Crossing over, as related to genetics and genomics, refers to the exchange of DNA between paired homologous chromosomes (one from each parent) that occurs during the development of egg and sperm cells (meiosis). This process results in new combinations of alleles in the gametes (egg or sperm) formed, which ensures genomic variation in any offspring produced.
Crossing Over. Crossing over is a cellular process that happens during meiosis when chromosomes of the same type are lined up. When two chromosomes — one from the mother and one from the father — line up, parts of the chromosome can be switched. The two chromosomes contain the same genes, but may have different forms of the genes. The mother's form of a gene, let's say, could be moved to the father's chromosome, and vice versa. This is a very interesting and important biological activity; different combinations of different gene forms are then potentially passed down to offspring. This genetic variation helps to increase the diversity of a species. And diversity strengthens a species' ability to respond to changing environments over time, and therefore evolve.
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
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
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%. Which of the following is true? A. Plants get nutrients from the soil in order to live and grow. B. Plants get energy from the soil in order to to liv … An individual should be more concerned with their diastolic blood pressure reading since this is measures how hard their heart is working when it is a … The field of blank can help solve the problem it is a broad field that involves the study of blank resources such as forests crops and fisheries Which reaction is an example of a reflex? A basketball player throws the ball into the hoop describe the energy stored which has taken please Describe the characteristics of the five groups of pathogenic microorganisms. 1. the current radius of the sun is approximately 696,000 km. what is the diameter? write the formula used to calculate the diameter of a circle, and … What are the scientists doing incorrectly? what could they do to make their work in the lab safer? why is it important to follow these procedures? par … Which statement explains one way that materials form? A: They are mined from rocks B: They are produced from molten materials C: They are grown in a l … How could mould growing on a tomato that is sitting on a countertop be used as evidence for the spontaneous generation theory |
Which method do scientist use to confirm crossing over has occurred?
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