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What are Refuges?
Refuges are places where non-Bt cotton is planted and grown near to, or within, fields of Bt cotton. (More details are given on the Regulations page. This regulations page has links to the U.S. Environmental Protection Agency and Monsanto Company webpages which detail the rules for how large and where refuges must be, and what kind of insecticide use, if any, is permitted within refuges.) The idea behind having refuges is that we do not want insects that are able to grow in the presence of the toxin (that is, Bt-resistant insects) to mate with one another, and thereby increase the number of insects that are resistant. If we can prevent resistant insects from mating with each other, then resistance will not develop easily in the insect population and the effectiveness of Bt can be prolonged.

How Can Refuges Work—What are the Limits?
In classical genetics, there are three main patterns to inheritance: dominant, incomplete dominant, and recessive. This is due to there being pairs of chromosomes: there are normally two copies (called alleles) of each gene, one on each chromosome of the pair. (Allele refers to the specific DNA sequence found at the site of one gene; the gene site is sometimes called the "locus" and the plural is "loci".) If only one locus needs to be affected for the insect to become resistant, then the resistance trait would be dominant; if both need to be affected, then the trait is said to be recessive. Incomplete dominant would take a value between these two extremes (i.e., dominant and recessive.). In systems so far studied, resistance to Bt toxin appears to be recessive—it is inherited in a pattern consistent with both copies of the gene (i.e., both alleles) needing to be affected.

Here's What Happens
Growing Bt cotton is subject to several regulations. Information concerning these, and links to further information, is provided at this site. Go to Growing Bt Cotton in Arizon

Imagine R stands for Resistant and S for Susceptible (the normal situation is for the insects to be susceptible to the Bt toxin because both alleles are S).
For the alleles studied so far, an insect that is Resistant needs to have both genes conferring Bt resistance, so it needs to have 2 R alleles.

If an insect has two S alleles (two normal alleles), then it is not resistant.

If an insect has one S and one R allele, it is not resistant either (because inheritance is recessive. Another word to describe having two different alleles is heterozygous (e.g., the RS individuals are 'heterozygous' or 'heterozygotes' for a specific locus; heterozygotes is the noun, heterozygous is the adjective.).

Here's what happens in some of the different mating scenarios:

RR mates with SS: offspring will get one R allele from the RR parent, and one S allele from the SS parent, so will be RS — and will be susceptible to the high dose of toxin in Bt plants.

RS mates with RS: offspring will get either an R or S from one parent, and an R or S from the other parent, so it can be RR or RS or SR or SS (since order doesn't matter this usually is written as RR or RS or SS.

Note there are two ways of being RS; so there will be 1/4 RR, 1/2 RS, and 1/4 SS. It's the RR's we want to make sure don't get to populate the fields because these are going to be able to survive on the Bt cotton.

When Bt cotton is first used, the frequency of the R allele is low (or so we expect). Thus, most of the individuals in the refuges are SS. If the refuges are sufficiently close to the Bt-cotton fields, then the extremely rare RR insects that survive on Bt-cotton will mate with SS insects. The progeny of such matings will be RS and thus will not survive on Bt-cotton (because Bt-cotton produces a high does of Bt toxin). The death of the RS individuals in the Bt-cotton fields will eliminate some of the R alleles and contribute to slowing the evolution of resistance.

Thus, a refuge of appropriate size and placement will provide sufficient numbers of SS individuals to minimize matings among the RS and RR individuals.

So, what follows from the above?

1. A refuge must be close enough to every Bt-cotton field so the insects from the refuge can mate with the insects surviving in the Bt-cotton fields.
2. There must be MANY MORE adult insects from the refuge than from the Bt-cotton fields. This helps ensure that an insect surviving in a Bt-cotton field would be very likely to mate with SS individual from a refuge.
3. The populations of pests in the Bt-cotton and non-Bt cotton fields should develop in synchrony to maximize the probability of mating between individuals from the refuges and the few individuals that may survive in the Bt-cotton fields.
4. It is best if the Bt-cotton plants can kill nearly all (preferably all) the RS insects.
   
   One way to know what percentages of insects are RS or SS or RR is to monitor the frequency of the R allele by sampling the insects and testing them in a laboratory. This is sometimes feasible if the frequency of the R allele is high enough in the field and if we know the dose of Bt toxin that kills the SS and RS individuals and does not kill the RR individuals (such a dose is called a discriminating dose).

5. Careful monitoring for resistance,
6. A plan of action to take if the frequency of the resistance alleles increases sufficiently to impair the performance of Bt cotton.

Learning more

Link to pages explaining:

• Inheritance—see The Biology Project on Monohybrid crosses at http://www.biology.arizona.edu/mendelian_genetics/mendelian_genetics.html