The Addition Rule Applied to Mendelian Inheritance

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  • 0:05 Cards & Inheritance
  • 0:32 Probability Review
  • 2:35 Addition Rule
  • 5:12 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
In this lesson, you're going to learn some basics concepts related to probability and how to apply the addition rule to problems related to Mendelian inheritance.

Cards & Inheritance

If you're a fan of card games like me, then you know a standard deck of cards contains 52 cards. What's the probability that you'll pull out a ten of hearts randomly from the deck? Well, that ten is the one and only ten of hearts in a deck of 52 cards. That means the probability of pulling that card out is 1/52.

That wasn't too hard, was it? Using simple concepts of probability just like this, let's find out how we can apply the addition rule to Mendelian inheritance.

Probability Review

Before we get to the harder parts of this lesson, I need to make sure you know some very basic concepts related to probability. If you have a deck of 52 cards and all the cards are aces of spades, what is the probability that you'll pull out an ace of spades? 1. If an event is certain to occur, it has a probability of 1.

Using this same exact deck, what's the probability you'll pull out an ace of hearts? 0, because the deck is stacked solely with aces of spades. If an event is certain to not occur, then it has a probability of 0.

Now let's switch back to a normal, standard deck of 52 cards. You know that the probability you'll pull out a two of diamonds is 1/52, a three of diamonds is 1/52, a ten of spades is 1/52, and so on for every one of the different cards in the 52 card deck. The probabilities of all the possible outcomes for an event have to add up to 1. The probabilities 1/52 + 1/52 + 1/52… for all the possible outcomes (the different cards you can pull out) will add up to 1.

Now, let's turn our attention to a couple of coins. One is a penny, and the other is a nickel. You're probably aware that you can flip each coin onto its head or tail. You also know that the outcome of each coin's flip is independent of the outcome of the other coin's flip, whether it occurs simultaneously or not. This means that each coin toss is an independent event because the outcome of any toss of any coin is independent of the outcome of any of its prior tosses or simultaneous tosses of another coin.

This is similar to Mendel's second law, the law of independent assortment, which boils down to the fact that the alleles of one gene segregate into gametes independently of the alleles of another gene. This means that if you flip a penny and get tails, it won't influence the outcome of flipping the nickel. Except with respect to genetics, we're talking about a pair of alleles of a gene, as opposed to a pair of sides of a coin.

Addition Rule

Let's apply your knowledge of probability to Mendelian inheritance using pea plants.

The P generation is the parental generation which gives rise to an F1, or first filial generation monohybrid cross in our example. The F1 hybrids have a genotype of Aa. When the F1 hybrids either self-pollinate or cross-pollinate with other F1 hybrids, they produce the F2 generation, the second filial generation, where a genotype of aa leads to wrinkled seeds.

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