Module 6 Mendelian Genetics

3.6.5 page 2

Read

Probabilities are usually expressed as real numbers with values from zero to one. You may also express them as fractions or percentages. A probability of zero means NO CHANCE, while a probability of one means it’s a SURE THING. Anywhere in between, the chance will differ; low values indicate it is unlikely, high values indicate it is likely. There are two general rules when considering probability. They are the rules of addition and product. Deciding when to use which rule depends on whether the probabilities are linked or independent.

 

Addition Rule

If two outcomes are mutually exclusive (you can have one but not the other), the probability that either will occur is their sum.

 

Dice Example

• The probability that we will roll a 3 on a single die is 1/6 [6 for the 6 sides, or possibilities]

• The probability that we will roll a 4 is the same (1/6).

• Thus, the probability that we will roll either a 3 or a 4 is 1/6+1/6 = 1/3

Having many exclusive outcomes makes the likelihood, or probability, increase.

 

Multiplication Rule

If two outcomes are independent (not linked), the probability that both will occur is their product.

 

Dice Example:

• The probability that we will roll a 6 on a single die is 1/6, and the probability that we will roll a 6 on a second die is the same.

• However, the probability that we will roll two sixes on a pair of dice (at once) is 1/6x1/6 = 1/36.

Having many independent outcomes occur at once makes the likelihood, or probability, decrease.

 

Example:

Which rules will be applied?

• The multiplication rule will be applied.

How will it be applied?

• Take the probability of the first event, times the second, times the third.

So what is the probability of “Heads - Tails - Tails” in a coin toss?

• (½) x (½) x (½) = 1/8 chance or 0.125 or 12.5%

Watch and Listen

Return again to the video on Classical Genetics and Di-hybrid Crosses (about 10 minutes left). Using the navigation bar on the right edge of the video, start watching the section titled “Bio Bit: Di-hybrid Crosses in Canaries”. Continue viewing until the end the video. Answer the following questions for your own understanding, and save your work in your course folder.

1. What is the probability of rolling a three on one die?

2. What does the sum of all possibilities equal?

3. What is the probability of rolling a six on two different dice at the same time? To figure this out, what “rule” did you need to use?

4. How can you use this rule and two smaller Punnett squares to predict the offspring of a Di-hybrid cross?

5. If you assume that the parents are heterozygous for all three traits in the film, what is the probability of having a child with widow’s peak, hitchhiker’s thumb, and free earlobes?