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

K12 Standards

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Life Science

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Test Cross

- Une croix d'essai est une techniquequi peut être utilisée pour déterminer le génotyped'un organisme, exprimant un trait dominant,comme la couleur pourpre des fleurs des petits pois. 
Parce que la version dominante, ou allèle d'un gène,sera toujours exprimé sur l'allèle récessif,les organismes à génotype homozygote dominant,deux allèles dominants apparaîtront identiquesaux organismes à génotype hétérozygote,un allèle dominant. 
Une plante de petits pois à fleurs violettes de génotypeinconnu peut être croisée à une plante de pois qui ale phénotype récessif, les fleurs blanches. 
Et ainsi est connu pour avoirle génotype homozygote récessif, petit p, petit p. 
Dans un scénario, si tous les descendantsdu croisement à l'essaiprésentent le phénotype dominant, le parent inconnudoit être homozygote-dominant pour les fleurs violettes. 
Tous les descendants sont hétérozygotes,recevant un allèle récessif du parent récessif et un allèledominant de l'autre parent. 
Toutefois, si la progéniture comprend un nombre égald'organismes présentant le phénotype dominant etd'organismes présentant le phénotype récessif,le parent inconnu doit être hétérozygote. 
Tous les descendants reçoivent encore leur allèle récessif,du parent récessif. 
La moitié de la progéniture a reçu un allèle dominant,du parent hétérozygote,et présente le phénotype dominant,tandis que l'autre moitié a reçu une seconde copiede l'allèle récessif du parent hétérozygote,et présente le phénotype récessif.

Miko, Ilona. 2008. « Croisements d’essai ». Éducation à la nature 1 (1) : 136. [<a href="https://www.nature.com/scitable/topicpage/test-crosses-585">Source</a>]

Reid, James B. et John J. Ross. 2011. « Les gènes de Mendel : vers une caractérisation moléculaire complète » Genetics 189 (1) : 3-10. [<a href="https://doi.org/10.1534/genetics.111.132118">Source</a>]

croisement test

Les allèles sont des formes différentes du même gène. Les humains et les autres organismes diploïdes héritent de deux allèles de chaque gène, un de chaque parent.
 
 Un allèle est récessif si ses effets sont masqués par un autre allèle au même emplacement de gène. Par exemple, les plants de petits pois peuvent avoir des fleurs violettes ou blanches. Dans ce cas, les fleurs blanches sont récessives puisqu’une seule copie de l’allèle pour les fleurs violettes donnera lieu à un plant avec des fleurs violettes, même si elles ont également l’allèle pour les fleurs blanches.
 
 Si un organisme présente un trait récessif connu, il est simple de déterminer le génotype de l’organisme — sa paire d’allèles hérités. Un seul génotype, pp (deux allèles récessifs), produit des fleurs blanches. 
 
 Pour un organisme ayant un trait dominant, comme un plant de petits pois avec des fleurs violettes, déterminer le génotype n’est pas aussi simple. Deux génotypes, PP et Pp, produisent des fleurs violettes.
 
 Les scientifiques utilisent des croisements de contrôle pour déterminer les génotypes d’organismes présentant des traits dominants simples. Un croisement de contrôle implique la reproduction, ou le croisement, de l’organisme en question avec un autre qui affiche l’équivalent récessif de son trait dominant. 
 
 Dans un croisement de contrôle pour un plant de petits pois avec des fleurs violettes, le plant est multiplié avec un plant de petits pois qui a des fleurs blanches (génotype pp). La couleur de fleur de la progéniture résultante révèle si le parent avec des fleurs violettes est homozygote (PP) ou hétérozygote (Pp). 
 
 Si le plant est homozygote, tous les descendants hériteront d’un de ses allèles P dominants et de l’un des allèles récessifs p de l’autre parent. Puisque les allèles dominants masquent les allèles récessifs, tous les descendants auront des fleurs violettes, le trait dominant.
 
 Cependant, si le plant est hétérozygote, environ 50 % de la progéniture héritera de son allèle récessif p, avec un autre allèle récessif de l’autre parent. Par conséquent, environ la moitié de la progéniture de ce croisement aura des fleurs blanches. De cette façon, les croisements de contrôle peuvent révéler des génotypes parentaux inconnus.

Croisement test

Les allèles sont des formes différentes d’un même gène. Les humains et les autres organismes diploïdes héritent de deux allèles de chaque gène, un de chaque parent.

Un allèle est ...

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KEY TERMS AND CONCEPTS

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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.

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Using Test Crosses to Identify Genotypes

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