Test cross

Last updated May 08, 2024

Under the law of dominance in genetics, an individual expressing a dominant phenotype could contain either two copies of the dominant allele (homozygous dominant) or one copy of each dominant and recessive allele (heterozygous dominant).^[1] By performing a test cross, one can determine whether the individual is heterozygous or homozygous dominant.^[1]

In a test cross, the individual in question is bred with another individual that is homozygous for the recessive trait and the offspring of the test cross are examined.^[2] Since the homozygous recessive individual can only pass on recessive alleles, the allele the individual in question passes on determines the phenotype of the offspring.^[3] Thus, this test yields 2 possible situations:

If any of the offspring produced express the recessive trait, the individual in question is heterozygous for the dominant allele.^[1]
If all of the offspring produced express the dominant trait, the individual in question is homozygous for the dominant allele.^[1]

History

The first uses of test crosses were in Gregor Mendel’s experiments in plant hybridization. While studying the inheritance of dominant and recessive traits in pea plants, he explains that the “signification” (now termed zygosity) of an individual for a dominant trait is determined by the expression patterns of the following generation.^[4]

Rediscovery of Mendel’s work in the early 1900s led to an explosion of experiments employing the principles of test crosses. From 1908-1911, Thomas Hunt Morgan conducted test crosses while determining the inheritance pattern of a white eye-colour mutation in Drosophila.^[5] These test cross experiments became hallmarks in the discovery of sex-linked traits.

Test cross types

A test cross involves crossing an individual organism with a dominant genotype or phenotype with another organism exhibiting a recessive genotype or phenotype. To better grasp the concept of test crossing, let's explore various types of crosses involving one or more genes of interest.

Monohybrid crosses (single-gene test cross)

Monohybrid crossing is a fertilization process which entails two purebred parents that consist of a difference in only one characteristic, where the resulting offspring become monohybrids. It is utilized to test only one type of gene or phenotype.^[6]

Monohybrid, also called “single gene test cross”, is used to observe how homozygous offspring express heterozygous genotypes inherited from their parents.

The implantation of monohybrid crossing includes signifying the alleles by using characters – recessive allele often is indicated with a lower-case letter, and the dominant allele is indicated with an upper-case letter. The phenotype and the genotype of both parents being crossed are noted, including the genotype of the gametes from the parental generation. The predictions of the combinations of the gametes will be constructed on a Punnett square.^{[ citation needed ]}

In conducting a monohybrid cross, Mendel initiated the experiment with a pair of pea plants exhibiting contrasting traits, one being tall and the other dwarf. Through cross-pollination, the resulting offspring plants manifested the tall trait. These first-generation hybrids were termed F1, with their offspring referred to as Filial or F1 progeny.

Mendel observed that characteristics that were absent in F1 generation had reappeared in the F2 generation. He referred the suppressed characteristic as recessive and the expressed characteristics as dominant characteristic.^{[ citation needed ]}

Dihybrid crosses (two-gene test cross)

Mendel furthermore determined to explore the outcome of crossing two plants that were hybrids for a single trait. This investigation aimed to determinate the inheritance patterns of two characteristics simultaneously and to verify the hypothesis that the inheritance of one trait would remain independent of the other. Termed a dihybrid cross or “two-gene test cross”, this experiment was grounded in the principle of segregation. When conducting a dihybrid test cross, two dominant phenotypic characteristics are selected and crossed with parents displaying double recessive traits. The phenotypic characteristics of the F1 generation are then analyzed. In such a test cross, if the individual being tested is heterozygous, a phenotypic ratio of 1:1:1:1 is typically observed.^[7]

To test Mendel’s idea, he performed complex crosses with plants that were purebred for two characteristics: seed color (yellow and green), seed shape (round and wrinkled). He crossed plants with wrinkled and yellow seeds and plants with round and green seeds. Due to earlier cross testing with monohybrids, Mendel anticipated that the round and yellow seeds were dominant with the purebred crossing, and that is what he observed.^[8]

Applications in model organisms

Microscopic image of Caenorhabditis elegans, a free-living, transparent nematode (roundworm).

Test crosses have a variety of applications. Model organisms, such as Caenorhabditis elegans and Drosophila melanogaster, are commonly used for test crosses. Basic procedures for performing test crosses in these organisms are provided below:

C. elegans

To perform a test cross with C. elegans, place worms with a known recessive genotype with worms of an unknown genotype on an agar plate. Allow the male and hermaphrodite worms time to mate and produce offspring. Using a microscope, the ratio of recessive versus dominant phenotype will elucidate the genotype of the dominant parent.^[9]

D. melanogaster

To perform a test cross with D. melanogaster, select a trait with a known dominant and recessive phenotype. Red eye colour is dominant and white is recessive. Obtain virgin females with white eyes, young males with red eyes, and put them into a single tube. Once offspring begin to appear as larvae, remove parental lines and observe the phenotype of adult offsprings.^[10]

Limitations

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.^[11] A large number of offspring are also required to have reliable data due to statistics.^[12] Test crosses are only useful if dominance is complete. Incomplete dominance is when the dominant allele and recessive allele come together to form a blend of the two phenotypes in the offspring. Test crosses are also not applicable with codominant genes, where both phenotypes of a heterozygote trait will be expressed. Another limitation is for epistatic mutations where the expression of a gene will be overpowered by the expression of another gene.^[13] A trait can also be determined by multiple genes, known as polygenic inheritance. Genes also have different levels of penetrance, which determines how much they will be expressed. In addition, the environment affects the expression of numerous genes, therefore making the test cross inapplicable in many cases.

As more advanced techniques to determine genotype emerge, the test cross is becoming less prevalent in genetics. Genetic testing and genome mapping are modern advances which allow for more efficient and detailed information about one’s genotype to be determined.^[14] Test crosses, however, are still used to this day and have created an excellent foundation for the development of more sophisticated techniques.

Related Research Articles

An allele, or allelomorph, is a variant of the sequence of nucleotides at a particular location, or locus, on a DNA molecule.

The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a specific gene depends on the number of copies of each chromosome found in that species, also referred to as ploidy. In diploid species like humans, two full sets of chromosomes are present, meaning each individual has two alleles for any given gene. If both alleles are the same, the genotype is referred to as homozygous. If the alleles are different, the genotype is referred to as heterozygous.

Heredity, also called inheritance or biological inheritance, is the passing on of traits from parents to their offspring; either through asexual reproduction or sexual reproduction, the offspring cells or organisms acquire the genetic information of their parents. Through heredity, variations between individuals can accumulate and cause species to evolve by natural selection. The study of heredity in biology is genetics.

<span class="mw-page-title-main">Mendelian inheritance</span> Type of biological inheritance

Mendelian inheritance is a type of biological inheritance following the principles originally proposed by Gregor Mendel in 1865 and 1866, re-discovered in 1900 by Hugo de Vries and Carl Correns, and later popularized by William Bateson. These principles were initially controversial. When Mendel's theories were integrated with the Boveri–Sutton chromosome theory of inheritance by Thomas Hunt Morgan in 1915, they became the core of classical genetics. Ronald Fisher combined these ideas with the theory of natural selection in his 1930 book The Genetical Theory of Natural Selection, putting evolution onto a mathematical footing and forming the basis for population genetics within the modern evolutionary synthesis.

In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and the second is called recessive. This state of having two different variants of the same gene on each chromosome is originally caused by a mutation in one of the genes, either new or inherited. The terms autosomal dominant or autosomal recessive are used to describe gene variants on non-sex chromosomes (autosomes) and their associated traits, while those on sex chromosomes (allosomes) are termed X-linked dominant, X-linked recessive or Y-linked; these have an inheritance and presentation pattern that depends on the sex of both the parent and the child. Since there is only one copy of the Y chromosome, Y-linked traits cannot be dominant or recessive. Additionally, there are other forms of dominance, such as incomplete dominance, in which a gene variant has a partial effect compared to when it is present on both chromosomes, and co-dominance, in which different variants on each chromosome both show their associated traits.

The Punnett square is a square diagram that is used to predict the genotypes of a particular cross or breeding experiment. It is named after Reginald C. Punnett, who devised the approach in 1905. The diagram is used by biologists to determine the probability of an offspring having a particular genotype. The Punnett square is a tabular summary of possible combinations of maternal alleles with paternal alleles. These tables can be used to examine the genotypical outcome probabilities of the offspring of a single trait (allele), or when crossing multiple traits from the parents. The Punnett square is a visual representation of Mendelian inheritance, a fundamental concept in genetics which is discovery of Gregor Mendel. For multiple traits, using the "forked-line method" is typically much easier than the Punnett square. Phenotypes may be predicted with at least better-than-chance accuracy using a Punnett square, but the phenotype that may appear in the presence of a given genotype can in some instances be influenced by many other factors, as when polygenic inheritance and/or epigenetics are at work.

An F1 hybrid (also known as filial 1 hybrid) is the first filial generation of offspring of distinctly different parental types. F1 hybrids are used in genetics, and in selective breeding, where the term F1 crossbreed may be used. The term is sometimes written with a subscript, as F₁ hybrid. Subsequent generations are called F₂, F₃, etc.

Equine coat color genetics determine a horse's coat color. Many colors are possible, but all variations are produced by changes in only a few genes. Bay is the most common color of horse, followed by black and chestnut. A change at the agouti locus is capable of turning bay to black, while a mutation at the extension locus can turn bay or black to chestnut.

A monohybrid cross is a cross between two organisms with different variations at one genetic locus of interest. The character(s) being studied in a monohybrid cross are governed by two or multiple variations for a single location of a gene. Then carry out such a cross, each parent is chosen to be homozygous or true breeding for a given trait (locus). When a cross satisfies the conditions for a monohybrid cross, it is usually detected by a characteristic distribution of second-generation (F₂) offspring that is sometimes called the monohybrid ratio.

Non-Mendelian inheritance is any pattern in which traits do not segregate in accordance with Mendel's laws. These laws describe the inheritance of traits linked to single genes on chromosomes in the nucleus. In Mendelian inheritance, each parent contributes one of two possible alleles for a trait. If the genotypes of both parents in a genetic cross are known, Mendel's laws can be used to determine the distribution of phenotypes expected for the population of offspring. There are several situations in which the proportions of phenotypes observed in the progeny do not match the predicted values.

Dihybrid cross is a cross between two individuals with two observed traits that are controlled by two distinct genes. The idea of a dihybrid cross came from Gregor Mendel when he observed pea plants that were either yellow or green and either round or wrinkled. Crossing of two heterozygous individuals will result in predictable ratios for both genotype and phenotype in the offspring. The expected phenotypic ratio of crossing heterozygous parents would be 9:3:3:1. Deviations from these expected ratios may indicate that the two traits are linked or that one or both traits has a non-Mendelian mode of inheritance.

Inbreeding depression is the reduced biological fitness that has the potential to result from inbreeding. The loss of genetic diversity that is seen due to inbreeding, results from small population size. Biological fitness refers to an organism's ability to survive and perpetuate its genetic material. Inbreeding depression is often the result of a population bottleneck. In general, the higher the genetic variation or gene pool within a breeding population, the less likely it is to suffer from inbreeding depression, though inbreeding and outbreeding depression can simultaneously occur.

The term transheterozygote is used in modern genetics periodicals in two different ways. In the first, the transheterozygote has one mutant (-) and one wildtype allele (+) at each of two different genes. In the second, the transheterozygote carries two different mutated alleles of the same gene. This second definition also applies to the term "heteroallelic combination".

Out-crossing or out-breeding is the technique of crossing between different breeds. This is the practice of introducing distantly related genetic material into a breeding line, thereby increasing genetic diversity.

Marker assisted selection or marker aided selection (MAS) is an indirect selection process where a trait of interest is selected based on a marker linked to a trait of interest, rather than on the trait itself. This process has been extensively researched and proposed for plant- and animal- breeding.

A hereditary carrier, is a person or other organism that has inherited a recessive allele for a genetic trait or mutation but usually does not display that trait or show symptoms of the disease. Carriers are, however, able to pass the allele onto their offspring, who may then express the genetic trait.

Particulate inheritance is a pattern of inheritance discovered by Mendelian genetics theorists, such as William Bateson, Ronald Fisher or Gregor Mendel himself, showing that phenotypic traits can be passed from generation to generation through "discrete particles" known as genes, which can keep their ability to be expressed while not always appearing in a descending generation.

Zygosity is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism.

Classical genetics is the branch of genetics based solely on visible results of reproductive acts. It is the oldest discipline in the field of genetics, going back to the experiments on Mendelian inheritance by Gregor Mendel who made it possible to identify the basic mechanisms of heredity. Subsequently, these mechanisms have been studied and explained at the molecular level.

This glossary of genetics and evolutionary biology is a list of definitions of terms and concepts used in the study of genetics and evolutionary biology, as well as sub-disciplines and related fields, with an emphasis on classical genetics, quantitative genetics, population biology, phylogenetics, speciation, and systematics. Overlapping and related terms can be found in Glossary of cellular and molecular biology, Glossary of ecology, and Glossary of biology.

References

1 2 3 4 Gai, J.; He, J. (2013), "Test Cross", Brenner's Encyclopedia of Genetics, Elsevier, pp. 49–50, doi:10.1016/b978-0-12-374984-0.01529-1, ISBN 978-0-08-096156-9 , retrieved 2020-10-25
↑ Griffiths JF, Gelbart WM, Lewontin RC, Wessler SR, Suzuki DT, Miller JH (2005). Introduction to Genetic Analysis . New York: W.H. Freeman and Co. pp. 34–40, 473–476, 626–629. ISBN 0-7167-4939-4.
↑ Freeman, S; Harrington, M; Sharp, J (2014). "Using a Testcross to Confirm Predictions". Biological Science (Custom Edition for the University of British Columbia). Toronto, Ontario: Pearson Canada. p. 260.
↑ Mendel, Gregor; Bateson, William (1925). Experiments in plant-hybridisation. Cambridge, Mass.: Harvard University Press. pp. 323–325.
↑ "Thomas Hunt Morgan and the Discovery of Sex Linkage | Learn Science at Scitable". www.nature.com. Retrieved 2020-10-25.
↑ "12.2C: The Punnett Square Approach for a Monohybrid Cross". Biology LibreTexts. 2018-07-12. Retrieved 2024-02-15.
↑ "Test Crosses | Learn Science at Scitable". www.nature.com. Retrieved 2024-02-15.
↑ "Gregor Mendel and the Principles of Inheritance | Learn Science at Scitable". www.nature.com. Retrieved 2024-02-15.
↑ Fay, David S. (2018). "Classical genetic methods". WormBook: The Online Review of C. Elegans Biology. WormBook: 1–58. doi:10.1895/wormbook.1.165.1. PMC 4127492 . PMID 24395816.
↑ Lawrence, Peter A. (1995). The making of a fly : the genetics of animal design. Oxford [England]: Blackwell Science. ISBN 0-632-03048-8. OCLC 24211238.
↑ Orias, Eduardo (2012). "Chapter 10 - Tetrahymena thermophila Genetics: Concepts and Applications". Methods in Cell Biology. Vol. 109. Elsevier. pp. 301–325. doi:10.1016/B978-0-12-385967-9.00010-4. ISBN 978-0-12-385967-9. PMID 22444149.
↑ Lobo, I. "Genetics and Statistical Analysis | Learn Science at Scitable". www.nature.com. Retrieved 2020-10-25.
↑ "Epistasis". www.genome.gov. Retrieved 2024-02-15.
↑ Özgüç, Meral (2011). "Genetic testing: predictive value of genotyping for diagnosis and management of disease". EPMA Journal. 2 (2): 173–179. doi:10.1007/s13167-011-0077-y. ISSN 1878-5077. PMC 3405385 . PMID 23199147.

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[:0-1] 1 2 3 4 Gai, J.; He, J. (2013), "Test Cross", Brenner's Encyclopedia of Genetics, Elsevier, pp. 49–50, doi:10.1016/b978-0-12-374984-0.01529-1, ISBN 978-0-08-096156-9 , retrieved 2020-10-25

[Griffiths2-2] Griffiths JF, Gelbart WM, Lewontin RC, Wessler SR, Suzuki DT, Miller JH (2005). Introduction to Genetic Analysis . New York: W.H. Freeman and Co. pp. 34–40, 473–476, 626–629. ISBN 0-7167-4939-4.

[3] Freeman, S; Harrington, M; Sharp, J (2014). "Using a Testcross to Confirm Predictions". Biological Science (Custom Edition for the University of British Columbia). Toronto, Ontario: Pearson Canada. p. 260.

[4] Mendel, Gregor; Bateson, William (1925). Experiments in plant-hybridisation. Cambridge, Mass.: Harvard University Press. pp. 323–325.

[5] "Thomas Hunt Morgan and the Discovery of Sex Linkage | Learn Science at Scitable". www.nature.com. Retrieved 2020-10-25.

[6] "12.2C: The Punnett Square Approach for a Monohybrid Cross". Biology LibreTexts. 2018-07-12. Retrieved 2024-02-15.

[7] "Test Crosses | Learn Science at Scitable". www.nature.com. Retrieved 2024-02-15.

[8] "Gregor Mendel and the Principles of Inheritance | Learn Science at Scitable". www.nature.com. Retrieved 2024-02-15.

[9] Fay, David S. (2018). "Classical genetic methods". WormBook: The Online Review of C. Elegans Biology. WormBook: 1–58. doi:10.1895/wormbook.1.165.1. PMC 4127492 . PMID 24395816.

[10] Lawrence, Peter A. (1995). The making of a fly : the genetics of animal design. Oxford [England]: Blackwell Science. ISBN 0-632-03048-8. OCLC 24211238.

[11] Orias, Eduardo (2012). "Chapter 10 - Tetrahymena thermophila Genetics: Concepts and Applications". Methods in Cell Biology. Vol. 109. Elsevier. pp. 301–325. doi:10.1016/B978-0-12-385967-9.00010-4. ISBN 978-0-12-385967-9. PMID 22444149.

[12] Lobo, I. "Genetics and Statistical Analysis | Learn Science at Scitable". www.nature.com. Retrieved 2020-10-25.

[13] "Epistasis". www.genome.gov. Retrieved 2024-02-15.

[14] Özgüç, Meral (2011). "Genetic testing: predictive value of genotyping for diagnosis and management of disease". EPMA Journal. 2 (2): 173–179. doi:10.1007/s13167-011-0077-y. ISSN 1878-5077. PMC 3405385 . PMID 23199147.

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