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. The probability for each possible combination of traits is calculated by multiplying the probability for each individual trait. While the forked-line method is a diagrammatic approach to keeping track of probabilities in a cross, the probability method gives the proportions of offspring expected to exhibit each phenotype or genotype without the added visual assistance.
To fully demonstrate the power of the probability method, however, we can consider specific genetic calculations. For instance, for a tetrahybrid cross between individuals that are heterozygotes for all four genes, and in which all four genes are sorting independently in a dominant and recessive pattern, what proportion of the offspring will be expected to be homozygous recessive for all four alleles?
Rather than writing out every possible genotype, we can use the probability method. Privacy Policy. Skip to main content. The last three of these factors reshuffle alleles within a population, giving offspring combinations which differ from their parents and from others.
Variation allows some individuals within a population to adapt to the changing environment. Because natural selection acts directly only on phenotypes, more genetic variation within a population usually enables more phenotypic variation. Other new alleles may be immediately detrimental such as a malformed oxygen-carrying protein and organisms carrying these new mutations will die out.
Neutral alleles are neither selected for nor against and usually remain in the population. Genetic variation is advantageous because it enables some individuals and, therefore, a population, to survive despite a changing environment. 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.
Migration is the movement of organisms from one location to another. Although it can occur in cyclical patterns as it does in birds , migration when used in a population genetics context often refers to the movement of individuals into or out of a defined population.
What effect does migration have on relative allele frequencies? If the migrating individuals stay and mate with the destination individuals, they can provide a sudden influx of alleles. After mating is established between the migrating and destination individuals, the migrating individuals will contribute gametes carrying alleles that can alter the existing proportion of alleles in the destination population.
How do populations respond to all these forces? As relative allele frequencies change, relative genotype frequencies may also change. Each genotype in the population usually has a different fitness for that particular environment. In other words, some genotypes will be favored, and individuals with those genotypes will continue to reproduce. Other genotypes will not be favored: individuals with those genotypes will be less likely to reproduce.
What type of genotype would be unfavorable? Unfavorable genotypes take many forms, such as increased risk of predation, decreased access to mates, or decreased access to resources that maintain health. Overall, the forces that cause relative allele frequencies to change at the population level can also influence the selection forces that shape them over successive generations. For example, if moths with genotype aa migrate into a population composed of AA and Aa individuals, they will increase the relative allele frequency of a.
However, if the aa genotype has a clear disadvantage to survival e. This page appears in the following eBook. Aa Aa Aa. Genetic variation describes naturally occurring genetic differences among individuals of the same species. This variation permits flexibility and survival of a population in the face of changing environmental circumstances. Consequently, genetic variation is often considered an advantage, as it is a form of preparation for the unexpected. But how does genetic variation increase or decrease?
And what effect do fluctuations in genetic variation have on populations over time? Mating patterns are important. Random forces lead to genetic drift. If the individuals at either end of the range reconnect and continue mating, the resulting genetic intermixing can contribute to more genetic variation overall.
However, if the range becomes wide enough that interbreeding between opposite ends becomes less and less likely, and the different forces acting at either end become more and more pronounced, and the individuals at each end of the population range may eventually become genetically distinct from one another. Here is an example of migration affecting relative allele frequency:.
The overall effect.
0コメント