1. Punnett Squares
Note Frame 11

2. Key Term
Punnett Square
Crossing
Complete dominance

3. Punnett Squares
Punnett Square is a way to predict all possible combinations of alleles from crossing parents
Used to predict the genotype and phenotype of offspring
Need to know genotypes of parents

4. Complete Dominance is when the dominant allele will mask the expression of the recessive allele

5. Example:
In pea plants, the colour of the flower purple (P) is dominant over white (p).
Use a Punnett square to find the possible outcomes of crossing a male heterozygous plant with a female homozygous recessive plant.

6. Get info from the question
How to Make a Punnett Square
Write symbols and what they stand for.
Get info from the question
“purple (P) is dominant over white (p)”
Purple = P
White = p

7. Get info from the question
2. Write the genotypes of each parent
Get info from the question Pp
Male heterozygous plant pp
Female homozygous recessive plant
Male = Pp
Female = pp

8. 3. Draw a Punnett Square ♂ Pp
Male= ♀
Female= pp

9. ♂ P Pp p ♀ p p Male= Female= pp
4. Write parent genotypes on top and side – one allele in each box ♂ P Pp p
Male= ♀ p p
Female= pp

10. ♂ Pp p p p P P P ♀ p p p p p p Male= Female= pp
5. Cross the male and female alleles to find the genotype of the offspring ♂
Male= Pp p p p P P P ♀
Female= pp p p p
**Always write capital letter first! p p p

11. ♂ Pp p P ♀ p p P p p Purple White p p P p p Purple White Male= Female=
6. Write the phenotype for each possible offspring ♂ Pp
Male= p P ♀
Female= pp p p P p p
Purple
White
Purple = P
White = p p p P p p
Purple
White

12. ♂ Pp p P ♀ p p P p p Purple White p p P p p Purple White Male= Female=
7. Write the answer as a ratio or percent (whatever you find easier!) ♂ Pp
Male= p P ♀
Female= pp p p P p p
Purple to white
2:2  1:1
Purple
White p p P p
50% purple
50% white p
Purple
White

13. Practice 1
In humans, dimples (D) is dominant over no-dimples (d).
Use a Punnett square to find the possible outcomes of crossing a male and female who are both heterozygous for dimples.

14. ♂ d D ♀ DD Dd D Dd dd d Dimples = D Male = Dd No dimples = d
Female = Dd ♂ d D ♀ DD Dd D
Dimples to non-dimple
3:1
Dimples
Dimples Dd dd
75% dimple
25% non-dimple d
Dimples
No Dimples

15. Practice 2
In humans, being able to roll your tongue (R) is dominant over not being able to roll your tongue (r).
Use a Punnett square to find the possible outcomes of crossing a male who is homozygous recessive and female who is heterozygous for tongue rolling.

16. ♂ r r ♀ Rr Rr R rr rr r Roll = R Male = rr No roll = r Female = Rr
Roll to no-roll
1:1
Roll
Roll rr rr
50% Roll
50% no-roll r
No roll
No roll

17. Practice 3
In humans, long eyelashes (L) is dominant over short eyelashes (l).
Use a Punnett square to find the possible outcomes of crossing a male who has short eyelashes with a female who is homozygous with long eyelashes.

18. ♂ l l ♀ Ll Ll L Ll Ll L Long lashes = L Male = ll Short lashes = l
Female = LL ♂ l l ♀ Ll Ll L
Long lashes : short
1:0
Long
Long Ll Ll
100% Long L
Long
Long