# Allele is passed on is equal to the sum of the

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allele is passed on is equal to the sum of the probabilities of the two events: Either G or g is passed on by this parent, but not both. This is an example of a mutually exclusive event and is analogous to tossing a coin. Either heads or tails can occur, but never both from the same toss of the coin. The preceding equation illus- trates the sum rule that applies to mutually exclusive events.The combined probability of two or more mutually exclusive events occurring is equal to the sum of their indi- vidual probabilities. Now let’s consider two events that are not mutually exclusive, but instead are independent of one another and happen simultaneously. This is exactly the situation when the gametes of two heterozygous parents join at fertilization, each donating just one allele for each trait to a single offspring. For example, assume a Gg mother and a Gg father have one child.What is the probability of that child having a gg genotype? The probability of the mother’s egg carrying a g allele is 0.5. The probability of the father’s sperm containing a g allele is also 0.5. The allele that the mother contributes has no effect on the one that the father contributes; thus, we are describing two independent events happening simultaneously. To calculate the probability that any two specific outcomes—say a g from mother and a g from father—will result, we must multiply the probabilities of each of the events that happen individually: or This is known as the product rule . It states that the joint probability that both of two independent events will occur is the product of the individual probabilities of each. The joint probability in this case is equal to 0.25; in other words, there is a 25% chance that any given offspring of this union will be homozygous for the g allele.This is precisely what we saw when we applied the Punnett analysis to a hybrid cross. Probability is a more formal way of determining the likelihood that an individual conception will yield a specific genotype.Keep in mind that probability cannot determine the actual outcome of any single event; it can only give the odds that a given outcome might occur. Thus, our expectant parents can make an educated guess about the likeli- hood of each of their children inheriting a gene for cystic fibrosis, but they cannot know for sure until their baby is tested. The more times an event occurs—the more times we toss a coin, or the more babies that are born to our heterozygous couple—the more likely it becomes that the ratio of different outcomes matches probabilities.Toss a coin 2 times or 4 times, and you may get two heads or four tails. But toss a coin 100 times, and the number of times you get heads will approach 50, approximately the probability of 0.5.

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