Probability & Genetics Punnet Squares

Probability Probability Probability The likelihood that a particular event will occur. The likelihood that a particular event will occur. Used to predict the results of an event. Used to predict the results of an event. Ex. What is the likelihood that when you flip a coin you’ll get heads? Ex. What is the likelihood that when you flip a coin you’ll get heads?

How Does Probability Relate to Genetics? Remember that Mendel counted the offspring from every cross that he carried out. Remember that Mendel counted the offspring from every cross that he carried out. When Mendel crossed two plants that were hybrid for stem height (Tt) he found that When Mendel crossed two plants that were hybrid for stem height (Tt) he found that ¾ of the F 1 plants had tall stems. ¾ of the F 1 plants had tall stems. ¼ of the plants had short stems. ¼ of the plants had short stems. He got these numbers over and over. He got these numbers over and over. The probability was ¾ tall and ¼ short. The probability was ¾ tall and ¼ short. Mendel used probabilities to predict the results of a genetic cross. Mendel used probabilities to predict the results of a genetic cross.

Punnett Square Shows all the possible outcomes or combinations of alleles for a genetic cross Helps to determine the probability that an offspring will have a certain trait

Reminder of Terms Alleles Alleles Two forms of a gene (dominant & recessive) Two forms of a gene (dominant & recessive) Dominant Allele Dominant Allele Stronger of two genes expressed in the hybrid Stronger of two genes expressed in the hybrid Represented by a capital letter (R) Represented by a capital letter (R) Recessive Allele Recessive Allele Gene that is masked and shows up less often in a cross Gene that is masked and shows up less often in a cross Represented by a lowercase letter (r) Represented by a lowercase letter (r)

Phenotypes vs. Genotypes Genotype Genotype Actual gene combination for a trait Actual gene combination for a trait Ex. TT, Tt, tt Ex. TT, Tt, tt Phenotype Phenotype Physical appearance, or its visible traits that result from a genotype Physical appearance, or its visible traits that result from a genotype Ex. Short or Tall Ex. Short or Tall

Genotype & Phenotype in Flowers Genotype of alleles: R = red flower r = yellow flower All genes occur in pairs, so 2 alleles affect a characteristic Possible combinations are: GenotypesRR Rrrr PhenotypesRED RED YELLOW

Types of Genotypes  Homozygous Genotype  An organism that has 2 identical alleles  Either 2 dominant or 2 recessive genes  AKA: Purebred  Ex. RR Homozygous dominant  Ex. rr Homozygous recessive  Heterozygous Genotype  An organism that has 1 dominant allele and one recessive allele  AKA: Hybrid  AKA: Hybrid  Ex. Rr

Setting up a Punnett Square 1.Draw a large square, and then divide it into 4 equal sections (also squares).

Using a Punnett Square 2.Find two parents to mate that you know their genotype Ex. A red flower (genotype Rr) and a white flower (genotype rr). Rr x rrEx. A red flower (genotype Rr) and a white flower (genotype rr). Rr x rr 3.Place one of the parents on top, and one on the left.

Using a Punnett Square 4.Take each letter in each column and combine it with each letter from each row in the corresponding square.

Using a Punnett Square 5.Use the possible genotypes to determine the probability (chance) that you will have a particular phenotype or genotype. Ex. Red phenotype (2/4 or 50%) Ex. Red phenotype (2/4 or 50%) White phenotype (2/4 or 50%) White phenotype (2/4 or 50%) Phenotype Ratio: 1:1 Genotype Ratio: 1Rr:1rr

Punnett Square Work Construct a Punnett Square for Black Mice (BB) X Black Mice (Bb). Note: bb=white Construct a Punnett Square for Black Mice (BB) X Black Mice (Bb). Note: bb=white

Black Mice (BB) X Black Mice (Bb). Black Mice (BB) X Black Mice (Bb). B b BB BB Bb BB

Black Mice (BB) X Black Mice (Bb). Black Mice (BB) X Black Mice (Bb). B b BB BB Bb BB What were the genotypes produced by the cross? What were the genotypes produced by the cross? BB and Bb BB and Bb What were the phenotypes produced by the cross? What were the phenotypes produced by the cross? All black All black What were the two homozygous genotypes? Were there two? What were the two homozygous genotypes? Were there two? BB BB What was the heterozygous genotype? What was the heterozygous genotype? Bb Bb

Black Mice (BB) X Black Mice (Bb). Black Mice (BB) X Black Mice (Bb). B b BB BB Bb BB What was the genotypic ratio? What was the genotypic ratio? 1BB: 1Bb 1BB: 1Bb What was the phenotypic ratio? What was the phenotypic ratio? All black All black

What do we mean by a Punnett Square?

Genetic Practice Problems

Breed the P 1 generation tall (TT) x dwarf (tt) pea plants tall (TT) x dwarf (tt) pea plants t t TT

Solution: t t TT Tt All Tt = tall (heterozygous tall) produces the F 1 generation tall (TT) vs. dwarf (tt) pea plants

Breed the F 1 generation tall (Tt) vs. tall (Tt) pea plants tall (Tt) vs. tall (Tt) pea plants T t Tt

Solution: TT Tt tt T t Tt produces the F 2 generation 1/4 (25%) = TT 1/2 (50%) = Tt 1/4 (25%) = tt 1:2:1 genotype 3:1 phenotype 3:1 phenotype tall (Tt) x tall (Tt) pea plants

Back to Mendel’s Peas

Trait: Seed Shape Alleles: R – Roundr – Wrinkled Cross: Round seeds x Wrinkled seeds RR x rr Crossing Mendel’s Parental R R rr Rr Genotype:Rr Genotype: Rr PhenotypeRound Phenotype: Round Genotypic Ratio:All alike Genotypic Ratio: All alike Phenotypic Ratio: All alike

Crossing Mendel’s P Generation Homozygous dominant x Homozygous recessive Offspring all Heterozygous (hybrids) Offspring called F 1 generation Genotypic & Phenotypic ratio is ALL ALIKE

Trait: Seed Shape Alleles: R – Roundr – Wrinkled Cross: Round seeds x Round seeds Rr x Rr F 1 X F 1 Cross R r rR RR rrRr Genotype:RR, Rr, rr Genotype: RR, Rr, rr PhenotypeRound & wrinkled Phenotype: Round & wrinkled G.Ratio:1:2:1 G.Ratio: 1:2:1 P.Ratio: 3:1

F 1 X F 1 Cross Review Heterozygous x heterozygous Offspring: 25% Homozygous dominant RR 50% Heterozygous Rr 25% Homozygous Recessive rr Offspring called F 2 generation Genotypic ratio is 1:2:1 Phenotypic Ratio is 3:1

What Do the Peas Look Like?

How Do We Find Out if Something is Homozygous Dominant or Heterozygous? Perform a Test Cross Take a hybrid from the F 2 generation and cross with a pure There are two possible testcrosses: Homozygous dominant x Hybrid Homozygous recessive x Hybrid

Trait: Seed Shape Alleles: R – Roundr – Wrinkled Cross: Round seeds x Round seeds RR x Rr Test Cross (1 st ) R R rR RR RrRR Rr Genotype:RR, Rr Genotype: RR, Rr PhenotypeRound Phenotype: Round Genotypic Ratio:1:1 Genotypic Ratio: 1:1 Phenotypic Ratio: All alike

Trait: Seed Shape Alleles: R – Roundr – Wrinkled Cross: Wrinkled seeds x Round seeds rr x Rr Test Cross (2nd) r r rR Rr rrRr rr Genotype:Rr, rr Genotype: Rr, rr PhenotypeRound & Wrinkled Phenotype: Round & Wrinkled G. Ratio:1:1 G. Ratio: 1:1 P.Ratio: 1:1

Testcross Review Homozygous x heterozygous(hybrid) Offspring: 50% Homozygous RR or rr 50% Heterozygous Rr Phenotypic Ratio is 1:1 Called Test Cross because the offspring have SAME genotype as parents

Summary Questions What is meant by probability? How is probability related to genetics? What is meant by probability? How is probability related to genetics? How are Punnett squares useful to geneticists? How are Punnett squares useful to geneticists? What is the difference between a phenotype and a genotype? Give an example of each. What is the difference between a phenotype and a genotype? Give an example of each.

Applying What We’ve Learned In pea plants, the allele for round seeds (R) is dominant over the allele for wrinkled seeds (r). In pea plants, the allele for round seeds (R) is dominant over the allele for wrinkled seeds (r). Construct a Punnett square that shows a cross between a heterozygous plant with round seeds (Rr) and a homozygous plant with wrinkled seeds (rr). Construct a Punnett square that shows a cross between a heterozygous plant with round seeds (Rr) and a homozygous plant with wrinkled seeds (rr). What is the probability that an offspring will have wrinkled seeds? What is the probability that an offspring will have wrinkled seeds?