6.3 Mendel and Heredity
Section 6-5: “Punnet Squares & Probability”
Write everything that is underlined
6.3 Mendel and Heredity
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.
6.3 Mendel and Heredity
Using Punnett Squares
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
6.3 Mendel and Heredity
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 SquareFather (Rr)
Mother (Rr)
6.3 Mendel and Heredity
“…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).
(1) 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.
6.3 Mendel and Heredity
Answer
Parents’ allele combinations for Pea Pod shape:
- Father RR - Mother rr
Punnett Square for types of possible offspring:
6.3 Mendel and Heredity
“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”?
6.3 Mendel and Heredity
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.
6.3 Mendel and Heredity
…
(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)?
i. Write the allele combination (“genotype”) for the offspring: ___ ___
ii. Fill out a Punnett Square for two of these offspring.
6.3 Mendel and Heredity
Answer …
Parents allele combinations (genotypes):
Father’s genotype: Rr
Mother’s genotype: Rr
Punnett Square for types of possible offspring:
6.3 Mendel and Heredity
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.
6.3 Mendel and Heredity
And, FINALLY!
What are the probabilities for each of the two possible phenotypes (appearances)?
Dominant phenotype (Round): ?
Recessive phenotype (wrinkled): ?
6.3 Mendel and Heredity
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%
6.3 Mendel and Heredity
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
6.3 Mendel and Heredity
Answer….
(1) 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
6.3 Mendel and Heredity
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
6.3 Mendel and Heredity
Punnett Squares: We show the possible offspring combinations with a diagram called a Punnett Square
PUNNETT SQUARES- We show the possible offspring combinations with a
diagram called a “Punnett Square”
6.3 Mendel and Heredity
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 “
Top Related