Table of Contents

• 1: Introduction
• The Law of OR
• 2: All Life’s a Game
• 3: The Law of OR
• 4: Some Fine Print
• 5: What if you violate the fine print?
• The Law of AND
• 6: More game shows
• 7: What are the chances?
• 8: The Law of AND
• 9: More Fine Print
• 10: Examples of independent events
• 11: What if you violate the fine print?
• AND and OR together
• 12: Summary
• 13: Practice with dice
• 14: Practice with coins
• Biological Applications
• 15:What else can probability do?
• 16: To Curl or not to Curl
• 17: Surviving the Numbers Game
• 18: Gaining an Edge
• 19: Hanging on for Dear Life

To curl or not to curl

Earlier I said that the process of flipping coins was a lot like creating and combining gametes. For any given gene, the mother "flips a coin" to determine which of her two copies her egg gets, and the father "flips a coin" to determine which of his two copies each sperm gets, and when you put the two coins together, you know the offspring's genotype, which determines its phenotype.

So, we can use the exact same methods as with coins to figure out genotypes. For example:

Let's consider a gene with two alleles, "C" for straight hair, and "c" for super-curly hair. As you know, the allele with the lower-case letter is recessive, so only people with two "c" alleles will actually have super-curly hair. (Note: this is a completely made-up gene, with no claim to reality).

So imagine a mother and father who both have the genotype Cc, and therefore straight hair. What is the probability that they will have a child with super-curly hair?

This is the same as asking, what is the probability of flipping flipping two coins and getting two tails?

P(coin1=tails AND coin2=tails) = P(coin1=tails) × P(coin2=tails) = 0.5 × 0.5 = 0.25.

Or, to put it in terms of alleles:

P(mother=c AND father=c) = P(mother=c) × P(father=c) = 0.5 × 0.5 = 0.25.

Here are some more problems with the same gene -- find the probability that