Map distance between two genes is the number of recombinant individuals divided by the total number of individuals in the progeny and multiplied by 100. In the exposed example, the map distance between genes is 20 cM.
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Progeny distribution provides evidence about genes.
If heter0zyg0us individuals, whose genes assort independently, are test crossed, they produce a progeny with equal phenotypic frequencies 1:1:1:1.
But if they express different proportions, we can assume genes are linked.
According to this informations, we might say the genes in the exposed example are linked.
Now, we might verify which are the recombinant gametes produced by the di-hybrid.
We will be able to recognize them by looking at the phenotypes with lower frequencies in the progeny.
The phenotypes with higher frequencies are the parentals.
- 401: Fan, Pink ⇒ Parental
- 399: Flesh, Rainbow ⇒ Parental
- 98: Flesh, Pink ⇒ Recombinant
- 102: Fan, Rainbow ⇒ Recombinant
To know the distance between genes, we need to use the recombination frequency value. So now, let us calculate the recombination frequency, P.
P = Recombinant number / Total of individuals
P = 98 + 102 / 98 + 102 + 401 + 399
P = 200 / 1000
P = 0.2
The recombination frequency is 0.2
The genetic distance, GD, will result from multiplying that frequency by 100 and expressing it in map units or centiMorgan (MU or cM).
P = 0.2
GD (cM) = P x 100 = 0.2 x 100 = 20cM
The map distance between these genes is 20 cM.
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