Probability is the likelihood that a specific event will occur. A probability may be expressed as a decimal, a percentage, or a fraction. Probability is determined by the following equation:

P b = number oftimes aneventisexpectedto happen y numberoftimesan eventcouldhappen

For example, in Mendel's experiments, the dominant trait of yellow seed color appeared in the F2 generation 6,022 times. The recessive trait of green seed color appeared 2,001 times. The total number of individuals was 8,023 (6,022 + 2,001). Using the probability equation above we can determine that the probability that the dominant trait will appear in a similar cross is ii = 075


Expressed as a percentage, the probability is 75 percent. Expressed as a fraction, the probability is 3/4.

The probability that the recessive trait will appear in an F2 generation is


Expressed as a percentage, the probability is 25 percent. Expressed as a fraction, the probability is 1/4. Fractions can also be expressed as ratios. For example, the ratio 1:3 represents the same probability that 1/4 does. Probability tells us that there are three chances in four that an offspring of two heterozygous individuals will have the dominant trait and one chance in four that the offspring will have only the recessive trait.

The results predicted by probability are more likely to occur when there are many trials. For example, many coin tosses should yield a result of heads 50 percent of the time and tails 50 percent of the time. However, if you toss a coin only a few times, you might not get this result. But each time a coin is tossed, the probability of landing tails is 50 percent. Only after many, many tries would you be likely to get the percentage of heads predicted on the basis of probability, that is, 50 percent heads and 50 percent tails.

Materials paper sack containing 20 jelly beans of three different colors (with an unknown number of each color)

Calculating Probability

Materials paper sack containing 20 jelly beans of three different colors (with an unknown number of each color)


1. Obtain a sack of 20 jelly beans from your teacher. Do not look into the sack. Do not eat the jelly beans. There are three possible colors of jelly beans that can be pulled from the sack. Pull one jelly bean out, and record the color. Return the jelly bean to the sack, and shake the bag to mix the jelly beans.

2. Repeat step 1 until you have examined 20 jelly beans.

3. Determine the probability of getting a jelly bean of a specific color with a single draw. Do this for each of the three colors of jelly beans. Compare your results with those of the rest of the class.

Analysis Does anyone have the same probabilities that you do? Are any probabilities very close to yours? Are any probabilities very different from yours? From these observations, determine how many jelly beans of each color are in your sack.

figure 9-7

A pea plant homozygous for purple flowers that is crossed with a pea plant homozygous for white flowers will produce only purple-flowering offspring. Note that all of the offspring, called monohybrids, are heterozygous for flower color.

figure 9-8

Crossing a guinea pig homozygous for black coat color with one heterozygous for black coat color produces all black-coated monohybrid offspring. Note that half of the monohybrid offspring are predicted to be homozygous for coat color.

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