypes offspring can inherit from two parental genotypes. If a trait&rsquo;s inheritance pattern ...

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punnett squares for predicting traits

Punnett squares help predict which traits offspring may inherit from two parents. They show possible genotypes, which are pairs of alleles, and can also show the chance of a phenotype, or visible trait, when the inheritance pattern is known. These grids work best for traits controlled by a single gene locus and passed on independently during meiosis.

Punnett squares were developed in the early 1900s by Reginald Punnett. His work came after Gregor Mendel’s pea plant experiments revealed the basic laws of inheritance. Mendel studied traits such as pea and pod shape and color, flower color and position, and plant size. For flower color, purple and white are different traits controlled by different alleles at the same gene locus.

A Punnett square organizes genetic information in a grid. Each box represents one possible fertilization event, or one possible offspring genotype, from two parental gametes. These diagrams are often shown as 2x2 squares for one trait or 4x4 squares for two traits. Alleles are usually written with the first letter of the dominant trait. For pea color, Y stands for the dominant yellow allele, and y stands for the recessive green allele.

To build a 2x2 Punnett square, one parent’s genotype is placed across the top and the other parent’s genotype is placed down the side. One allele goes above each column, and one allele goes next to each row. The boxes then combine one allele from each parent. The full square shows the chances of each genotype and helps predict the trait an offspring may show.

Punnett squares still matter in genetics, counseling, and breeding. Genetic counselors can use them to estimate a child’s risk for inherited disease. For example, if one parent has cystic fibrosis and the other neither has nor carries it, the child will be a carrier but will not be at risk for cystic fibrosis. Breeders also use Punnett squares to choose organisms with useful traits for future breeding.

These grids do have limits. They do not work well for complex inheritance patterns, such as linked genes on the same chromosome, because those genes are not independently assorted. They are also not a good fit for traits like height, which can depend on several genes and environmental factors such as diet. Punnett squares are also not useful for traits acquired from only one parent.

Punnett Squares for Predicting Traits

genetic lingo

Genotype, Phenotype, and Inheritance

Genotype, phenotype, and inheritance help explain how traits and disease risk are passed from parents to offspring. In diploid organisms, including humans, most other animals, and many plants, somatic cells carry a duplicate set of chromosomes. These chromosome pairs are homologous, with similar length, centromere position, and gene locations. Diploid organisms inherit two versions of each gene, one from each parent. These versions are called alleles, and they sit at the same relative locus,...

punnett squares

Punnett squares help predict which traits offspring may inherit from two parents. They show possible genotypes, which are pairs of alleles, and can also show the chance of a phenotype, or visible trait, when the inheritance pattern is known. These grids work best for traits controlled by a single gene locus and passed on independently during meiosis. Punnett squares were developed in the early 1900s by Reginald Punnett. His work came after Gregor Mendel’s pea plant experiments revealed the...

monohybrid crosses

Mendel’s Pea Plant Inheritance Ratios

Gregor Mendel used pea plants to study how traits are passed from parents to offspring. His breeding experiments in the 1850s and 1860s showed that inheritance follows clear patterns. He crossed true-breeding plants that differed in one trait and found that dominant traits can hide recessive traits. Mendel focused on seven pea plant traits, and each trait had two forms controlled by a single gene locus. He noticed that true-breeding plants always produced offspring with the same phenotype, or...

dihybrid crosses

Mendel's Law of Independent Assortment

Mendel’s law of independent assortment explains how two traits can be inherited separately. Gregor Mendel tested this idea with pea plants that differed in two characteristics, such as pea color and pea shape. He used true-breeding parents that were homozygous for both traits but showed different phenotypes. In one example, Mendel crossed round, yellow peas with wrinkled, green peas. The parental generation had RRYY and rryy genotypes. Their first-generation offspring were all dihybrids,...

law of segregation

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Mendel’s pea plant crosses show how traits can disappear in one generation and return in the next. When he crossed plants, one parental trait sometimes seemed to vanish in the F1 generation and reappear in the F2 generation. Mendel used this pattern to explain why some traits are dominant and others are recessive. He proposed that traits are controlled by alleles, which are different versions of a gene. An organism inherits two copies of each gene, one from each parent. In his model, a...

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Mendel’s Dihybrid Cross and Trait Ratios

Mendel’s dihybrid cross shows how two traits are inherited at the same time. A dihybrid cross is a cross in which both parents are homozygous for two distinct traits, and the F1 generation is heterozygous for both traits. Mendel used pea plants with round yellow peas and wrinkled green peas as an example. One parent had genotype YYRR, and the other had genotype yyrr. In the F1 generation, every plant showed the dominant traits, so the plants were YyRr and had yellow, round peas. The F2...

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epistasis

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nature and nurture

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pleiotropy

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sex-linked disorders

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x-linked traits

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x-inactivation

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Barr bodies and X-chromosome balance help explain how female mammalian cells control gene activity from the X chromosome. The human X chromosome carries far more genes than the Y chromosome. Because males usually have one X chromosome and females have two, females would make twice as many of some proteins without a control system. To prevent that imbalance, female mammalian cells inactivate nearly all the genes on one X chromosome during early embryonic development. The inactive X condenses...

non-nuclear inheritance

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chromosomal theory of inheritance

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Genes map to chromosomes, and this idea explains how Mendel’s inheritance patterns work. Gregor Mendel published his pea plant breeding results in 1866. He showed that physical traits follow predictable patterns. At the time, scientists did not yet know that genes existed. Mendel used the word "factors" for the hereditary units he studied. The details behind Mendel’s laws of segregation and independent assortment were still unknown for many years. In the late 1800s, better microscopes and...

key terms and concepts

Learn how Punnett squares predict offspring genotypes and traits, from Mendel’s pea plants to genetic counseling, breeding, and simple inheritance patterns.

Punnett Squares for Predicting Traits | JoVE K12

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heredity-inheritance-and-variation-of-traits

Heredity: Inheritance and Variation of Traits

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Inheritance and Variation of Traits

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Florida Department of Education

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Punnett Squares for Predicting Traits

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