How Does Meiosis In Parent Cells Produce Variation In The Genetic Makeup Of Offspring?

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 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.

Although he did not know it, Walther Flemming actually observed spermatozoa undergoing meiosis in 1882, merely he mistook this process for mitosis. Nonetheless, Flemming did notice that, unlike during regular cell division, chromosomes occurred in pairs during spermatozoan development. This observation, followed in 1902 by Sutton'southward meticulous measurement of chromosomes in grasshopper sperm cell evolution, provided definitive clues that prison cell sectionalization in gametes was not just regular mitosis. Sutton demonstrated that the number of chromosomes was reduced in spermatozoan jail cell division, a process referred to equally reductive division. Every bit a event of this process, each gamete that Sutton observed had one-half the genetic information of the original cell. A few years later, researchers J. B. Farmer and J. E. Southward. Moore reported that this process—otherwise known as meiosis—is the primal ways by which animals and plants produce gametes (Farmer & Moore, 1905).

The greatest impact of Sutton'southward work has far more than to do with providing bear witness for Mendel'south principle of contained array 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 prison 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 cell could contain a mixture of the parental traits, but not necessarily the same mixture as in other daughter cells.

To illustrate this concept, consider the diversity derived from just iii hypothetical chromosome pairs, every bit shown in the following example (Hirsch, 1963). Each pair consists of two homologues: one maternal and one paternal. Hither, capital letters correspond the maternal chromosome, and lowercase letters correspond 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 array, they tin 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 calculation based on the number of chromosomes in an organism will also provide the number of possible combinations of chromosomes for each gamete. In detail, Sutton pointed out that the independence of each chromosome during meiosis ways that there are 2^north possible combinations of chromosomes in gametes, with "n" being the number of chromosomes per gamete. Thus, in the previous instance of three chromosome pairs, the calculation is two³, which equals viii. Furthermore, when y'all consider all the possible pairings of male and female person gametes, the variation in zygotes is (two^north)², which results in some adequately large numbers.

But what nigh chromosome reassortment in humans? Humans have 23 pairs of chromosomes. That ways 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 consequence is (2²³)ⁱⁱ possible combinations. However, some of these combinations produce the same genotype (for example, several gametes can produce a heterozygous individual). Equally a result, the chances that two siblings will accept the same combination of chromosomes (assuming no recombination) is about (three/8)²³, or one in half dozen.27 billion. Of class, there are more than than 23 segregating units (Hirsch, 2004).

While calculations of the random assortment of chromosomes and the mixture of different gametes are impressive, random array is not the just source of variation that comes from meiosis. In fact, these calculations are ideal 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. Because recombination occurs every time gametes are formed, we can expect that it will always add to the possible genotypes predicted from the ii^north adding. In addition, the variety of gametes becomes even more unpredictable and complex when we consider the contribution of cistron linkage. Some genes will always cosegregate into gametes if they are tightly linked, and they will therefore testify a very low recombination charge per unit. 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 go mixed up. Sexual reproduction actually expands the variety created by meiosis, considering it combines the unlike varieties of parental genotypes. Thus, because of independent assortment, recombination, and sexual reproduction, at that place are trillions of possible genotypes in the human species.

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

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