When you see a dominant trait, the underlying genetic make-up can still be ambiguous. See how researchers use test crosses to find out the genotype behind the phenotype.
It is not always possible to determine what genes an organism is carrying by simply looking at its appearance. After all, gene expression is a complex process that is dependant on many environmental and hereditary factors. For example, Gregor Mendel's experiments with pea plants showed how dominant traits can mask recessive ones, thus causing him to muse how "rash it must be... to draw from the external resemblances of hybrids conclusions as to their internal nature" (Mendel, 1866).
Today, scientists use the word "phenotype" to refer to what Mendel termed "external resemblance" and the word "genotype" to refer to an organism's "internal nature." Thus, to restate Mendel's musing in modern terms, we cannot infer an organism's genotype by simply observing its phenotype. Indeed, Mendel showed that phenotypic traits can be hidden in one generation, yet reemerge in subsequent generations. This occurs because some alleles are dominant over others, which means that their phenotype will mask the phenotype associated with the recessive alleles.
Because of dominance, there is not a one-to-one correspondence between the alleles that an organism possesses (i.e., its genotype) and the organism's observed phenotype. Consider, for instance, the genes that code for eye and body color in the fruit fly Drosophila melanogaster. In these flies, the brown-eye allele (b) is recessive to the normal red-eye allele (B). Similarly, the ebony body color allele (e) is recessive to the normal (yellow-brown) body color allele (E). Because ebony has 100% penetrance, a fly that has dark black body color has the homozygous genotype ee. However, a fly that has a normal body color may have the homozygous genotype EE or the heterozygous genotype Ee.
Things get slightly more complex when considering two genes. For instance, a wild-type fly (with red eyes and a yellow body) has one of four possible genotypes: EEBB, EEBb, EeBB, and EeBb. There is no way to tell these genotypes apart visually, but there is a well-established experimental technique to determine the fly's genetic makeup. Specifically, to detect the underlying genotype of an organism with a dominant phenotype, one must do a type of breeding analysis called a test cross.
The test cross is another fundamental tool devised by Gregor Mendel. In its simplest form, a test cross is an experimental cross of an individual organism of dominant phenotype but unknown genotype and an organism with a homozygous recessive genotype (and phenotype). In order to understand how test crosses work, it helps to consider several examples, including those that involve just one gene of interest, as well as those that involve multiple genes.
Single-Gene Test Crosses
Recall that in the fruit fly Drosophila melanogaster, the ebony-body allele (e) is recessive to the normal yellow-body allele (E). Say you are given a male fly with a yellow body. How could you use a test cross to determine this fly's genotype?
In order to set up your test cross, you must first realize that the male fly has one of two possible genotypes: Ee or EE. Because the male exhibits the dominant body color phenotype, you must cross it with a female with the homozygous recessive phenotype and genotype. Thus, the male fly is crossed with an ebony-bodied female of genotype ee. Depending on the male fly's underlying genotype, this cross will yield one of two possible sets of outcomes, as depicted in Tables 1 and 2.
Table 1: Outcome if Male Fly Is Heterozygous (Ee)
Table 2: Outcome if Male Fly Is Homozygous (EE)
In the first outcome (Table 1), the male had the genotype Ee. By the principle of segregation, he made two types of gametes—one that contained E, and another that contained e—in equal frequencies. The female "tester" fly, on the other hand, had genotype ee, so she made only one type of gamete (which contained e). Thus, the progeny of this cross would be expected to be 50% ebony bodied (ee) and 50% yellow bodied (Ee), thus reflecting the type and frequency of their father's gametes.
In contrast, in the second outcome (Table 2), the male fly had the genotype EE, so he made only one type of gamete (E). Meanwhile, the female "tester" had genotype ee, so she also made only one variety of gamete (e). The progeny of this cross would thereby be expected to be 100% yellow heterozygotes (Ee), again reflecting the type and frequency of their father's gametes.
Two-Gene Test Crosses
Test crosses operate under the same principle no matter whether you are considering one gene or multiple genes; in all cases, you are crossing an individual of a dominant phenotype but unknown genotype to an individual that is homozygous recessive for all relevant genes. Because the "tester" individual makes one known type of gamete, the ratios of phenotypes among the progeny of the cross indicate the type and frequencies of gametes made by the individual with the unknown genotype. Once you know the gametes that this individual produces, you can "reconstruct" the individual's genotype.
Consider again the fruit fly Drosophila melanogaster, and recall that the ebony-body allele (e) is recessive to the normal yellow-body allele (E), while the brown-eye allele (b) is recessive to the normal red-eye allele (B). If you are given a male with a yellow body and red eyes, how can you determine its genotype?
In this example, there are now four possible genotypes that are associated with the dominant phenotype of yellow body/red eyes. These four genotypes can produce one, two, two, and four different gametes, respectively (Table 3). Moreover, in combination with the single gamete from the "tester" parent, these gametes will produce one, two, two, or four progeny phenotypes.
Table 3: Possible Male Gametes and Their Frequency
|Case #||Possible Genotype||Frequency of EB Allele||Frequency of Eb Allele||Frequency of eB Allele||Frequency of eb Allele|
Now, say you carry out the test cross and obtain 400 progeny. You sort these progeny by phenotype and discover that you have 200 flies with a yellow body and red eyes, as well as 200 progeny with a yellow body and brown eyes. These progeny must have the genotypes described in Table 4.
Table 4: Offspring Phenotype and Genotype and Corresponding Parental Gametes
|Phenotype||Frequency||Genotype||Gamete from Tester Parent||Gamete from Parent with Unknown Genotype|
| Yellow body, |
|0.5||EeBb||eb (1)||EB (0.5)|
|Yellow body, brown eyes||0.5||Eebb||eb (1)||Eb (0.5)|
You know that the homozygous recessive tester parent produces only one type of gamete (eb). Thus, the yellow-bodied, red-eyed progeny must be heterozygous at both loci (EeBb) due to the receipt of an EB allele from the unknown parent. Meanwhile, the yellow-bodied, brown-eyed progeny must be heterozygous at the body color locus but homozygous recessive at the eye color locus (Eebb). This could only happen if the progeny received an Eb gamete from the individual with the unknown genotype. Thus, you can deduce that the fly with the unknown genotype produced two types of gametes, EB and Eb, in equal frequencies. This means that you can reconstruct the fly's genotype as EEBb (case 2 in Table 3).
In sum, a test cross is a device that can be used to infer the Mendelian alleles present in parental gametes based on the observation of offspring phenotypes. Specifically, the ratio of phenotypes in a set of offspring reveals missing information about one of the parent's genotypes. Test crosses may also be used to determine whether two genes are linked, as well as to determine the underlying genotype if an allele's penetrance is less than 100%.
References and Recommended Reading
Mendel, G. Versuche über Plflanzen-hybriden. Verhandlungen des naturforschenden Ver-eines in Brünn, Bd. IV für das Jahr 1865, Abhand-lungen, 3-47 (1866) (Bateson translation)
Pierce, B. Genetics: A Conceptual Approach, 2nd ed. (New York, W. H. Freeman, 2006)
Sadava, D., et al. Life: The Science of Biology, 8th ed. (New York, W. H. Freeman/Sinaeur Associates, 2008)
A test cross is a cross between an individual with an unknown genotype with a homozygous recessive genotype. The test cross research was initially used by Gregor Johann Mendel. It is used to know whether the trait which is dominant is heterozygous or homozygous.What are examples of test crosses? ›
The typical example of the test cross is the origin experiment Mendel conducted himself, to determine the genotype of a yellow pea. As seen in the image below, the alleles Y and y are used for the yellow and green versions of the allele, respectively. The yellow allele, Y, is dominant over the y allele.What are some weaknesses with using a test cross? ›
There are many limitations to test crosses. It can be a time-consuming process as some organisms require a long growing time in each generation to show the necessary phenotype. A large number of offspring are also required to have reliable data due to statistics. Test crosses are only useful if dominance is complete.What is the result of test cross? ›
A test cross is the hypothetical breeding between an individual with an unknown genotype and a known homozygous recessive individual. If the offsprings resemble the person being tested, it means the individual is homozygous dominant. Note: As we know alleles are alternative forms of a gene at a locus.What is a test cross and what is its purpose? ›
The test cross is performed to determine the genotype of a dominant parent if it is a heterozygous or homozygous dominant. On the basis of the results obtained in the ratio of the offspring, it can be predicted that the parent is having which genotype.Why do people test cross? ›
A test cross can be used to determine the genotype of another individual by looking at offspring phenotypic ratios. A test cross is completed always with a homozygous recessive individual because it makes it easy to understand where all the alleles in the offspring came from.How do you identify a test cross? ›
To identify whether an organism exhibiting a dominant trait is homozygous or heterozygous for a specific allele, a scientist can perform a test cross. The organism in question is crossed with an organism that is homozygous for the recessive trait, and the offspring of the test cross are examined.What is a test cross simple? ›
In its simplest form, a test cross is an experimental cross of an individual organism of dominant phenotype but unknown genotype and an organism with a homozygous recessive genotype (and phenotype).What problem are geneticists solving with a test cross? ›
To determine the genotype of a specific individual, a test cross can be performed, in which the individual with an uncertain genotype is crossed with an individual that is homozygous recessive for all of the loci being tested.Which of the following is correct about test cross? ›
This cross was first performed by Gregor Mendel and is used to detect the unknown genotype of a particular organism. Hence it is a valuable tool in genetics. In all the given options the cross involves a homozygous recessive genotype ww. Thus the correct answer is option D.
What is TRUE about a testcross? - It is a mating between a hybrid individual and a homozygous recessive individual.What is a test cross quizlet? ›
Test cross. A genetic cross in which a test organism showing the dominant trait is crossed with one showing the recessive trait; used to determine whether the test organism is homozygous dominant or heterozygous.Which of the following statements best describes a test cross? ›
Answer and Explanation: The correct answer is (a) One individual has the dominant phenotype and the other has the recessive phenotype. A testcross is an experiment performed to determine whether the genotype of an organism with a dominant phenotype is homozygous or heterozygous.Can we complete test crosses with humans? ›
Mendel's test cross
The test cross, invented by Mendel, is a deliberate genetic cross between an individual of unknown genotype (homozygous dominant or heterozygous) for a dominant phenotype, and a homozygous recessive individual. Deliberate genetic crosses are not possible in studying human inheritance.
synonyms: test-cross. type of: cross, crossbreeding, crossing, hybridisation, hybridization, hybridizing, interbreeding.How do you do a 3 point test cross? ›
- Test cross offspring.
- Step 1: Identify the parental gametes.
- Step 2: Classify the recombinants.
- Step 3: Determine recombinant gamete frequency.
- Step 4: Add in the double crossover gametes.
The purpose of the test cross is to determine if an individual is homozygous dominant/recessive or heterozygous dominant/recessive. Explain the events in meiosis that explain the law of segregation and the law of independent assortment. It is observed only for genes located on separate chromosomes.What does test cross mean in biology? ›
test·cross ˈtes(t)-ˌkrȯs. : a genetic cross between a homozygous recessive individual and a corresponding suspected heterozygote to determine the genotype of the latter.What is a test cross simple definition? ›
In its simplest form, a test cross is an experimental cross of an individual organism of dominant phenotype but unknown genotype and an organism with a homozygous recessive genotype (and phenotype).What is test cross in biology in short answer? ›
A test cross is a way to explore the genotpye of an organism. Early use of the test cross was as an experimental mating test used to determine what alleles are present in the genotype.
(i) A cross between individual with unknown genotype for a particular trait with a recessive plant for that trait is called test cross.Which of the following statements best describe a test cross? ›
Answer and Explanation: The correct answer is (a) One individual has the dominant phenotype and the other has the recessive phenotype. A testcross is an experiment performed to determine whether the genotype of an organism with a dominant phenotype is homozygous or heterozygous.How does a cross test work? ›
The definition of a testcross is a cross performed to determine the genotype of an organism showing the dominant trait. To do a testcross, cross the phenotypically dominant organism and an unknown genotype with a recessive organism to determine the genotype of the dominant organism.