Punnett Square Part 2 A punnett square is used to show the possible allele (gene) combinations for the offspring of 2 parents. The four boxes represent the four possible gene combinations of the offspring

Monohybrid Cross A monohybrid cross = cross involving only 1 trait

How to do a monohybrid cross A heterozygous plant with green peas (Gg) is crossed with a plant with yellow peas (gg) Step 1 : Choose the letter for the alleles (green is dominant; yellow is recessive) G = green (capital letter = dominant) g = yellow (lower case letter = recessive)

How to do a monohybrid cross Step 2: Write the genotypes of the parents Heterozygous plant with green peas = Gg Plant with yellow peas = gg You will cross Gg x gg

How to do a monohybrid cross Step 3: Set up the punnett square with a parent on each side G g g g

How to set up a monohybrid cross Step 4 : fill out the 4 boxes G g G G G g g g G g Gg g g Gg g g Gg g g g g g g g g g g

How to do a monohybrid cross Step 5: look at the 4 boxes from step 4 and determine the genotypes of the four offspring Genotypic ratios: 2 Gg (2 out of the 4 boxes or 2/4 = 50%) 2 gg (2 out of the 4 boxes or 2/4 = 50%) Gg gg

How to do a monohybrid cross Step 6: Look at the genotypes from Step 5 and determine the phenotypes: Green (G) is dominant over yellow (g) Plants that have a G will have green peas Phenotypic ratios: 2 Gg or 2 out of the 4 - 50% will be green 2 gg or 2 out of the 4 – 50% will be yellow

Practice a monohybrid cross Cross an individual with blue eyes with an individual with homozygous brown eyes. Brown eyes (B) is dominant to blue eyes (b). B B b b

Practice a monohybrid cross Cross an individual with blue eyes with an individual with homozygous brown eyes. Brown eyes (B) is dominant to blue eyes (b). B B Bb Genotypic ratio: Bb : 4/4 = 100% Phenotypic ratio: Bb: 4/4 = Brown eyes b b

Practice a monohybrid cross A child is diagnosed with a recessive genetic disease. Neither parent has the disease. What are the genotypes of the parents? N ? nn N ?

Practice a monohybrid cross A child is diagnosed with a recessive genetic disease. Neither parent has the disease. What are the genotypes of the parents? N n NN Nn nn N n

Review of Genetic Voc. Terms Allele A form of a gene Homozygous Both alleles are the SAME Heterozygous Alleles are Different Homozygous Dominant AA Homozygous Recessive aa Heterozygous Aa Genotypic Ratio 1/4 AA = 25% Phenotypic Ratios ¼ Purple (25% purple)