In genetics, the "or rule" checks the probability (P) of the occurrence of one or another event, meaning events that are mutually exclusive, because in this case both are exclusive, that is: either one happens or the other happens necessarily.
MATHEMATICALLY, THIS RULE RESULTS IN THE SUM OF THE CONDITIONS.
A good example where it is possible to evidence this event is when we analyze the roll of only one die, and we want to verify the probability of more than one episode, stated as follows: What is the probability of an even number in the release of one given away?
By interpreting the situation, we have:
Even numbers of a die → 2, 4 and 6
The probability of one of these numbers coming out is equal to the product of the division represented by the probable event possibility (numerator / dividend), by the total possible possibilities (denominator / divider).
- Probability of getting out the number 2 P (2) = 1/6
- Probability of getting the number 4 P (4) = 1/6
- Probability of getting out the number 6 P (6) = 1/6
However, the questioning involves the three events, so we must add them up.
P (2 or 4 or 6) = 1/6 + 1/6 +1/6 = 3/6 = 1/2, percentage equal to 50%
Practical example applied in Genetics
What is the probability of obtaining, in a cross of hybrid peas for seed texture (smooth and wrinkled), a homozygous recessive or heterozygous plant for this trait?
Problem Interpretation:
Pea genotype and phenotype
- Dominant homozygotes → RR / smooth
- Recessive homozygous → rr / wrinkled
- Heterozygous (hybrid) → Rr / smooth
Problem resolution:
Crossing of the parietal generation: Rr x Rr
Descendants of this generation: RR / Rr / Rr / rr
- Probability of emerging a homozygous recessive plant
P(rr) = 1/4
- Probability of emerging a heterozygous plant
P(Rr) = 2/4
Therefore, the probability in question represents the sum of P(rr) + P(Rr)
P(rr or Rr) = 1/4 + 2/4 = 3/4, percentage equal to 75%
Result = 3/4 or 75%
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By Krukemberghe Fonseca
Graduated in Biology
Would you like to reference this text in a school or academic work? Look:
RIBEIRO, Krukemberghe Divine Kirk da Fonseca. "Rule of the “OR""; Brazil School. Available in: https://brasilescola.uol.com.br/biologia/regra-ou.htm. Accessed on June 28, 2021.
