How Does Recombination Frequency Affect Genetic Linkage?

[TytoAlba95]

Homework Statement

In a linkage map, two genes A and B, are 70 cM apart. If individuals heterozygous for both the genes are test crossed number of progeny with parental phenotype will be:

1. equal to the number of progeny with recombinant phenotype
2. more than the number of progeny with recombinant phenotype
3. less than the number of progeny with recombinant phenotype
4. could be more or less than the number of progeny with recombinant phenotype depending on whether the genes are linked in cis or trans, respectively2. Solution
The answer is 1.

3. Attempt
Since the genes are 70 cM apart on a linkage map. The recombinant frequency is more than 50% (In case of true linkage the recombinant frequency should always be less than 50%, i.e. is less than parental frequency, I don't know the reason why.) So this is a case of independent assortment. So in a test cross, the frequency would be 1:1.

 

[Ygggdrasil]

Homework Statement

In a linkage map, two genes A and B, are 70 cM apart. If individuals heterozygous for both the genes are test crossed number of progeny with parental phenotype will be:

1. equal to the number of progeny with recombinant phenotype
2. more than the number of progeny with recombinant phenotype
3. less than the number of progeny with recombinant phenotype
4. could be more or less than the number of progeny with recombinant phenotype depending on whether the genes are linked in cis or trans, respectively2. Solution
The answer is 1.

3. Attempt
Since the genes are 70 cM apart on a linkage map. The recombinant frequency is more than 50% (In case of true linkage the recombinant frequency should always be less than 50%, i.e. is less than parental frequency, I don't know the reason why.) So this is a case of independent assortment. So in a test cross, the frequency would be 1:1.

If two genes are 70cM apart on a linkage map, this suggests that, on average, there will be 0.7 crossover events between the two genes during a single generation. This means that some gametes will show 0 crossover events (no recombination) or 1 crossover event (recombination). However, two crossover events occurring between the two genes is also possible, which would result in the gametes showing no recombination between the parental alleles. In order to account for the fact that an odd number of crossover events results in recombination while an even number of crossover events results in no recombination, one can sum over the probabilities to obtain a simple formula for the recombination frequency:
$$P[\text{recombination}|d \text{ cM}] = \frac{1-e^{(-2d/100)}}{2}$$
https://en.wikipedia.org/wiki/Centimorgan

With this information, can you calculate the recombination frequency between the two parental alleles?