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

Um quadro de Punnett, um diagrama simples com o nome de seu inventor, é uma forma gráfica para prever a probabilidade de que a prole possa herdar uma característica de um único gene, como intolerância à lactose, dados os genótipos parentais. Usando esta grade, as contribuições genéticas de um pai que, neste caso é homozigoto recessivo, com alelos denotados como dois ls minúsculos são colocados no topo. Os do outro, heterozigoto com alelos mistos, representado por um L maiúsculo dominante junto com um l minúsculo recessivo são escritos no lado esquerdo.Combinando essas linhas e cabeçalhos de coluna nas caixas, resultam em dois genótipos possíveis em ambos os fenótipos para uma criança. Uma chance 50%de ser heterozigoto, e ser capaz de digerir lactose, e uma chance de 50%de ser homozigoto recessivo, e intolerante à lactose.

Edwards, AWF "Reginald Crundall Punnett: Primeiro Professor de Genética Arthur Balfour, Cambridge, 1912." Genética 192, nº 1 (1º de setembro de 2012): 3. [<a href="https://doi.org/10.1534/genetics.112.143552" target="__blank">Fonte</a>]

Reid, James B. e John J. Ross. "Genes de Mendel: Rumo a uma caracterização molecular completa." Genética 189, nº 1 (1º de setembro de 2011): 3. [<a href="https://doi.org/10.1534/genetics.111.132118" target="__blank">Fonte</a>]

quadrados de punnett

<h4>Visão Geral</h4>
 
 Um quadrado de Punnett exibe os possíveis genótipos que a progenia pode herdar de dois genótipos parentais. Se o padrão hereditário de uma característica (por exemplo, dominante ou recessivo) for conhecido, os quadrados de Punnett também podem ser usados para determinar a probabilidade de herdar um fenótipo. Os quadrados de Punnett são aplicáveis em situações em que a hereditariedade de características é determinada por um único locus genético e as características são herdadas independentemente. No entanto, eles não podem prever probabilidades de características para cenários de hereditariedade genética mais complexos.
 
 <h4>Os Quadrados de Punnett Exibem a Probabilidade de Herdar Uma ou Mais Características</h4>
 
 Os quadrados de Punnett são representações visuais que mostram os possíveis genótipos da progenia resultantes de um cruzamento entre dois genótipos parentais. Eles podem retratar a hereditariedade de um ou vários fenótipos, ou características, embora outras ferramentas sejam mais apropriadas para investigar a hereditariedade de mais de duas características.
 
 Os quadrados de Punnett podem ser usados para determinar a probabilidade de a progenia herdar um genótipo específico, ou par de alelos que causem uma característica em particular (ou seja, fenótipo ou traço), desde que o fenótipo seja causado por um único locus genético e seja independentemente combinado durante a meiose. Por outras palavras, os quadrados de Punnett são úteis para determinar probabilidades de hereditariedade em cenários onde a probabilidade de herdar uma característica não afeta a probabilidade de herdar outra. Embora existam muitas exceções a essas suposições (por exemplo, características codificadas por genes vizinhos no mesmo cromossoma), elas são verdadeiras para muitas características vegetais e animais.
 
 <h4>Experiências de Mendel Inspiraram a Criação do Quadrado de Punnett</h4>
 
 Os quadrados de Punnett foram criados no início da década de 1900 por Reginald Punnett, várias décadas depois de as experiências inovadoras de Gregor Mendel terem revelado as leis fundamentais da hereditariedade. Hoje, os quadrados de Punnett são frequentemente usados para ilustrar os princípios subjacentes às experiências de Mendel.
 
 Mendel estudou a hereditariedade de várias características da planta da ervilha, incluindo forma e cor da ervilha e vagem, cor e posição das flores, e tamanho da planta. Mendel também examinou os fenótipos herdados para cada característica. Por exemplo, roxo e branco são possíveis fenótipos para a característica da cor da flor. Em plantas da ervilha, as flores roxas e brancas são determinadas por variantes genéticas distintas, ou alelos, no locus do gene da cor das flores.
 
 <h4>Os Quadrados de Punnett são Grelhas que Organizam Informações Genéticas</h4>
 
 Cada caixa em um quadrado de Punnett representa um possível evento de fertilização, ou genótipo da progenia, decorrente de dois gâmetas parentais. Os quadrados de Punnett são tipicamente organizados em configurações 2x2 ou 4x4 para visualizar a hereditariedade de uma ou duas características, respectivamente.
 
 Em relação à nomenclatura, os alelos são indicados pela primeira letra do fenótipo causado pelo alelo dominante. Por exemplo, como o amarelo é o fenótipo dominante da cor da ervilha, os alelos que codificam a cor da ervilha são denotados com a letra ‘y’ em itálico. Letras maiúsculas e minúsculas representam alelos dominantes e recessivos, respectivamente. Assim, Y representa o alelo amarelo dominante e y denota o alelo verde recessivo.
 
 Para criar um quadrado de Punnett 2x2 examinando uma característica, um genótipo parental é listado acima do diagrama, com um alelo sobre cada coluna. O outro genótipo parental é exibido verticalmente à esquerda do diagrama, com um alelo ao lado de cada linha. Cada caixa do quadrado de Punnett contém os dois alelos parentais (um de cada progenitor) correspondentes à linha e coluna da caixa, representando um possível resultado de fertilização. O conteúdo do quadrado de Punnett completo pode ser usado para determinar a probabilidade de a progenia herdar um fenótipo em particular.
 
 <h4>Os Quadrados de Punnett são Ferramentas Informativas para Conselheiros Genéticos e Criadores</h4>
 
 Apesar de terem sido criados há mais de 100 anos, os quadrados de Punnett ainda têm várias aplicações relevantes. Para um casal que receba aconselhamento genético, os quadrados de Punnett podem ajudar a determinar o risco de os seus filhos terem uma doença hereditária. Por exemplo, se um dos progenitores tiver fibrose quística (dois alelos recessivos e causais) e o outro não tiver nem carregar, os seus filhos serão portadores (ou seja, terão um alelo causal), mas não terão risco de fibrose quística. Os quadrados de Punnett também podem ajudar os criadores de animais e plantas a selecionar organismos com características específicas para a reprodução contínua.
 
 Embora os quadrados de Punnett sejam úteis em muitos contextos, eles não podem descrever com precisão hereditariedade genética complexa. Por exemplo, características codificadas por genes vizinhos no mesmo cromossoma são muitas vezes herdadas juntas de um dos progenitores, um fenómeno chamado linkage. Essas características não são independentemente combinadas, pelo que um quadrado de Punnett não pode prever com precisão os seus padrões hereditários. Algumas características, como a altura, são inadequadas para os quadrados de Punnett porque são determinadas por vários genes e afetadas por condições ambientais (por exemplo, dieta). Os quadrados de Punnett também são ineficazes em prever a hereditariedade das características adquiridas de apenas um dos progenitores.

Quadrados de Punnett

Visão geral

Um quadrado de Punnett exibe os possíveis genótipos que a prole pode herdar de dois genótipos parentais. Se o padrão de herança de uma característica ...

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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

Mendel’s Pea Cross Ratios

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...

law of independent assortment

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...

probability laws

Inheritance Risk with Sum and Product Rules

Inheritance risk can be calculated with the sum rule and the product rule. The sum rule applies to mutually exclusive events, or events that cannot happen at the same time. The product rule applies to independent events, or events that do not affect one another. These rules describe theoretical probability, which is the chance of an event before it happens. Empirical probability is different because it is based on events that have already occurred. These probability rules are useful in...

test cross

Using Test Crosses to Identify Genotypes

Test crosses are used to identify the genotype of an organism with a dominant trait. A genotype is the pair of alleles inherited for a gene, and alleles are different forms of the same gene. In diploid organisms, including humans, each gene usually has two alleles, one from each parent. An allele is recessive when its effect is hidden by another allele at the same gene location. Pea plants provide a clear example with flower color. White flowers are recessive, because one copy of the purple...

polygenic traits

Polygenic Traits in Human Height

Polygenic traits are traits controlled by more than one gene. Human height is a clear example of this kind of inheritance. Because many genes work together, the final trait can vary a lot from person to person. Researchers have found hundreds of loci, or gene locations, that influence height. There are likely many more that have not yet been identified. Many of these genes affect cartilage in the growth plates of the long bones in the arms and legs, either directly or indirectly. Height is...

pedigree analysis

Reading Family Traits in Pedigrees

Pedigrees are family trees used to track how a trait passes through generations. They help scientists and students see whether a trait is dominant or recessive, whether it is autosomal or sex-linked, what genotypes family members may have, and what phenotypes may appear in future generations. For families with a history of inherited disease, this information can be important for family planning. In plants and animals, inheritance is often studied with crosses. Monohybrid crosses can show...

multiple allele traits

Blood Type and Allele Interactions

Multiple alleles can shape traits through blood type, sickle cell anemia, incomplete dominance, and codominance. Multiple allelism means a gene has three or more allelic forms. Diploid organisms, including humans, usually carry only two alleles for each gene, but many human genes exist in multiple forms in the population. Human blood type is a clear example of multiple allelism. The blood type gene has three alleles: I A , I B , and i. These alleles help determine which blood type a person has...

epistasis

Gene Masking in Rabbit Fur Color

Epistasis describes how one gene can mask or change the effect of another gene. In high school genetics, this helps explain why many traits do not follow a simple single-gene pattern. Instead, alleles at different locations can work together to shape the final phenotype. Mendel studied seven unrelated pea traits, but most traits involve multiple gene interactions. Epistasis often happens when one gene interferes with the expression of another gene. It can also occur when genes are part of the...

nature and nurture

Genes and Environment Shape Height

Genes and environment both shape human height and other traits. Genes are the inherited instructions we get from our parents. Environment includes factors such as diet, stress, and social conditions. Many phenotypes, or observable traits, reflect both influences. Some also show up later in life, including cancer and other diseases. Height is a clear example of this interaction. Scientists estimate that 70-90% of height variation comes from genetic differences among people. The remaining 10-30%...

pleiotropy

SOX10 and Multiple Traits in Pleiotropy

Pleiotropy is when one gene affects more than one trait. The SOX10 gene is a clear example because it can influence pigmentation, hearing, and intestinal movement. These traits may seem unrelated, but they can all be linked to the same gene change. In humans, a deleted region in SOX10 can cause Waardenburg Syndrome Type 4, or WS4. WS4 is an autosomal recessive disorder, which means a person usually inherits two altered copies of the gene. Common signs include a white forelock,...

sex-linked disorders

X- and Y-Linked Disorder Patterns

X- and Y-linked disorders come from mutations in genes on the sex chromosomes. These genes, like those on autosomes, are needed for normal body function. When a mutation causes a biological problem, the condition is called sex-linked. Y-linked disorders come from changes in the Y chromosome. They affect only males, because only males carry a Y chromosome. Mutations in this small chromosome can affect male sexual function and secondary sex characteristics. One example is Y-chromosome...

x-linked traits

X-Linked Inheritance in Families

X-linked inheritance explains how traits on the X chromosome are passed through families. In most mammalian species, females have two X sex chromosomes, while males have one X and one Y. A mutation on one X chromosome may be masked in females by a normal allele on the other X. In males, that same mutation is more likely to show because there is no second normal X to balance it. Traits caused by changes on the X chromosome are called X-linked traits. Color blindness is a well-studied example.

x-inactivation

Barr Bodies and X-Chromosome Balance

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

Maternal DNA in Mitochondria and Chloroplasts

Maternal DNA in mitochondria and chloroplasts explains how some genes are inherited outside the nucleus. Most DNA is in the nucleus, but mitochondria and chloroplasts in the cell cytoplasm also carry their own DNA and can copy it independently of nuclear DNA. Mitochondria are found in both plant and animal cells. They are called the powerhouses of eukaryotic cells because they break down glucose to make energy for cellular activity. Mitochondrial DNA has about 37 genes, and many help with...

chromosomal theory of inheritance

How Genes Map to Chromosomes

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

HS-LS3-1

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New York State Science Learning Standards

Inheritance and Variation of Traits

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Overview

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Monohybrid Crosses

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Punnett Squares for Predicting Traits

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