How does independent assortment cause genetic variation

When these changes occur, they may provide either a beneficial phenotypic change or a nonbeneficial one. Over time, processes such as natural selection favor organisms with mutations that are beneficial. If these changes provide enough of a benefit in the sexual reproduction of an organism, the mutations will become more prevalent in the population as a whole. Figure The sea anemone order Actiniaria reproduces via asexual reproduction.

In organisms like the sea anemone that reproduce without meiosis, the opportunity for introducing genetic variation into a population is very limited. Courtesy of Laszlo Ilyes. Some rights reserved. One significant advantage for genetic variation produced by sexual reproduction over the consistency of asexual reproduction is seen with viral disease. In many species that produce asexually, such as the sea anemone, a single virus may have devastating effects on a population Figure In sexually reproducing organisms with a varied gene pool, a virus will likely have a less detrimental effect because some of the genetic variations that arise may provide some degree of resistance to the virus.

In humans, for example, gene variants that confer resistance to viruses have probably been favored by natural selection. For example, a study in from the University of Milan analyzed 52 populations worldwide and identified human genes that modulate susceptibility to viral infections. Polar bodies do not function as sex cells. During fertilisation, 1 gamete from each parent combines to form a zygote.

Because of recombination and independent assortment in meiosis, each gamete contains a different set of DNA. This produces a unique combination of genes in the resulting zygote. Recombination or crossing over occurs during prophase I. Homologous chromosomes — 1 inherited from each parent — pair along their lengths, gene by gene. Breaks occur along the chromosomes, and they rejoin, trading some of their genes. The chromosomes now have genes in a unique combination.

Independent assortment is the process where the chromosomes move randomly to separate poles during meiosis. A gamete will end up with 23 chromosomes after meiosis, but independent assortment means that each gamete will have 1 of many different combinations of chromosomes. This reshuffling of genes into unique combinations increases the genetic variation in a population and explains the variation we see between siblings with the same parents.

Visit the Learn Genetics website to go on an animated tour of the basics. For the F2 generation, the law of segregation requires that each gamete receive either an R allele or an r allele along with either a Y allele or a y allele. The law of independent assortment states that a gamete into which an r allele sorted would be equally likely to contain either a Y allele or a y allele.

Thus, there are four equally likely gametes that can be formed when the YyRr heterozygote is self-crossed as follows: YR, Yr, yR, and yr. These are the offspring ratios we would expect, assuming we performed the crosses with a large enough sample size.

Independent assortment of 2 genes : This dihybrid cross of pea plants involves the genes for seed color and texture. Because of independent assortment and dominance, the dihybrid phenotypic ratio can be collapsed into two ratios, characteristic of any monohybrid cross that follows a dominant and recessive pattern. Ignoring seed color and considering only seed texture in the above dihybrid cross, we would expect that three-quarters of the F 2 generation offspring would be round and one-quarter would be wrinkled.

Similarly, isolating only seed color, we would assume that three-quarters of the F 2 offspring would be yellow and one-quarter would be green. The sorting of alleles for texture and color are independent events, so we can apply the product rule.

These proportions are identical to those obtained using a Punnett square. When more than two genes are being considered, the Punnett-square method becomes unwieldy. It would be extremely cumbersome to manually enter each genotype.

For more complex crosses, the forked-line and probability methods are preferred. To prepare a forked-line diagram for a cross between F 1 heterozygotes resulting from a cross between AABBCC and aabbcc parents, we first create rows equal to the number of genes being considered and then segregate the alleles in each row on forked lines according to the probabilities for individual monohybrid crosses.

We then multiply the values along each forked path to obtain the F 2 offspring probabilities. Note that this process is a diagrammatic version of the product rule. The values along each forked pathway can be multiplied because each gene assorts independently. For a trihybrid cross, the F 2 phenotypic ratio is Independent assortment of 3 genes : The forked-line method can be used to analyze a trihybrid cross.

Here, the probability for color in the F2 generation occupies the top row 3 yellow:1 green. The probability for shape occupies the second row 3 round:1 wrinked , and the probability for height occupies the third row 3 tall:1 dwarf. This occurs when there is some aberration in the centromere , and spindle fibers cannot attach to the chromosome to segregate it to distal poles of the cell. Consequently, the lost chromosome never properly groups with others into a new nuclear envelope , and it is left in the cytoplasm , where it will not be transcribed.

Also, chromosomes don't always separate equally into daughter cells. This sometimes happens in mitosis, when sister chromatids fail to separate during anaphase.

One daughter cell thus ends up with more chromosomes in its nucleus than the other. Likewise, abnormal separation can occur in meiosis when homologous pairs fail to separate during anaphase I.

This also results in daughter cells with different numbers of chromosomes. The phenomenon of unequal separation in meiosis is called nondisjunction. If nondisjunction causes a missing chromosome in a haploid gamete, the diploid zygote it forms with another gamete will contain only one copy of that chromosome from the other parent, a condition known as monosomy.

Conversely, if nondisjunction causes a homologous pair to travel together into the same gamete, the resulting zygote will have three copies, a condition known as trisomy Figure 3. The term " aneuploidy " applies to any of these conditions that cause an unexpected chromosome number in a daughter cell. Aneuploidy can also occur in humans. For instance, the underlying causes of Klinefelter's syndrome and Turner's syndrome are errors in sex chromosome number, and Down syndrome is caused by trisomy of chromosome However, the severity of phenotypic abnormalities can vary among different types of aneuploidy.

In addition, aneuploidy is rarely transferred to subsequent generations, because this condition impairs the production of gametes. Overall, the inheritance of odd chromosome number arises from errors in segregation during chromosome replication. Often, it is these very exceptions or modifications of expected patterns in mitosis and meiosis that enrich our understanding of how the transfer of chromosomes is regulated from one generation to the next.

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