Punnett squares and probability Predicting Genetic Outcomes Punnett squares and probability

Patterns of inheritance A dominant allele is always expressed A recessive allele is only visible when paired with another recessive allele.

Alleles – Patterns of inheritance To test for patterns of inheritance, Mendel crossed two organisms with different traits Each had two distinct forms Each allele is represented by a letter CAPITAL = dominant lower = recessive Homozygous - if the two alleles for a trait are the same (AA or aa) Heterozygous - if the two alleles for a trait are different (Aa) TT Tt tt Dominant Trait Recessive Trait

Predicting Inheritance To determine the chances of inheriting a given trait, scientists use Punnett squares and symbols to represent the genes. UPPERCASE letters are used to represent dominant genes. lowercase letters are used to represent recessive genes.

Predicting Inheritance For example: T = represents the gene for TALL in pea plants t = represents the gene for short in pea plants So: TT & Tt both result in a TALL plant, because T is dominant over t. t is recessive. tt will result in a short plant. Remember there are two alleles for every trait!

PUNNETT SQUARES A diagram that predicts the outcome of a genetic cross Considers all the possible combinations of gametes 1ST DRAW A BIG SQUARE AND DIVIDE IT IN 4’S WHY E COLI? What do you like to eat? WHEN YOU EAT YOU NEED ENZYMES TO BREAK DOWN THE FOOD YOU EAT EXPLAIN WHAT ENZYMES ARE THEY BREAK DOWN FOOD LIKE GLUCOSE AND LACTOSE WHAT ARE GLUCOSE AND LACTOSE?

PUNNETT SQUARES The genes from one parent go here. The genes from the other parent go here.

PUNNETT SQUARES T t

PUNNETT SQUARES T t

PUNNETT SQUARES T t

PUNNETT SQUARES T t Tt

PUNNETT SQUARES T t Tt

PUNNETT SQUARES T t Tt F1 generation

Interpreting the Results The genotype for all the offspring is Tt. The genotype ratio is: Tt – 4:4 = 100% heterozygous The phenotype for all the offspring is tall. The phenotype ratio is: tall – 4:4 = 100% tall T t Tt

PUNNETT SQUARES T t TT Tt tt F2 generation

Interpreting the Results This time the ratios are different! Tt. The genotype ratio is: TT – 1:4 Tt – 2:4 tt – 1:4 1:2:1 2 – homozygous 2 – heterozygous The phenotype ratio is: TT, Tt, Tt = 3 tall tt = 1 short 3:1 – tall : short T t TT Tt tt

PunnetT Squares Simple dominance: one allele is completely dominant P=Aa x Aa A=red, a=clear A a   A a F1 Genotype= AA: Aa: aa 1: 2: 1 F1 Phenotype= A (red): a (clear) 3: 1 AA Aa aa

Laws of probability help explain genetic events Genetic ratios are most properly expressed as probabilities: Probabilities range from 0 - an event is certain NOT to happen 1.0 - an event is certain to happen

Predicting Outcomes-probability The likelihood that a specific event will occur Probability = # of one kind of possible outcome total # of all possible outcomes a coin lands on “heads” 1 outcome Total possible outcomes = 2 heads or tails Possibility that the coin will land on heads = 1/2

Product law For simultaneous outcomes (this AND that) What is the chance that you will roll snake eyes with two dice? (1 and 1) Chance of rolling 1 with first die = 1/6 Chance of rolling 1 with second die = 1/6 Chance of rolling two 1’s = 1/6 X 1/6 = 1/36

What fraction would we expect to be Probabilities Multiplication Rule The probability that two independent events, A and B, are realized simultaneously is given by the product of their separate probabilities What fraction would we expect to be Round AND Green 3/4 x 1/4 = 3/16

Sum law For outcomes that can occur more than one way (this OR that) What is the chance that you will roll either a 1 or a 6 with one die? Chance of rolling 1 = 1/6 Chance of rolling 6 = 1/6 Chance of rolling 1 or 6 = 1/6 + 1/6 = 2/6 = 1/3

What fraction would we expect to be Probabilities Addition Rule The probability that one or the other of two mutually exclusive events, A or B, is the sum of their separate probabilities What fraction would we expect to be (Round and Green) OR (wrinkled and yellow) 3/16 + 3/16 = 6/16

Dihybrid cross F1 produces equal amounts of 4 possible genotypes F2 reveals even more genotypic possibilities (9:3:3:1) Dihybrid cross is equivalent to two monohybrid crosses (12:4 or 3:1) Illustrates the Law of Independent Assortment

Why we look the way we look... Mendel and Heredity