Probability in Genetics:
The physicist, Helmholtz stated that ‘all science is measurement’. The statement is true but the methods of measurement differ from one science to other. In transmission genetics, the measurement is in the form of ratios of different phenotypes and genotypes. These genetic ratios arise from probability relationships. These are:
1. Chance segregation of genes into gametes
2. Chance assortment of genes into gametes
3. Chance combination of gametes to form zygotes
Definition of Probability: Probability of occurrence of an event is like the likelihood that an event occurs in a very large number of trials. It may also be called ‘the law of chance’. It is defined as ‘the ratio of the number of times a particular event occurs to the total number of trials during which the event could have happened.
If there are (n) number of trials and an event occurs on an average (m) times out of these (n) trials, the probability of the event will be m/n.
Rules of Probability:
The determination of genetic ratios derives essentially from two basic laws of probability.
1. Product Rule or Multiplication Law:
The product rule states that ‘the probability of two independent events, occurring simultaneously is the product of the probabilities of occurrence of each of these individually’. Thus, the probability that a couple will have two girls in a sequence is 1/4. The probability of a first child being a girl is 1/2 and the probability of a second child being a girl is also 1/2.
2. Sum Rule or Addition Rule:
The sum rule of probability applies to different types of events that are mutually exclusive or alternative. When the occurrence of one event preludes the possibility of the occurrence of another event, the two events are said to be mutually exclusive. It means that when one event occurs, the other doesn’t. The probability of occurrence of one of the two mutually exclusive events is the sum of their individual probability.