Coming from LINKED GENES
===In a normal dihybrid test cross (aabb x AaBb) we would expect the following:=== [image:] Normally we would expect the resulting offspring (above) in a ratio of 1:1:1:1, however, if these two genes are linked, then two of the father’s gametes and thus two of the resulting offspring are the product of crossing over. But which ones are the recombinants? '''Crossing over is a relatively infrequent event'''. The further apart two genes are the more likely crossing over will occur between them. However, as the probability of crossing over increases so does the probability that they will cross back over (a double cross over which would restore the original arrangement). Therefore the theoretical maximum number of recombinants is 50%. However, in practice the number of recombinants is almost always less than 50%. If we go back to our original test cross (aabb x AaBa), we might see the following numbers of offspring: [image:] If the genes were not linked, we would expect approximately equal numbers of each (a ratio of 1:1:1:1). However, because the genes are linked, we know that two of the offspring types are the result of crossing over (recombinants). Because recombination (crossing over) is a relatively infrequent event, we know that the recombinants will occur less frequently than the normal (parental) types. If we look at our resulting offspring, we can see that two types are under-represented (the smaller set of numbers)- these must be the recombinants. This also tells us that the gametes AB and ab are the result of crossing over and that the gametes Ab and aB are the parental types. This means that that the genes were originally arranged as follows: [image:] The recombinants are only produced when crossing over occurs somewhere between the two genes. [image:] We can also work out the frequency of crossing over. This is sometimes referred to as the crossover value (COV): [image:] Students studying NCEA Level 2 Biology are not required to calculate recombinant frequencies.