BIOL 2500 - Topic 3 (part 5)

Using probabilities

It can be used to predict phenotypic ratios in dihybrid crosses and to predict gamete combinations

Predicting the phenotypic traits

You combine the probabilities for each trait to get the phenotypic ratio using the product rule and fork diagrams

What happens once we have the predicted gamete genotypes

We can set up a punnett square to perform the cross

Punnett squares

1.) It helps us summarize genotypic frequencies and genotypes

2.) It can also translate genotypes into phenotypes to get the phenotypic ratio

Inheritance of two genes on separate chromosomes or very apart on the same chromosome

The probability of them being inherited together is low

What two things are evidence of independent assortment

1.) Gamete ratio —> 1:1:1:1

2.) Phenotypic ratio —> 9:3:3:1

Punnett squares vs. fork diagrams

1.) Punnett squares are good for monohybrid, dihybrid, and sometimes trihybrid crosses

2.) Fork diagrams are more efficient when using increasing numbers of traits

Determining whether to use punnett squares or fork diagrams

1.) n = number of genes with 2 alleles

2.) 2ⁿ = The number of phenotypes

3.) 3ⁿ = The number of genotypes

Probability

The proportion of times a particular event/outcome is expected to occur

ex.) p(Heads) = 1/2

Independence

The probability of an outcome that does not depend on the outcome of another or previous outcome

ex.) p(Heads) is independent from the p(Heads) of the next toss

Product rule

Probability that 2 independent outcomes will BOTH occur (do this by multiplying the separate probabilities)

Sum rule

The probability that either one OR the other of two independent outcomes will occur, therefore making them mutually exclusive (usually do this by adding their individual probabilities)

Mutually exclusive

It means that you can’t have one if you have the other

Key words to know when you use the sum rule

1.) Either or

2.) No more than

3.) At least

Assuming independent assortment of genes when calculating the probability of getting a specific genotype

You can do separate monohybrids for each gene that encodes the trait

Conditional probabilities

Probability prediction that is dependent on another previous event that has taken place

Example of conditional probability

What is the probability that F2 yellow-seeded progeny plants are heterozygous like their parents

Binomial probability

It simplifies the use of BOTH the product and sum rules in cases where there are multiple outcomes/events