In What Phase Does Crossing Over Occur The Genetic Makeup Of An Organism Is Its

Figure 2: Examples of polytene chromosomes

Pairing of homologous chromatids results in hundreds to thousands of individual chromatid copies aligned tightly in parallel to produce giant, "polytene" chromosomes.

© 2007 Nature Publishing Group Novikov, D. et al. High-pressure treatment of polytene chromosomes improves structural resolution. Nature Methods iv, 483 (2007). All rights reserved.

Although he did not know it, Walther Flemming actually observed spermatozoa undergoing meiosis in 1882, only he mistook this process for mitosis. All the same, Flemming did notice that, unlike during regular prison cell division, chromosomes occurred in pairs during spermatozoan evolution. This ascertainment, followed in 1902 by Sutton'southward meticulous measurement of chromosomes in grasshopper sperm prison cell development, provided definitive clues that jail cell division in gametes was not just regular mitosis. Sutton demonstrated that the number of chromosomes was reduced in spermatozoan cell partitioning, a process referred to as reductive segmentation. As a result of this process, each gamete that Sutton observed had half the genetic data of the original cell. A few years afterward, researchers J. B. Farmer and J. Eastward. S. Moore reported that this process—otherwise known every bit meiosis—is the cardinal means by which animals and plants produce gametes (Farmer & Moore, 1905).

The greatest touch on of Sutton'due south piece of work has far more than to do with providing show for Mendel'south principle of contained assortment than anything else. Specifically, Sutton saw that the position of each chromosome at the midline during metaphase was random, and that there was never a consistent maternal or paternal side of the cell division. Therefore, each chromosome was independent of the other. Thus, when the parent cell separated into gametes, the set of chromosomes in each girl jail cell could contain a mixture of the parental traits, merely not necessarily the same mixture as in other daughter cells.

To illustrate this concept, consider the diverseness derived from only three hypothetical chromosome pairs, as shown in the following example (Hirsch, 1963). Each pair consists of two homologues: i maternal and one paternal. Here, majuscule messages represent the maternal chromosome, and lowercase letters represent the paternal chromosome:

• Pair 1: A and a
• Pair 2: B and b
• Pair 3: C and c

When these chromosome pairs are reshuffled through independent assortment, they can produce eight possible combinations in the resulting gametes:

• A B C
• A B c
• A b c
• A b C
• a B C
• a B c
• a b C
• a b c

A mathematical adding based on the number of chromosomes in an organism volition as well provide the number of possible combinations of chromosomes for each gamete. In item, Sutton pointed out that the independence of each chromosome during meiosis means that there are iiⁿ possible combinations of chromosomes in gametes, with "n" existence the number of chromosomes per gamete. Thus, in the previous example of 3 chromosome pairs, the calculation is 2³, which equals eight. Furthermore, when you consider all the possible pairings of male and female gametes, the variation in zygotes is (2ⁿ)², which results in some adequately large numbers.

Just what nearly chromosome reassortment in humans? Humans have 23 pairs of chromosomes. That means that one person could produce two²³ different gametes. In addition, when you calculate the possible combinations that emerge from the pairing of an egg and a sperm, the result is (2²³)ⁱⁱ possible combinations. Nonetheless, some of these combinations produce the same genotype (for example, several gametes can produce a heterozygous individual). As a result, the chances that two siblings will have the same combination of chromosomes (assuming no recombination) is well-nigh (3/eight)²³, or one in 6.27 billion. Of course, there are more than 23 segregating units (Hirsch, 2004).

While calculations of the random array of chromosomes and the mixture of different gametes are impressive, random assortment is non the only source of variation that comes from meiosis. In fact, these calculations are platonic numbers based on chromosomes that actually stay intact throughout the meiotic process. In reality, crossing-over betwixt chromatids during prophase I of meiosis mixes up pieces of chromosomes between homologue pairs, a phenomenon called recombination. Considering recombination occurs every time gametes are formed, we can expect that it will e'er add to the possible genotypes predicted from the 2^{due north} calculation. In addition, the diverseness of gametes becomes even more unpredictable and complex when we consider the contribution of gene linkage. Some genes volition always cosegregate into gametes if they are tightly linked, and they will therefore prove a very low recombination rate. While linkage is a force that tends to reduce independent assortment of certain traits, recombination increases this assortment. In fact, recombination leads to an overall increase in the number of units that assort independently, and this increases variation.

While in mitosis, genes are more often than not transferred faithfully from one cellular generation to the next; in meiosis and subsequent sexual reproduction, genes get mixed up. Sexual reproduction actually expands the variety created by meiosis, because it combines the unlike varieties of parental genotypes. Thus, considering of independent assortment, recombination, and sexual reproduction, there are trillions of possible genotypes in the human species.

In What Phase Does Crossing Over Occur The Genetic Makeup Of An Organism Is Its,

Source: http://www.nature.com/scitable/topicpage/mitosis-meiosis-and-inheritance-476

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