Nevertheless, multicellular organisms that exclusively depend on asexual reproduction are exceedingly rare. Why is sexuality and meiosis so common? This is one of the important unanswered questions in biology and has been the focus of much research beginning in the latter half of the twentieth century.
There are several possible explanations, one of which is that the variation that sexual reproduction creates among offspring is very important to the survival and reproduction of the population. Thus, on average, a sexually-reproducing population will leave more descendants than an otherwise similar asexually-reproducing population. The only source of variation in asexual organisms is mutation.
This is the ultimate source of variation in sexual organisms, but, in addition, those different mutations are continually reshuffled from one generation to the next when different parents combine their unique genomes and the genes are mixed into different combinations by the process of meiosis.
Meiosis is the division of the contents of the nucleus, dividing the chromosomes among gametes. The process of meiosis produces unique reproductive cells called gametes, which have half the number of chromosomes as the parent cell. Fertilization, the fusion of haploid gametes from two individuals, restores the diploid condition. Thus, sexually-reproducing organisms alternate between haploid and diploid stages. However, the ways in which reproductive cells are produced and the timing between meiosis and fertilization vary greatly.
There are three main categories of sexual life cycles: diploid-dominant, demonstrated by most animals; haploid-dominant, demonstrated by all fungi and some algae; and the alternation of generations, demonstrated by plants and some algae. The Sexual Life Cycle : In animals, sexually-reproducing adults form haploid gametes from diploid germ cells.
Felsenstein, J. The evolutionary advantage of recombination. Genetics 78 , — Otto, S. Resolving the paradox of sex and recombination. Nature Reviews Genetics 3 , — link to article. Origins of New Genes and Pseudogenes. Evolutionary Adaptation in the Human Lineage. Genetic Mutation. Negative Selection. Sexual Reproduction and the Evolution of Sex. Haldane's Rule: the Heterogametic Sex.
Hybrid Incompatibility and Speciation. Hybridization and Gene Flow. Why Should We Care about Species? Citation: Otto, S. Nature Education 1 1 What, then, are the true costs and benefits of sex?
Aa Aa Aa. The Importance of Sexual Reproduction. Indeed, theoretical models developed in the s and s demonstrate that genes promoting sex and recombination increase in frequency only when all of the following conditions hold true: The population is under directional selection. This means that increased variation can improve the response to selection.
Fitness surfaces are negatively curved. This means that sex and recombination can restore variation eliminated by past selection. The surface curvature is not too strong. If too strong, the recombination load is severe. Genetics 78 , — Otto, S. Article History Close. Share Cancel. Revoke Cancel. Keywords Keywords for this Article. Save Cancel. Flag Inappropriate The Content is: Objectionable.
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No topic rooms are there. Or Browse Visually. Other Topic Rooms Genetics. Independent assortment allows the calculation of genotypic and phenotypic ratios based on the probability of individual gene combinations.
Use the probability or forked line method to calculate the chance of any particular genotype arising from a genetic cross. The independent assortment of genes can be illustrated by the dihybrid cross: a cross between two true-breeding parents that express different traits for two characteristics. Consider the characteristics of seed color and seed texture for two pea plants: one that has green, wrinkled seeds yyrr and another that has yellow, round seeds YYRR. Therefore, the F 1 generation of offspring all are YyRr.
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. Genetics is the study of genes and how traits are inherited—or passed down—from one generation to the next.
Join our community of educators and receive the latest information on National Geographic's resources for you and your students. Skip to content. Image genetic variation In many species, special genetic variations give animals a camouflaged appearance to blend in with their environment, like this Catalpa Sphinx moth Ceratomia catalpae which uses its textured wings to blend in with a tree's bark.
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