Do Somatic Cells Have The Same Genetic Makeup

Three photomicrographs show polytene chromosomes. The chromosomes look like horizontal tubes composed of white, grey, and black bands against a black background. They look like thick, striated lengths of rope.

Figure ii: 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.

Although he did not know it, Walther Flemming really observed spermatozoa undergoing meiosis in 1882, only he mistook this procedure for mitosis. Yet, Flemming did notice that, unlike during regular cell partition, chromosomes occurred in pairs during spermatozoan development. This observation, followed in 1902 by Sutton's meticulous measurement of chromosomes in grasshopper sperm prison cell development, provided definitive clues that prison cell division in gametes was not just regular mitosis. Sutton demonstrated that the number of chromosomes was reduced in spermatozoan prison cell sectionalisation, a procedure referred to as reductive division. As a result of this process, each gamete that Sutton observed had ane-half the genetic information of the original prison cell. A few years later, researchers J. B. Farmer and J. Due east. S. Moore reported that this process—otherwise known equally meiosis—is the primal means by which animals and plants produce gametes (Farmer & Moore, 1905).

The greatest bear upon of Sutton's work has far more to do with providing evidence for Mendel'southward principle of independent assortment than annihilation else. Specifically, Sutton saw that the position of each chromosome at the midline during metaphase was random, and that at that place 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 daughter prison cell could incorporate a mixture of the parental traits, but non necessarily the aforementioned mixture every bit in other daughter cells.

To illustrate this concept, consider the multifariousness derived from just three hypothetical chromosome pairs, as shown in the following example (Hirsch, 1963). Each pair consists of two homologues: 1 maternal and 1 paternal. Here, upper-case letter letters represent the maternal chromosome, and lowercase letters represent the paternal chromosome:

Pair 1: A and a
Pair two: B and b
Pair three: C and c

When these chromosome pairs are reshuffled through independent assortment, they can produce viii 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 calculation based on the number of chromosomes in an organism volition besides provide the number of possible combinations of chromosomes for each gamete. In particular, Sutton pointed out that the independence of each chromosome during meiosis means that there are ii^northward possible combinations of chromosomes in gametes, with "northward" beingness the number of chromosomes per gamete. Thus, in the previous instance of 3 chromosome pairs, the calculation is 2³, which equals 8. Furthermore, when you lot consider all the possible pairings of male and female gametes, the variation in zygotes is (2ⁿ)^two, which results in some fairly large numbers.

But what about chromosome reassortment in humans? Humans take 23 pairs of chromosomes. That means that one person could produce 2²³ different gametes. In addition, when you calculate the possible combinations that sally from the pairing of an egg and a sperm, the result is (2²³)ⁱⁱ possible combinations. However, some of these combinations produce the same genotype (for example, several gametes can produce a heterozygous individual). As a result, the chances that ii siblings will have the aforementioned combination of chromosomes (assuming no recombination) is about (3/8)²³, or one in half dozen.27 billion. Of form, there are more 23 segregating units (Hirsch, 2004).

While calculations of the random array of chromosomes and the mixture of different gametes are impressive, random array is not the only source of variation that comes from meiosis. In fact, these calculations are ideal numbers based on chromosomes that really stay intact throughout the meiotic process. In reality, crossing-over between chromatids during prophase I of meiosis mixes upwards pieces of chromosomes betwixt homologue pairs, a phenomenon chosen recombination. Because recombination occurs every fourth dimension gametes are formed, we tin can expect that it will ever add to the possible genotypes predicted from the 2^northward calculation. In add-on, the variety of gametes becomes even more unpredictable and complex when we consider the contribution of factor linkage. Some genes will always cosegregate into gametes if they are tightly linked, and they will therefore show a very low recombination rate. While linkage is a force that tends to reduce independent assortment of certain traits, recombination increases this array. In fact, recombination leads to an overall increment 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 side by side; in meiosis and subsequent sexual reproduction, genes get mixed upwardly. Sexual reproduction actually expands the multifariousness created by meiosis, because information technology combines the different varieties of parental genotypes. Thus, because of independent assortment, recombination, and sexual reproduction, there are trillions of possible genotypes in the homo species.

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

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