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 “