# PUNNETT SQUARES pg 92 PUNNETT SQUARES Punnett Square

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PUNNETT SQUARES ~pg. 92

PUNNETT SQUARES Punnett Square - a diagram used to predict the possible offsprings of a cross Created by Reginald Punnett The alleles of one parent are written across the top of the square (one allele per box) and those of the other parent are written on the side

Maternal Paternal B b B BB Bb bb Genotype Frequency: 25% BB, 50% Bb, 25% bb Phenotype Frequency: 75% Black Fur, 25% White Fur

EXAMPLE: GUINEA PIG DIAGRAM In guinea pigs, black fur (B) is dominate to white fur (b). A heterozygous female mates with a white male. What are the genotype and phenotype ratios of the offspring? (Make a punnett square) B b b Bb bb Genotype: 50% Bb, 50% bb Phenotype: 50% Black, 50% White

EXAMPLE: FLOWER PETAL DIAGRAM In a certain flower type, red colored petals (R) is R R dominate to yellow colored petals (r). A Rr Rr r homozygous red and a homozygous yellow flower are crossed. r Rr Rr What are the genotype and phenotype ratios of the offspring? (Make a Genotype: 100% Rr punnett square) Phenotype: 100% Red colored petals

MAKE UP A PROBLEM THAT REQUIRES A PUNNETT SQUARE TO SOLVE. WRITE IT OUT AND THEN SWITCH WITH A NEIGHBOR. SOLVE EACH OTHERS PROBLEM.

WHY DOES THE PUNNETT SQUARE WORK? It all goes back to meiosis…each side represents a sperm or egg. The boxes simply give you the statistical chance that a certain sperm will fertilize a certain egg. Consider a pea plant that is both round and tall = Rr. Tt When this plant’s cells go through meiosis, the alleles segregate – each sperm/egg receives a random combination… These possible combinations include: RT, Rt, r. T, rt