1. 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

2. Monohybrid Cross
A monohybrid cross = cross involving only 1 trait

3. 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)

4. 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

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

6. 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

7. 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

8. 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 % will be green
2 gg or 2 out of the 4 – 50% will be yellow

9. 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

10. 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

11. 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 ?

12. 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

13. 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)