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10/19/2013 Predicting Traits Monohybrid and Dihybrid Punnett Squares Date _____________ Punnett Squares • Monohybrid crosses –Examines the inheritance of one trait –Can be homozygous-homozygous, heterozygous-heterozygous, or heterozygous-homozygous • Dihybrid crosses –Examines the inheritance of two traits Monohybrid Punnett Squares Parent Offspring’s alleles go alleles go here here • The principles of Mendelian genetics can be used to predict the inherited traits of offspring • A punnett square can be used to predict genetic combinations in the offspring that result from different parental allele combinations Crossing Individuals • When you are tracing the probability of traits being passed down through families, there are three terms to refer to these generations. – P generation = parental generation; first in the line – F1 generation = first filial generation; comes from the P generation – F2 generation = second filial generation; comes from the F1 generation • Could be considered the “grand children” Interpreting Results T = tall plant t = short plant • 1:4 ratio (25%) that an offspring will carry two dominant alleles – Homozygous dominant – What will this offspring look like? • 1:4 ratio (25%) that an offspring will carry two recessive alleles – Homozygous recessive – What will this offspring look like? T t T t TT Tt Tt tt • 2:4 ratio OR 1:2 ratio (50%) that an offspring will carry one dominant allele and one recessive allele – Heterozygous – What will this offspring look like? 1 10/19/2013 Dihybrid Punnett Squares Allele Combinations • The setup is the hardest part! • You have to ensure that all allele combinations are calculated. • Let’s talk about pea traits: – R = round shape / r = wrinkled shape – Y = yellow color / y = green color • Calculate the possible traits of two parents that are heterozygous for both traits: – RrYy x RrYy • RrYy x RrYy RrYy RY Ry ry rY Interpreting Results RY RY RRYY Ry RRYy rY RrYY ry RrYy Ry RRYy Rryy RrYy Rryy rY RrYY RrYy rrYY rrYy ry RrYy Rryy rrYy rryy Basic Monohybrid Crosses 1. Aa x aa 2. BB x bb 3. Ee x Ee 4. Homozygous dominant x heterozygous 5. Homozygous recessive x homozygous recessive 6. Heterozygous x heterozygous • This is the hardest dihybrid cross to calculate! • All ratios will be out of 16 instead of 4. – – – – – – – – – 1:16 or 1/16 RRYY 2:16 or 2/16 RRYy 1:16 or 1/16 RRyy 2:16 or 2/16 RrYY 4:16 or 4/16 RrYy 2:16 or 2/16 Rryy 1:16 or 1/16 rrYY 2:16 or 2/16 rrYy 1:16 or 1/16 rryy RY Ry rY ry RY RRYY RRYy RrYY RrYy Ry RRYy RRyy RrYy Rryy rY RrYY RrYy rrYY rrYy ry RrYy Rryy rrYy rryy What will these genotypes look like? Word Problems – Mono. Crosses 1. A heterozygous round seeded plant is crossed with a homozygous round seeded plant. What are the possible genotypes and phenotypes of the offspring? 2. In pea plants, purple flowers are dominant to white flowers. If two white flowered plants are crossed, what percentage of their offspring will be white flowered? 3. Sickle-cell anemia is a recessive disorder in which the red blood cells are sickle-shaped and very rigid and fragile. They do not transport oxygen as well as normal red blood cells. Two individuals that carry the sickle-cell trait, but do not have it, want to have a child. They have come to you for genetic advice. What would you tell them about their child potentially having sickle-cell anemia? 4. In dragons, the allele for fire breathing is dominant. Dragons can be fire breathers or non-fire breathers. If a heterozygous fire-breathing dragon is crossed with one that does not breathe fire, what is the possibility that an offspring will be born a fire-breather? 2 10/19/2013 Focus Questions • What are Punnett squares used for? • What is the difference between a monohybrid and dihybrid cross? • Know how to set up a Punnett square and calculate percentages/ratios! 3