1. “Punnet Squares & Probability” Write everything that is underlined
Section 6-5:
“Punnet Squares & Probability”
Write everything that is underlined

2. Dominance: - Remember, some inherited versions of a gene
for a trait dominate the other(s)!
Ex: For flower color trait, the purple flower gene/allele
dominates the white flower gene/allele, so plants
with a mix of one purple allele and one white allele,
will be purple.

3. Punnett Squares Using Punnett Squares
- Predict the possible types of offspring of two parents
- Are used to illustrate (show) genetic crosses
- Genes/alleles for a trait are represented by a letter;
> for dominant alleles, a capital is used
> for recessive alleles, lower case
Ex: Round pea pods (dominant) = R
wrinkled pea pods (recessive) = r
copy punnett square 

4. Using Punnett Squares
- Write 1 parent’s gene combination across the top, and the 2nd across the side - Carry the letters down and across to fill the 4 boxes of the Punnett Square
Father (Rr)
Mother
(Rr)

5. “…test. This is only a test. Had this been a real ….”
Pea shapes follows Mendel’s law of dominance. Peas are either Wrinkled or Round. Round is dominant (R) and wrinkled is recessive).
Write the two allele combinations for pod shape for these parents: one that is true-breeding (homozygous) dominant and one that is true-breeding (homozygous) recessive.
Father: ___ ___ Mother: ___ ___
(2) Then, fill in all four boxes of a Punnett Square correctly
with the letters (representing the alleles) from the
parents.

6. Answer
Parents’ allele combinations for Pea Pod shape: - Father RR - Mother rr Punnett Square for types of possible offspring:

7. “This test will continue…”
(3) What would the pods of the offspring of a true-breeding Wrinkled-pod (homozygous dominant) parent and a true-breeding Round-pod (homozygous recessive) parent look like – ie, what are their “phenotypes”?

8. All of the offspring will have Round (R) pea pods.
Answer
All of the offspring will have Round (R) pea pods.
Why?
Because the dominant allele (R) “masks” the recessive (r), as if it weren’t there, even though it is.

9. …
(4) If two of these offspring were cross-bred, how many of their offspring would have the Round phenotype (appearance) and how many would have the wrinkled phenotype (appearance)?
Write the allele combination (“genotype”) for the offspring: ___ ___
Fill out a Punnett Square for two of these offspring.

10. Answer …
Parents allele combinations (genotypes): Father’s genotype: Rr Mother’s genotype: Rr Punnett Square for types of possible offspring:

11. How many would be Round, how many wrinkled?
Three out of four possible offspring would have Round pea pods: RR, Rr, and Rr.
One out of four would have a wrinkled pea pod:
rr.
Why?
Dominance, again, … baby.

12. And, FINALLY!
What are the probabilities for each of the two possible phenotypes (appearances)? Dominant phenotype (Round): ? Recessive phenotype (wrinkled): ?

13. This IS my FINAL answer!
The probabilities for each of the two “possible” phenotypes (appearances) are: Dominant phenotype (Round): ¾ or 3:4 or 75% Recessive phenotype (wrinkled): ¼ or 1:4 or 25%

14. Now YOU!
Do the same for pe pod shape for a heterozygous father and a homozygous recessive parent. List the parents’ genotypes; complete a Punnett Square for them; identify the phenotypes for the offspring; and list the probabilities for each phenotype. GO

15. ALL of the offspring (F1’s) would be Round.
Answer….
ALL of the offspring (F1’s) would be Round.
(Each would receive 1 dominant (round) and 1 recessive (wrinkled)
allele from their true-breeding parents. This is called “heterozygous”
– a mix: “1 Round (R) & 1 wrinkled (r) allele”, or “Rr”)
(2) Seventy-five percent (75%) of the F2 offspring would
be Round and twenty-five percent would be wrinkled.
(Each of the heterozygous F1 “rounds” (above) could give either a
dominant or a recessive allele to an F2 offspring. - Does this blow
your mind right in front of your face?)
F2 Parents: Rr Rr
Give either R R or r = RR or Rr
or r R or r = RR or rr

16. WHAT genes do the F2 offspring receive?!
F1 (parents): Rr Rr (both heterozygous) Give either R + R or r = ? pairs or r + R or r = ? pairs F2 (offspring): = RR or Rr (Four possible = Rr or rr outcomes) RR (1 way) = homozygous dominant = Round peas Rr (2 ways) = heterozygous (dominant) = Round peas rr (1 way) = homozygous recessive = wrinkled peas Outcomes: ¾ Round and ¼ wrinkled

17. Punnett Squares: We show the possible offspring combinations with a diagram called a Punnett Square

18. Dominant and one recessive Allele for pod shape.
Using Punnett Squares
Both parents have one
Dominant and one recessive
Allele for pod shape.
- that’s called “Heterozygous”
Offspring - one of four (1/4) has two dominant alleles
That’s “Homozygous Dominant”
One of four (1/4) has two recessive alleles
That’s “Homozygous Recessive”
Two of four (2/4 = ½) have a mix of one dominant and one recessive
That’s “H “