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? 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. Now, say you carry out the test cross and obtain progeny. You sort these progeny by phenotype and discover that you have flies with a yellow body and red eyes, as well as progeny with a yellow body and brown eyes. These progeny must have the genotypes described in Table 4. 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. Mendel, G. Sadava, D. Life: The Science of Biology , 8th ed. New York, W. Atavism: Embryology, Development and Evolution. Gene Interaction and Disease. Genetic Control of Aging and Life Span. Genetic Imprinting and X Inactivation. Genetic Regulation of Cancer. Obesity, Epigenetics, and Gene Regulation. Environmental Influences on Gene Expression. Gene Expression Regulates Cell Differentiation.
Genes, Smoking, and Lung Cancer. Negative Transcription Regulation in Prokaryotes. Operons and Prokaryotic Gene Regulation. Regulation of Transcription and Gene Expression in Eukaryotes. The Role of Methylation in Gene Expression. DNA Transcription. Reading the Genetic Code. Simultaneous Gene Transcription and Translation in Bacteria.
Chromatin Remodeling and DNase 1 Sensitivity. Chromatin Remodeling in Eukaryotes. RNA Functions. Citation: Miko, I. Classification 4. Cladistics 6: Human Physiology 1. Digestion 2. The Blood System 3. Disease Defences 4. Gas Exchange 5. Homeostasis Higher Level 7: Nucleic Acids 1. DNA Structure 2. Transcription 3. Translation 8: Metabolism 1. Metabolism 2. Cell Respiration 3. Photosynthesis 9: Plant Biology 1. Xylem Transport 2. Phloem Transport 3. Plant Growth 4. Plant Reproduction Genetics 1.
Meiosis 2. One allele can be dominant and mask the effect of a second recessive allele in a heterozygous organism that carries two different alleles at a specific locus. Recessive alleles only express their phenotype if an organism carries two identical copies of the recessive allele, meaning it is homozygous for the recessive allele. This means that the genotype of an organism with a dominant phenotype may be either homozygous or heterozygous for the dominant allele.
Therefore, it is impossible to identify the genotype of an organism with a dominant trait by visually examining its phenotype. 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.
If the test cross results in any recessive offspring, then the parent organism is heterozygous for the allele in question.
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