Punnett Squares Punnett Squares Way to show possible
Punnett Squares • Way to show possible genotypic and phenotypic outcomes for simple genetic crosses • Shows the probability for each outcome • There are two types: – Monohybrid cross-one trait crossed – Dihybrid cross – two traits crossed
P 1 generation—original parents F 1 generation—offspring of the parent plants (filial) • Ex. Your parents are P 1 and you are F 1 P 1 Short pea plant F 1 Tall pea plant All tall pea plants F 2 generation—offspring of the F 1 generation. Ex. Your children F 2 3 tall: 1 short
Mendel’s Experiment • Mendel crossed a tall plant with a short plant and remember he got…. • The offspring in the 1 st generation were all tall • The offspring in the second generation, a tall and a short, were 75% tall and 25% short Why? What happened?
• He concluded that tall (T) must be the dominant trait and short (t) must be recessive. • Remember he crossed a tall pea plant with a short pea plant. The F 1 generation were all tall. • So was the tall plant homozygous (TT) or heterozygous (Tt)? But what about the F 2 Generation? ! • You can do a Punnett square to figure it out!
Mendel’s Experiment Crossing tall homozygous dominant pea plants with short homozygous recessive. T = tall T T t =short t TT x tt Offspring Phenotype: 4/4 or 100 % tall Genotype: 4/4 or 100 % Tt t Tt Tt
Mendel’s Experiment In the next generation he got 3 tall plants and 1 short plant. Let’s see why. T = tall T t t =short T Tt x Tt Offspring Phenotype: 75 % tall, 25 % short Genotype: 25 % TT, 50 % Tt, 25 % tt t TT Tt Tt tt
Mendel’s Experiment • Crossing homozygous pea plants W=purple flower w=white flower W WW x ww W w Genotype: 4 Ww Phenotype: 4 purple 0 white w Ww Ww
• Crossing heterozygous pea plants W Ww x Ww w WW Ww Ww ww Genotype: 1 WW : 2 Ww : 1 ww Phenotype: 3 purple 1 white