The synaptonemal complex, a lattice of proteins between the homologous chromosomes, forms at specific locations, spreading to cover the entire length of the chromosomes. The tight pairing of the homologous chromosomes is called synapsis.
In synapsis, the genes on the chromatids of the homologous chromosomes are aligned with each other. The synaptonemal complex also supports the exchange of chromosomal segments between non-sister homologous chromatids in a process called crossing over. The crossover events are the first source of genetic variation produced by meiosis. A single crossover event between homologous non-sister chromatids leads to an exchange of DNA between chromosomes.
Following crossover, the synaptonemal complex breaks down and the cohesin connection between homologous pairs is also removed. At the end of prophase I, the pairs are held together only at the chiasmata; they are called tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible.
During metaphase I, the tetrads move to the metaphase plate with kinetochores facing opposite poles. The homologous pairs orient themselves randomly at the equator. This event is the second mechanism that introduces variation into the gametes or spores. In each cell that undergoes meiosis, the arrangement of the tetrads is different. The number of variations is dependent on the number of chromosomes making up a set.
There are two possibilities for orientation at the metaphase plate. The possible number of alignments, therefore, equals 2 n , where n is the number of chromosomes per set. Given these two mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition. In humans, there are over 8 million configurations in which the chromosomes can line up during metaphase I of meiosis.
It is the specific process of meiosis, resulting in four unique haploid cells, that results in these many combinations. This independent assortment, in which the chromosome inherited from either the father or mother can sort into any gamete, produces the potential for tremendous genetic variation.
Together with random fertilization, more possibilities for genetic variation exist between any two people than the number of individuals alive today. Sexual reproduction is the random fertilization of a gamete from the female using a gamete from the male. A sperm cell, with over 8 million chromosome combinations, fertilizes an egg cell, which also has over 8 million chromosome combinations. What are sources of variation within a species? What is independent assortment, and how does it contribute to variation within a species?
How is genetic variation increased by fertilization? Why is genetic variation important to living organisms? How would you explain why crossing-over is an important source of genetic variation?
Why is sexual reproduction a more important source of genetic variation than asexual reproduction? See all questions in Sources of Variation. Some species display geographic variation as well as variation within a population. Geographic variation, or the distinctions in the genetic makeup of different populations, often occurs when populations are geographically separated by environmental barriers or when they are under selection pressures from a different environment.
One example of geographic variation are clines: graded changes in a character down a geographic axis. Gene duplication, mutation, or other processes can produce new genes and alleles and increase genetic variation. New genetic variation can be created within generations in a population, so a population with rapid reproduction rates will probably have high genetic variation.
However, existing genes can be arranged in new ways from chromosomal crossing over and recombination in sexual reproduction. Overall, the main sources of genetic variation are the formation of new alleles, the altering of gene number or position, rapid reproduction, and sexual reproduction. Apply the law of segregation to determine the chances of a particular genotype arising from a genetic cross. Observing that true-breeding pea plants with contrasting traits gave rise to F 1 generations that all expressed the dominant trait and F 2 generations that expressed the dominant and recessive traits in a ratio, Mendel proposed the law of segregation.
The law of segregation states that each individual that is a diploid has a pair of alleles copy for a particular trait. Each parent passes an allele at random to their offspring resulting in a diploid organism. The allele that contains the dominant trait determines the phenotype of the offspring. In essence, the law states that copies of genes separate or segregate so that each gamete receives only one allele. For the F 2 generation of a monohybrid cross, the following three possible combinations of genotypes could result: homozygous dominant, heterozygous, or homozygous recessive.
The equal segregation of alleles is the reason we can apply the Punnett square to accurately predict the offspring of parents with known genotypes. The behavior of homologous chromosomes during meiosis can account for the segregation of the alleles at each genetic locus to different gametes. As chromosomes separate into different gametes during meiosis, the two different alleles for a particular gene also segregate so that each gamete acquires one of the two alleles.
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.
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