Each locus carries two alleles and we will consider the case where the two alleles are different each locus is heterozygous :. When this "individual" makes gametes, we can say that recombination occurs with a frequency or probability " r ". Thus recombination does not occur with a probability 1-r. When recombination does not occur the gametes produced will be A B and a b note only one letter is used at each locus because gametes are haploid ; when recombination does occur the gametes will be A b and a B.
These four gametes would be in the relative proportions Now consider linkage. If the A and B loci were on the same chromosome, to determine the proportions of the four gametes we would have to know the probability of recombination between the two loci.
This probability is r , so "given" an A allele, 1-r of the gametes would be AB, and r of the gametes would be Ab i. Similarly, given the a allele, 1-r of the gametes would be ab, and r of the gametes would be aB recombinants. Since there are two kinds of recombinant gametes resulting from crossover e.
For loci at opposite ends of long chromosomes, r can be very close to 0. Recombination can shuffle existing variation and lead to new variants. Note recombination will produce variation faster than mutation alone assuming "normal" mutation and recombination rates. We can thus think of extensive "latent" variation in chromosomes: there is the potential to generate extremes of variation given certain chance recombination and mating events.
Can a change in the number of genes affect health and development? Can changes in the number of chromosomes affect health and development? Can changes in the structure of chromosomes affect health and development? Can changes in noncoding DNA affect health and development? Can changes in mitochondrial DNA affect health and development?
What are complex or multifactorial disorders? What does it mean to have a genetic predisposition to a disease? What information can statistics provide about a genetic condition?
How are genetic conditions and genes named? Most organisms that reproduce sexually have two copies of each gene , because each parent cell or organism donates a single copy of its genes to its offspring. Additionally, genes can exist in slightly different forms, called alleles, which further adds to genetic variation. The combination of alleles of a gene that an individual receives from both parents determines what biologists call the genotype for a particular trait, such as hair texture.
The genotype that an individual possesses for a trait, in turn, determines the phenotype —the observable characteristics—such as whether that individual actually ends up with straight, wavy, or curly hair.
Genetic variation within a species can result from a few different sources. Mutations, the changes in the sequences of genes in DNA , are one source of genetic variation. Another source is gene flow , or the movement of genes between different groups of organisms. Finally, genetic variation can be a result of sexual reproduction , which leads to the creation of new combinations of genes.
Genetic variation in a group of organisms enables some organisms to survive better than others in the environment in which they live. Organisms of even a small population can differ strikingly in terms of how well suited they are for life in a certain environment. An example would be moths of the same species with different color wings. Moths with wings similar to the color of tree bark are better able to camouflage themselves than moths of a different color. As a result, the tree-colored moths are more likely to survive, reproduce, and pass on their genes.
This process is called natural selection , and it is the main force that drives evolution. 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.
New genetic variation can be created within generations in a population, so a population with rapid reproduction rates will probably have high genetic variation.
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