Name: using test crosses to identify genotypes
Uploaded: 2025-10-23T11:49:30.000Z
Duration: 1 min 39 s
Channel: JoVE

K12 Standards

Virginia  Standards of Learning

HIgh School

Life Science

LS.10

LS.10.c

Test Cross

- Um teste cruzado é uma técnica que pode ser usada para determinar o genótipo de um organismo, expressando um traço dominante, como, a cor da flor roxa em plantas de ervilha. 
Porque a versão dominante, ou alelo, de um gene sempre será expressa sobre o alelo recessivo, organismos com o genótipo homozigótico dominante, dois alelos dominantes aparecerão da mesma forma como organismos com o genótipo heterozigótico, um alelo dominante. 
Uma planta de ervilha de flor roxa de genótipo desconhecido pode ser cruzada com uma planta de ervilha com o fenótipo recessivo e flores brancas. 
Assim, conhecida pelo genótipo homozigoto-recessivo, p pequeno, p pequeno. 
Em um cenário, se todos os descendentes do teste cruzado exibirem o fenótipo dominante, o pai desconhecido deve ser homozigoto dominante para flores roxas. 
Todos os filhos são heterozigotos, recebendo um alelo recessivo do pai recessivo, e um alelo dominante do outro progenitor. 
No entanto, se os descendentes incluírem números iguais de organismos, exibindo o fenótipo dominante, e organismos exibindo o fenótipo recessivo, o pai desconhecido deve ser heterozigótico. 
Todos os filhos ainda recebem seu alelo recessivo, do pai recessivo. 
Metade dos descendentes receberam um alelo dominante do pai heterozigoto e exibem o fenótipo dominante, enquanto a outra metade recebeu uma segunda cópia do alelo recessivo do genitor heterozigoto, e exibe o fenótipo recessivo.

Miko, Ilona. 2008. "Cruzes de teste." Educação da Natureza 1 (1): 136. [<a href="https://www.nature.com/scitable/topicpage/test-crosses-585">Fonte</a>]

Reid, James B. e John J. Ross. 2011. "Genes de Mendel: Rumo a uma Caracterização Molecular Completa" Genética 189 (1): 3–10. [<a href="https://doi.org/10.1534/genetics.111.132118">Fonte</a>]

retrocruzamento

Alelos são formas diferentes do mesmo gene. Humanos e outros organismos diplóides herdam dois alelos de cada gene, um de cada progenitor.
 
 Um alelo é recessivo se os seus efeitos forem mascarados por outro alelo no mesmo local genético. Por exemplo, plantas de ervilha podem ter flores roxas ou brancas. Neste caso, as flores brancas são recessivas, uma vez que uma única cópia do alelo para flores roxas resultará em uma planta com flores roxas, mesmo que ela também tenha o alelo para flores brancas.
 
 Se um organismo exibe uma característica recessiva conhecida, determinar o genótipo do organismo—o seu par de alelos herdados—é simples. Apenas um genótipo, pp (dois alelos recessivos), produz flores brancas.
 
 Para um organismo com uma característica dominante—como uma planta de ervilha com flores roxas—determinar o genótipo não é tão simples. Dois genótipos, PP e Pp, produzem flores roxas.
 
 Os cientistas usam cruzamentos de teste para determinar os genótipos de organismos que exibem características dominantes simples. Um cruzamento de teste envolve a reprodução, ou cruzamento, do organismo em questão com outro exibindo a contraparte recessiva da sua característica dominante.
 
 Em um cruzamento de teste para uma planta de ervilha com flores roxas, a planta é cruzada com uma planta de ervilha que tem flores brancas (genótipo pp). A cor das flores da progenia resultante revela se o progenitor com flores roxas é homozigótico (PP) ou heterozigótico (Pp).
 
 Se a planta for homozigótica, toda a progenia herdará um dos seus alelos P dominantes e um dos alelos p recessivos do outro progenitor. Uma vez que alelos dominantes mascaram alelos recessivos, todos os descendentes terão flores roxas, o traço dominante.
 
 Se a planta for heterozigótica, no entanto, cerca de 50% da progenia herdará o seu alelo p recessivo, juntamente com outro alelo recessivo do outro prognitor. Portanto, cerca de metade dos descendentes deste cruzamento terão flores brancas. Desta forma, os cruzamentos de teste podem revelar genótipos parentais desconhecidos.

Retrocruzamento

Os alelos são formas diferentes do mesmo gene. Humanos e outros organismos diplóides herdam dois alelos de cada gene, um de cada pai.

Um alelo é ...

Education

JoVE Core

Biology

Biologia

Genetics

Genética

Classical and Modern Genetics

Genética Clássica e Moderna

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

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 allele is enough to make a plant purple, even if it also carries the white allele.

When a trait is recessive, the genotype is easy to determine. For white pea flowers, only one genotype produces the trait: pp, with two recessive alleles. The problem is different for dominant traits, such as purple flowers. Two genotypes, PP and Pp, can both produce purple flowers.

Scientists use a test cross to solve that problem. In this method, the organism with the dominant trait is bred with an organism that shows the recessive version of the same trait. For a purple-flowered pea plant, the cross is made with a white-flowered pea plant with the pp genotype. The offspring then show whether the purple parent is homozygous, meaning PP, or heterozygous, meaning Pp.

The offspring pattern gives the answer. If the purple plant is homozygous, every offspring inherits one dominant P allele and one recessive p allele, so they all have purple flowers. If the purple plant is heterozygous, about half of the offspring inherit the recessive p allele from that parent and become white. In this way, test crosses reveal unknown parental genotypes.

Using Test Crosses to Identify Genotypes

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 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...

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

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 a test cross uses a recessive partner to reveal whether a dominant trait is homozygous or heterozygous in pea plants.

Using Test Crosses to Identify Genotypes | JoVE K12

JoVE Core series brings biology to life through over 300 concise and easy-to-understand animated video lessons that explain key concepts in biology, plus more than 150 scientist-in-action videos that show actual research experiments conducted in today’s laboratories.

biology

A coleção JoVE Core dá vida à biologia através de mais de 300 vídeos de animação concisos e fáceis de entender que explicam conceitos-chave de biologia, além de mais de 150 vídeos de Cientistas em ação que mostram experimentos reais em pesquisa realizados nos laboratórios atualmente.

Next Generation Science Standards

High School

heredity-inheritance-and-variation-of-traits

Heredity: Inheritance and Variation of Traits

HS-LS3-1

new-york-state-science-learning-standards

New York State Science Learning Standards

Inheritance and Variation of Traits

HS-LS3-1.

Georgia Standards of Excellence

9-12

SB3.a

Florida Department of Education

SC.912.L.16

SC.912.L.16.1

myJoVE Corporation

The DNA Helix

Overview

Deoxyribonucleic acid, or DNA, is the genetic material responsible for passing traits from generation to generation in all organisms and most ...

DNA Packaging

Overview

Eukaryotes have large genomes compared to prokaryotes. In order to fit their genomes into a cell, eukaryotes must pack their DNA tightly inside ...

Organization of Genes

Overview

The genomes of eukaryotes can be structured in several functional categories. A strand of DNA is comprised of genes and intergenic regions. ...

Replication in Prokaryotes

Overview

DNA replication has three main steps: initiation, elongation, and termination. Replication in prokaryotes begins when initiator proteins bind to ...

Replication in Eukaryotes

Overview

In eukaryotic cells, DNA replication is highly conserved and tightly regulated. Multiple linear chromosomes must be duplicated with high ...

Proofreading

Synthesis of new DNA molecules starts when DNA polymerase links nucleotides together in a sequence that is complementary to the template DNA strand. DNA ...

Mutations

Overview

Mutations are changes in the sequence of DNA. These changes can occur spontaneously or they can be induced by exposure to environmental factors. ...

Transcription

Overview
Transcription is the process of synthesizing RNA from a DNA sequence by RNA polymerase. It is the first step in producing a protein from a gene ...

Translation

Lesson: Translation
Translation is the process of synthesizing proteins from the genetic information carried by messenger RNA (mRNA). Following ...

Bacterial Transformation

In 1928, bacteriologist Frederick Griffith worked on a vaccine for pneumonia, which is caused by Streptococcus pneumoniae bacteria. Griffith studied two ...

What is Gene Expression?

Overview
Gene expression is the process in which DNA&nbsp;directs the synthesis of functional products, that is, proteins. Cells can regulate gene ...

The Central Dogma

Overview

The central dogma of biology states that information encoded in the DNA is transferred to messenger RNA (mRNA), which then directs the synthesis ...

Transcription Factors

Tissue-specific transcription factors contribute to diverse cellular functions in mammals. For example, the gene for beta globin, a major component of ...

RNA Structure

Overview
The basic structure of RNA consists of a five-carbon sugar and one of four nitrogenous bases. Although most RNA is single-stranded, it can form ...

RNA Stability

Intact DNA strands can be found in fossils, while scientists sometimes struggle to keep RNA intact under laboratory conditions. The structural variations ...

Types of RNA

Overview
Three main types of RNA are involved in protein synthesis: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). These RNAs ...

MicroRNAs

MicroRNA (miRNA) are short, regulatory RNA transcribed from introns&mdash;non-coding regions of a gene&mdash;or intergenic regions&mdash;stretches of DNA ...

RNA Splicing

Splicing is the process by which eukaryotic RNA is edited before its translation into protein. The RNA strand transcribed from eukaryotic DNA is called ...

Epigenetic Regulation

Epigenetic mechanisms play an essential role in healthy development. Conversely, precisely regulated epigenetic mechanisms are disrupted in diseases like ...

RNA Interference

RNA interference (RNAi) is a process in which a small non-coding RNA molecule blocks the post-transcriptional expression of a gene by binding to its ...

Operons

Prokaryotes can control gene expression through operons&mdash;DNA sequences consisting of regulatory elements and clustered, functionally related ...

Proteins: From Genes to Degradation

The student will investigate and understand that organisms reproduce and transmit genetic information to new generations. Key ideas include DNA has a role in making proteins that determine organism traits;

LS.10.a

What is Meiosis?

Meiosis is the process by which diploid cells divide to produce haploid daughter cells. In humans, each diploid cell contains 46 chromosomes, half from ...

Meiosis I

Meiosis is a carefully orchestrated set of cell divisions, the goal of which&#x2014;in humans&#x2014;is to produce haploid sperm or eggs, each containing ...

Meiosis II

Meiosis II is the second and final stage of meiosis. It relies on the haploid cells produced during meiosis I, each of which contain only 23 ...

Crossing Over

Unlike mitosis, meiosis aims for genetic diversity in its creation of haploid gametes. Dividing germ cells first begin this process in prophase I, where ...

Nondisjunction

During meiosis, chromosomes occasionally separate improperly. This occurs due to failure of homologous chromosome separation during meiosis I or failed ...

The student will investigate and understand that organisms reproduce and transmit genetic information to new generations. Key ideas include the role of meiosis is to transfer traits to the next generation; and

LS.10.b

Genetic Lingo

Overview

An organism is diploid if it inherits two variants, or alleles, of each gene, one from each parent. These two alleles constitute the genotype ...

Punnett Squares

Overview

A Punnett square displays the possible genotypes offspring can inherit from two parental genotypes. If a trait&#x2019;s inheritance pattern ...

Monohybrid Crosses

Overview

In the 1850s and 1860s, Gregor Mendel investigated inheritance by performing monohybrid crosses in pea plants. He crossed two plants that were ...

Dihybrid Crosses

Overview

To determine whether traits are inherited together or separately, Gregor Mendel crossed pea plants that differed in two traits. These parental ...

Law of Segregation

When crossing pea plants, Mendel noticed that one of the parental traits would sometimes disappear in the first generation of offspring, called the F1 ...

Law of Independent Assortment

While Mendel&#x2019;s Law of Segregation states that the two alleles for one gene are separated into different gametes, a different question of how ...

Alleles are different forms of the same gene. Humans and other diploid organisms inherit two alleles of every gene, one from each parent.

An allele is ...

Pedigree Analysis

Overview

A pedigree is a diagram displaying a family&#x2019;s history of a trait. Analyzing pedigrees can reveal (1) whether a trait is dominant or ...

X-linked Traits

In most mammalian species, females have two X sex chromosomes and males have an X and Y. As a result, mutations on the X chromosome in females may be ...

Sex-linked Disorders

Like autosomes, sex chromosomes contain a variety of genes necessary for normal body function. When a mutation in one of these genes results in biological ...

Nature and Nurture

Many human characteristics, like height, are shaped by both nature&mdash;in other words, by our genes&mdash;and by nurture, or our environment. For ...

The student will investigate and understand that organisms reproduce and transmit genetic information to new generations. Key ideas include Punnett squares are mathematical models used to predict the probability of traits in offspring.

Using Test Crosses to Identify Genotypes

Description

Transcript

Tags

More videos by Standards