DEVELOPMENT AND PROBABILITY OF A PUNNETT SQUARE

LESSON OBJECTIVES Explain the Mendelian Principles of Dominance, Segregation, and Independent Assortment Using a punnett square, determine the probability of obtaining any one type of genotype and phenotype in a genetic cross

Gregor Mendel

In the 1860’s Austrian Monk Gregor Mendel, based on his experiments with the garden pea, put forward the major ideas of transmission genetics. He is called the “Father of Genetics”. Gregor Mendel

TRAITS -Expressed characteristics GENES - Carry information for traits - Mendel called them unit factors ALLELES - Alternate forms for the same gene - two alleles for each trait - a capital letter and its lower case represent the two alleles (TT = Tallness) (tt = shortness) TERMINOLOGY

TT = Homozygous Dominant (purebred) tt = Homozygous Recessive (purebred) Tt = Heterozygous (hybrid) Phenotype = Expressed Traits/Characteristics Genotype = Genetic Make Up of an Individual TERMINOLOGY

MONOHYBRID CROSS TT Tall Plant X tt Short Plant Tt Tall Planth

TT X tt Tall Plant Short Plant (Parent Generation) Tt, Tt, Tt, Tt All Tall Plants (1st Filial Generation or F1) MONOHYBRID CROSS

RESULTS Short trait disappeared All offspring tall CONCLUSION Mendel’s Law of Dominance Tallness allele (T) masked shortness allele (t) (T) called dominant, (t) called recessive MONOHYBRID CROSS

USING A PUNNETT SQUARE TT One Parent t t Other Parent Parent Generation - Outside F1 Generation - Inside

TT t t MONOHYBRID CROSS USING A PUNNETT SQUARE Results: All 4 F1 = Tt Phenotype = 4 Tall Plants Genotype = 4 Tt

TT Female Gametes TT t t tt Male Gametes MEIOSIS AND THE PUNNETT SQUARE Allele Represents a Gamete

SELFING F1 (Tt) GENERATION T t T t F2 Generation

F2 GENERATION RESULTS F2 Ratio 1 TT 2 Tt 3/4 tall 1/4 short1 tt F 2 Probability Genotype Phenotype 1/4 (25%) TT 2/4 (50%) Tt 1/4 (25%) tt ___________________________________________________________ 3/4 (75%) 1/4 (25%)

CONCLUSION Mendel’s Law of Segregation Alleles Separated and Recombined (as evidenced by reappearance of short trait)

PHENOTYPE GENOTYPE Tall 3 x 0.25 = 75% Short 1 x 0.25 = 25% TT = 0.25 Tt = 2 x 0.25 = 0.50 tt = 0.25

GENETICS AND PROBABILITY Mendel applied the mathematical concept of probability to biology. Probability is the likelihood that a particular event will occur.

GENETICS AND PROBABILITY Probability is determined by the following formula: The number of times a particular event occurs. Probability = The number of opportunities for the event to occur.

GENETICS AND PROBABILITY Two important rules of probability: 1. You only get the expected ratio for large numbers of trials. The larger the number of trials, the closer you get to the expected ratio.

GENETICS AND PROBABILITY Two important rules of probability: 2. Previous events do not affect future outcomes. (For example, for each flip of a coin the probability of getting heads is always 1/2, or 1:1)

GENETICS AND PROBABILITY The rules of probability apply to genetics as well as to flipping a coin. Expected genetic ratios may not show up when only a few organisms are considered. The larger the number of organisms examined, the closer the numbers will get to the expected values.

GENETICS AND PROBABILITY Genetic ratios do not indicate what the outcome of a single event will be. Because previous events do not affect future outcomes, it cannot be assumed that a particular event will occur because it seems overdue.

GENETICS AND PROBABILITY Despite such limitations, genetic ratios are still very useful because they make it possible to predict the most likely outcome for a large number of events.

PROBABILITY T t p (0.5) p (0.5) T p (0.5) t

F2 GENERATION RESULTS F 2 Probability Genotype Phenotype 1/4 (25%) TT 2/4 (50%) Tt 1/4 (25%) tt 3/4 (75%) 1/4 (25%)

Test Cross The cross of an organism with an unknown genotype and a homozygous recessive individual is called a test cross. It is used to determine the genotype of the individual showing the dominant trait.

Test Cross For example, cross a tall plant (unknown genotype) with a short plant. If any of the offspring show the recessive phenotype, then the unknown parent must be heterozygous.

Test Cross

Two-Factor Cross After crossing plants for one trait, Mendel crossed plants for two traits. A cross that involves two traits, such as seed shape and color, is called a two-factor cross or dihybrid cross.

Two-Factor Cross Mendel discovered that the two traits separate and recombine. This is known as Mendel’s Law of Independent Assortment.

Two-Factor Cross without independent assortment

Two-Factor Cross with independent assortment