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New Microsoft Word Document E- Content BSc I Sem. -II Back Cross and Test Cross Back Cross Backcrossing is a crossing of a hybrid with one of its parents or an individual genetically similar to its parent, in order to achieve offspring with a genetic identity which is closer to that of the parent. It is used in horticulture, animal breeding and in production of gene knockout organisms. Backcrossed hybrids are sometimes described with acronym “BC”, for example, an F1 hybrid crossed with one of its parents (or a genetically similar individual) can be termed a BC1 hybrid, and a further cross of the BC1 hybrid to the same parent (or a genetically similar individual) produces a BC2 hybrid. York radiate groundsel (Senecio eboracensis) is a naturally occurring hybrid species of Oxford ragwort (Senecio squalidus) and common groundsel (Senecio vulgaris). It is thought to have arisen from a backcrossing of the F1 hybrid with S. vulgaris. Again, the pure tall (TT) and pure dwarf (tt) pea plants when crossed in the parental generation, they produce all heterozygote (Tt) tall pea plants in the first filial generation. The cross between first filial heterozygote tall (Tt) pea plant and pure tall (TT) or pure dwarf (tt) pea plant of the parental generation is also an example for the back-crossing between two plants. In this case, the filial generation formed after the back cross may have a phenotype ratio of 1:1 if the cross is made with recessive parent or else all offspring may be having phenotype of dominant trait if back cross is with parent having dominant trait. Backcrossing may be deliberately employed in animals to transfer a desirable trait in an animal of inferior genetic background to an animal of preferable genetic background. In gene knockout experiments in particular, where the knockout is performed on easily cultured stem cell lines, but is required in an animal with a different genetic background, the knockout animal is backcrossed against the animal of the required genetic background. As the figure shows, each time that the mouse with the desired trait (in this case the lack of a gene (i.e. a knockout), indicated by the presence of a positive selectable marker) is crossed with a mouse of a constant genetic background, the average percentage of the genetic material of the offspring that is derived from that constant. The result, after sufficient reiterations, is an animal with the desired trait in the desired genetic background, with the percentage of genetic material from the original stem cells reduced to a minimum (in the order of 0.01%). This image demonstrates backcrossing of a heterozygous mouse from one genetic background onto another genetic background. In this example, the gene knockout is performed on 129/Sv cells and then backcrossed into the C57B/6J genetic background. With each successive backcross, there is an increase in the percentage of C57B/6J DNA that constitutes the genome of the offspring. Due to the nature of meiosis, in which chromosomes derived from each parent are randomly shuffled and assigned to each nascent gamete, the percentage of genetic material deriving from either cell-line will vary between offspring of a single crossing but will have an expected value. The genotype of each member of offspring may be assessed to choose not only an individual that carries the desired genetic trait, but also the minimum percentage of genetic material from the original stem cell line. A consomic strain is an inbred strain with one of its chromosomes replaced by the homologous chromosome of another inbred strain via a series of marker-assisted backcrosses. Test Cross In genetics, a test cross, first introduced by Gregor Mendel, involves the breeding of an individual with a phenotypically recessive individual, in order to determine the zygosity of the former by analyzing proportions of offspring phenotypes. Zygosity can either be heterozygous or homozygous. Those that are heterozygous have one dominant and one recessive allele. Individuals that are homozygous dominant have two dominant alleles, and those that are homozygous recessive have two recessive alleles. The genotype that an offspring has for each of its genes is determined by the alleles inherited from its parents. The combination of alleles is a result of the maternal and paternal chromosomes contributed from each gamete at fertilization of that offspring. During meiosis in gametes, homologous chromosomes experience genetic recombination and segregate randomly into haploid daughter cells, each with a unique combination of maternally and paternally coded genes. Dominant alleles will override the expression of recessive alleles. Punnett squares showing typical test crosses and the two potential outcomes. The individual in question may either be heterozygous, in which half the offspring would be heterozygous and half would be homozygous recessive, or homozygous dominant, in which all the offspring would be heterozygous. The genotype that an offspring has for each of its genes is determined by the alleles inherited from its parents. The combination of alleles is a result of the maternal and paternal chromosomes contributed from each gamete at fertilization of that offspring. During meiosis in gametes, homologous chromosomes experience genetic recombination and segregate randomly into haploid daughter cells, each with a unique combination of maternally and paternally coded genes. Dominant alleles will override the expression of recessive alleles. Test crosses are used to test an individual’s genotype by crossing it with an individual of a known genotype. Individuals that show the recessive phenotype are known to have a homozygous recessive genotype. Individuals that show the dominant phenotype, however, may either be homozygous dominant or heterozygous. The phenotypically dominant organism is the individual in question in a test cross. The purpose of a test cross is to determine if this individual is homozygous dominant or heterozygous. Test crosses involve breeding the individual in question with another individual that expresses a recessive version of the same trait. Analyzing the proportions of dominant and recessive offspring determines if the individual in question is homozygous dominant or heterozygous. If all offspring from the test cross display the dominant phenotype, the individual in question is homozygous dominant; if half the offspring display dominant phenotypes and half display recessive phenotypes, then the individual is heterozygous. Since the homozygous recessive individual can only pass on recessive alleles, the alleles the individual in question passes on determine the phenotypes of the offspring. Test Cross versus Backcross Test cross and backcross are two types of crosses introduced by Gregor Mendel. In test cross, a dominant phenotype is crossed with the homologous recessive genotype in order to discriminate between homologous dominant and heterozygous genotypes. In backcross, the F1 is crossed with one of the parents or genetically identical individuals to the parent. The main difference between test cross and the backcross is that test cross is used to discriminate the genotype of an individual which is phenotypically dominant whereas a backcross is used to recover an elite genotype from a parent which bears an elite genotype. The breeding of a dominant phenotype with the recessive phenotype is referred to as a test cross. Zygosity of the dominant phenotype can be identified by test cross. Zygosity is the degree of similarity between two alleles which determine a particular trait. The zygosity is identified by the proportion of phenotypes occurring in the offspring. It can be either homozygous or heterozygous. Homozygous individuals consist of either two dominant alleles or two recessive alleles. The heterozygous individuals contain both dominant and recessive alleles of the gene. If an individual exhibits the dominant phenotype, the genotype of that particular individual would be either homozygous dominant or heterozygous. In this situation, the exact genotype can be determined by performing a test cross with an individual exhibiting the recessive phenotype for that trait. The genotype of the recessive phenotype is always homozygous recessive for that particular trait. Therefore, the proportion of the phenotypes in the offspring may describe the zygosity of the dominant phenotype which is examined during the test cross. Punnett square of the test cross performed for the pod colour of a pea plant. Dominant allele for the pod colour is dominated by Y whereas the recessive is dominated by y. Here, yellow is the dominant colour of the pod whereas green is the recessive pod colour. The allele combination of the homologous dominant is YY, exhibiting the yellow colour pods. Yy is the allele combination of heterozygous, exhibiting the yellow colour pods. The allele combination of the homozygous recessive is yy, exhibiting the green colour pods. The genotype of the pea which exhibits the yellow pod colour can be either YY or Yy. The discrimination between YY and Yy can be achieved by mating that particular pea with a pea exhibiting the green colour pods (yy). If the genotype of the yellow colour pod is Yy, the offspring consists of 50% yellow colour pods and 50% green colour pods as in the first Punnett square in the above figure. On the other hand, if the genotype is YY, the offspring consists of only the yellow colour pods. Hence, the genotype of the dominant phenotype can be identified depending on the colour of the pods resulting in the offspring. The breeding of F1 hybrid with one of the two parents is referred to as a backcross. When F1 is bred with the homozygous dominant, the offspring produces a 100% dominant phenotype. When the F1 is bred with a recessive phenotype, the offspring produces 50% dominant and 50% recessive phenotypes. This cross produces an offspring which is genetically identical or closer to the parents of the F1. Hence, backcross is often used in horticulture and animal breeding in order to achieve genetically identical offspring carrying elite genotypes.
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