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

検定交雑は，エンドウの植物の紫色の花の色のような、優勢な形質を表す生物の遺伝子型を 決定するために使用できる技術です。優性バージョン，つまり遺伝子の対立遺伝子は，常に劣性対立遺伝子、ホモ接合型優性遺伝子型を持つ生物、2つの優性対立遺伝子を持つ有機体、1つの優性対立遺伝子と 同じように表現されます。未知の遺伝子型の紫色の花をつけたエンドウ豆植物は、劣性表現型、白い花を持つエンドウ豆植物と 交配することができます。このように、ホモ接合性劣性遺伝子型であるリトルp，リトルpを持つことが知られています。あるケースでは，試験交雑による すべての子孫が 優性表現型を示す場合、未知の親は紫色の花に関して 同型接合優性でなければならないのです。すべての子孫はヘテロ接合型で、劣性親から1つの劣性対立遺伝子を受け取り、他の親から1つの優性対立遺伝子を受け取ります。しかし、子孫が優性表現型を示す同数の生物と 劣性表現型を示す生物を含む場合、未知の親はヘテロ接合で なければなりません。すべての子孫は、劣性の親から 劣性の対立遺伝子を受け取ります。子孫の半分は、ヘテロ接合親から 優性対立遺伝子を受け，優性表現型を示し、残りの半分は、ヘテロ接合親から 劣性対立遺伝子の2番目のコピーを受け，劣勢表現型を示します。

ミコ、イロナ。2008.「テストクロス」 ネイチャーエデュケーション 1(1):136。[<a href="https://www.nature.com/scitable/topicpage/test-crosses-585">ソース</a>]

リード、ジェームズB.、ジョンJ.ロス。2011.「メンデルの遺伝子:完全な分子特性評価に向けて」遺伝学189(1):3–10。[<a href="https://doi.org/10.1534/genetics.111.132118">出典</a>]

試験交配

対立遺伝子とは、同じ遺伝子の異なる形のことです。人間をはじめとする2倍体の生物は、それぞれの親から1つずつ、すべての遺伝子の2つの対立遺伝子を受け継ぎます。
 
 ある対立遺伝子の効果が、同じ遺伝子の位置にある別の対立遺伝子によって隠される場合、その対立遺伝子は潜性となります。例えば、エンドウ豆の花には紫と白があります。この場合、紫の花の対立遺伝子を1コピー持っていれば、たとえ白の花の対立遺伝子を持っていても、紫の花の植物になるので、白の花は潜性です。
 
 ある生物が既知の潜性形質を示す場合、その生物の遺伝子型（遺伝に関する対立遺伝子のペア）を決定するのは簡単です。白い花を咲かせる遺伝子型は、pp(2つの潜性対立遺伝子)のみであることがわかります。
 
 紫の花を咲かせるエンドウ豆のように、顕性形質を持つ生物の場合、遺伝子型の決定はそれほど簡単ではありません。紫色の花を咲かせるのは、PPとPpという2つの遺伝子型です。 
 
 科学者は、単純な顕性形質を持つ生物の遺伝子型を決定するために試験交配を行います。試験交配とは、対象となる生物と、その顕性形質の潜性形質を持つ生物とを交配させることです。
 
 紫色の花を咲かせるエンドウ豆の試験交配では、白い花を咲かせるエンドウ豆（pp遺伝子型）と交配します。得られた子孫の花の色から、紫色の花を咲かせる親がホモ接合（PP）かヘテロ接合（Pp）
 かがわかります。
 
 もし植物がホモ接合であれば、すべての子孫はその顕性のP対立遺伝子の1つと、もう一方の親の潜性のp対立遺伝子の1つを受け継ぐことになります。顕性対立遺伝子は潜性対立遺伝子をマスクするのであるすべての子孫は顕性形質である紫色の花を持つことになります。 
 
 しかし、植物がヘテロ接合の場合、子孫の約50％は、その潜性のp対立遺伝子と、もう一方の親からの潜性対立遺伝子を受け継ぐことになります。したがって、この交配で生まれた子供の約半分は白い花を咲かせることになります。このように、試験交配では遺伝子型がわからない親の遺伝子型を明らかにできます。

対立遺伝子は、同じ遺伝子の異なる形態です。人間や他の二倍体生物は、各遺伝子の2つの対立遺伝子を、各親から1つずつ受け継ぎます。

対立遺伝子は...

Education

JoVE Core

Biology

生物学

Genetics

遺伝学

Classical and Modern Genetics

古典遺伝学と現代遺伝学

KEY TERMS AND CONCEPTS

主要用語と概念

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

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JoVE Core シリーズは、生物学の重要な概念を説明する 300 を超える簡潔でわかりやすいアニメーション ビデオ レッスンに加え、今日の研究室で行われる実際の研究実験を示す 150 以上の科学者の活動ビデオを通じて、生物学に命を吹き込みます。

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

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LS.10.b

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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

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