Punnett Square Catalyst Is it possible for two brown eyed people to have a blue eyed child? Explain your answer.

Objective 5 SWBAT solve simple genetics problems using monohybrid Punnett squares -I can identify homozygous and heterozygous genotypes -I can determine an organism’s genotype for a trait based on a given scenario -I can use a cross to predict the genotype/phenotype ratio of potential offspring -I can ‘work backwards’ to determine the parents of a given offspring.

Key Words Allele Homozygous Heterozygous Dominant Recessive Genotype Phenotype Cross Purebred Hybrid Probability Ratio Offspring Key Words

An EOCT Problem might look like this… Which of the following describes a phenotype? A Gg B homozygous C having freckles D heterozygous

Comparing Genotypes: BB Homozygous: -Same alleles -Purebred bb Heterozygous: -Different alleles -Hybrid Bb For Complete Dominance: A heterozygous genotype will code for the DOMINANT phenotype

Homozygous vs Heterozygous Practice For each genotype below, indicate whether it is a heterozygous (He) OR homozygous (Ho). TT _____ Bb _____ DD _____ Ff _____ tt _____ dd _____ Dd _____ ff _____ Tt _____ bb _____ BB _____ FF _____

Using Genotype to Determine Phenotype To be successful at this, you have to know which trait is DOMINANT! If it is a Complete Dominance example: Heterozygous genotype = Dominant phenotype If it is a Incomplete Dominance example: Heterozygous genotype = Blend of the two traits If it is a Codominant example: Heterozygous genotype = Spotted/patches of both traits

Think: What is the Heterozygous phenotype? Complete Dominance RR=red flower rr= white flower Incomplete Dominance RR= red flower Codominance RR= Red flower R’R’= white flower Rr= Rr= RR’=

Punnett squares let you predict the genotypes of the children

If you are looking at a single trait, you must use a single letter from the alphabet! Ex: eye color = letter b

Baby Steps to Solving Punnett Squares BABY STEPS: 1. determine the genotypes of the parent organisms *For long scenarios, make a T-chart to jot down notes about each parent

Green peas (G) are dominant to yellow peas (g) in pea plants Green peas (G) are dominant to yellow peas (g) in pea plants. A heterozygous pea plant is crossed with a yellow pea plant.

BABY STEPS: 2. write down your "cross" (mating): __________ x____________

Baby Steps #3 Draw a punnett-square

Baby Steps #4 “Split" the letters of the genotype for each parent & put them "outside" the p-square (This represents the process of Meiosis and the potential GAMETES!*) Tt x tt

Baby Steps #5 Determine the possible genotypes of the offspring by filling in the p-square

Baby Steps #6 Summarize results in a KEY *letters will vary depending on problem Genotypes Ratio Phenotype Ratio

Shortcut #1 Homozygous Dominant + Homozygous recessive = 100% heterozygous TT x tt 100 % Tt (100% dominant phenotype)

Shortcut #2: Heterozygous Cross (1:2:1 Rule) If you cross TWO HETEROZYGOUS, you will always get… 1 homozygous dominant (BB) 2 heterozygous (Bb) 1 homozygous recessive (bb) Bb x Bb

Shortcut #3: Half and Half Homozygous crossed with heterozygous = HALF AND HALF hh x Hh -50 % hh -50% Hh

Punnett Square Practice Problem In elephants, the gene for long trunks is dominant over the gene for short trunks. A mom who is homozygous recessive is crossed with a heterozygous dad. What are the genotypes and phenotypes of their offspring?