Traits & Probability Using Punnett Squares What Mendel Knew ● Traits are passed from parent to offspring through genes ● There are two forms to a gene, called alleles ● Each parent gives one allele to offspring ● One allele is dominant, the other is recessive R.C. Punnett ● One of the first geneticists, studied chickens ● Created the Punnett square to calculate possible genotypes in a cross ○ uses Mendel’s law of segregation (one allele from a parent) ○ Dominant alleles use a capital letter, recessive alleles use a lower-case letter Some Genetics Vocabulary ● Homozygous- an organism has two identical alleles ○ BB or bb ○ purebred, true-breeding ● Heterozygous- an organism has two different alleles ○ Bb ○ hybrid Some Genetics Vocabulary ● Genotype- genetic makeup of an organism ● Phenotype- physical characteristics determined by genotype How Punnett Squares Work ● Monohybrid cross- examines one trait Parent genotypes: TT x Tt ● One parent organism’s alleles are on the top of the box, the other parent is T T on the side TT TT ○ only one allele is placed by each T square (represents the law of t Tt Tt segregation) ● Squares are filled in with alleles Punnett Squares & Mendel’s Experiment ● Parental generation, FF (purple flowers) x ff (white flowers) F F ● genotypic ratio: 100% Ff f Ff Ff ● phenotypic ratio: 100% purple flowers f Ff Ff Punnett Squares & Mendel’s Experiment ● F1 generation, Ff (purple flowers) x Ff (purple flowers) F f ● genotypic ratio: F FF Ff 1 FF : 2 Ff : 1 ff ● phenotypic ratio: 3 purple flowers : 1 white f Ff ff flower Bonus Punnett Square!!! ● Heterozygous (Ff) x Homozygous recessive (ff) F f ● genotypic ratio: f Ff ff 2 Ff : 2 ff, 1:1 ● phenotypic ratio: 2 purple flowers : 2 white f Ff ff flowers, 1:1 Test Cross ● Cross between an organism with an unknown genotype and an organism with a recessive genotype ○ if all offspring has the dominant phenotype, the unknown is homozygous dominant ○ if half of the offspring has a dominant and recessive phenotypic ratio of 1:1, the unknown is heterozygous.