4
Monohybrid Cross - Pea Characters
Classical Genetics-Laboratory Exercise 4 Set up Punnet squares for each of the crosses listed below. Show you work. Always show the genotype
and phenotype of the offspring and its ratios then answer the questions
A. Round seeds are dominant to wrinkled seeds
1. In a cross between Rr x m, what percentage of offspring will be round?
2. In a cross between RR x Rr, what percentage of offspring will be round?
3. In a cross between Rrx Rr, what percentage of offspring will be round?
4. A homozygous round seeded plant is crossed with a homozygous wrinkled seeded plant. What are
the genotypes of the parents? What percentage of the offspring will also be homozygous?
B. Tall plant is dominant to short plant
1. ATT plant is crossed with a tt. What percentage of the offspring will be tal?
2. A Tt plant is crossed with a Tt plant. What percentage of the offspring will be short?
3. A heterozygous tall plant is crossed with a homozygous tall plant. What percentage of the offspring
will be homozygous?
C. Purple flowers are dominant to white flowers 1. two white flowered plants are cross, what percentage of their offspring will be white flowered?
2. A white flowered plant is crossed with a plant that is heterozygous for the trait. What percentage of the
offspring will have purple flowers?
3. Two plants, both heterozygous for the gene that controls flower color are crossed. What percentage of their offspring
1. In a cross between Rr x m, the percentage of offspring that will be round can be calculated using a Punnett square.
Rr x m
R m
r m
In this cross, 50% of the offspring will be round.
2. In a cross between RR x Rr, the percentage of offspring that will be round can also be calculated using a Punnett square.
RR x Rr
R R
R R
In this cross, 100% of the offspring will be round.
3. In a cross between Rr x Rr, the percentage of offspring that will be round can be calculated using a Punnett square.
Rr x Rr
R r
R r
In this cross, 75% of the offspring will be round.
4. A homozygous round seeded plant (RR) is crossed with a homozygous wrinkled seeded plant (rr). The genotypes of the parents are RR and rr.
When these two parents are crossed, all of the offspring will be heterozygous (Rr) for the round seed trait. So, 0% of the offspring will be homozygous.
B. Tall plant is dominant to short plant
1. In a cross between ATT plant and tt, the percentage of offspring that will be tall can be calculated using a Punnett square.
ATT x tt
AT tt
AT tt
In this cross, 100% of the offspring will be tall.
2. In a cross between Tt plant and Tt plant, the percentage of offspring that will be short can be calculated using a Punnett square.
Tt x Tt
T t
T t
In this cross, 25% of the offspring will be short.
3. A heterozygous tall plant (Tt) is crossed with a homozygous tall plant (TT). The percentage of offspring that will be homozygous can be calculated using a Punnett square.
Tt x TT
T T
t T
In this cross, 50% of the offspring will be homozygous.
C. Purple flowers are dominant to white flowers
1. In a cross between two white flowered plants, the percentage of their offspring that will be white flowered can be calculated using a Punnett square.
ww x ww
w w
w w
In this cross, 100% of the offspring will be white flowered.
2. In a cross between a white flowered plant (ww) and a plant that is heterozygous for the trait (Ww), the percentage of the offspring that will have purple flowers can be calculated using a Punnett square.
ww x Ww
w W
w w
In this cross, 0% of the offspring will have purple flowers.
3. In a cross between two plants that are heterozygous for the gene that controls flower color (Ww x Ww), the percentage of their offspring that will have purple flowers can be calculated using a Punnett square.
Ww x Ww
W W
W w
In this cross, 75% of the offspring will have purple flowers.
To solve these Punnett squares and determine the percentages of offspring with specific traits, we need to understand the principles of Mendelian genetics and the rules of inheritance.
A. Round seeds are dominant to wrinkled seeds:
1. In a cross between Rr x m, to determine the percentage of offspring that will be round, we need to make a Punnett square. The genotype of the parent Rr (round seed) is heterozygous, and the genotype of the other parent m (wrinkled seed) is recessive. The Punnett square would look like this:
| R | m |
---------------
R | RR | Rm |
---------------
r | Rm | rm |
From the Punnett square, we see that 50% of the offspring will have the genotype Rr, which is round. So, the percentage of offspring that will be round is 50%.
2. In a cross between RR x Rr, both parents have the round seed genotype. The Punnett square would look like this:
| R | R |
---------------
R | RR | RR |
---------------
r | Rr | Rr |
From the Punnett square, we can see that 100% of the offspring will have the genotype Rr, which is round. So, the percentage of offspring that will be round is 100%.
3. In a cross between Rr x Rr, both parents have the round seed genotype. The Punnett square would look like this:
| R | r |
---------------
R | RR | Rr |
---------------
r | Rr | rr |
From the Punnett square, we can see that 75% of the offspring will have the genotype Rr, which is round. So, the percentage of offspring that will be round is 75%.
4. A homozygous round seeded plant (RR) is crossed with a homozygous wrinkled seeded plant (rr). The genotypes of the parents are RR and rr, respectively. All the offspring will have the genotype Rr, which is heterozygous. So, the percentage of offspring that will also be homozygous (RR or rr) is 0%.
B. Tall plant is dominant to short plant:
1. In a cross between ATT plant (homozygous tall) and tt (homozygous short), all the offspring will have the genotype AtTt, which is heterozygous tall. So, the percentage of offspring that will be tall is 100%.
2. In a cross between Tt plant (heterozygous tall) and Tt plant (heterozygous tall), the Punnett square would look like this:
| T | t |
---------------
T | TT | Tt |
---------------
t | Tt | tt |
From the Punnett square, we can see that 25% of the offspring will have the genotype tt, which is short. So, the percentage of offspring that will be short is 25%.
3. In a cross between a heterozygous tall plant (Tt) and a homozygous tall plant (TT), the Punnett square would look like this:
| T | T |
---------------
T | TT | TT |
---------------
t | Tt | Tt |
From the Punnett square, we can see that 50% of the offspring will have the genotype TT, which is homozygous tall. So, the percentage of offspring that will be homozygous is 50%.
C. Purple flowers are dominant to white flowers:
1. In a cross between two white-flowered plants, which are homozygous recessive (ww), 100% of their offspring will also have the genotype ww and exhibit the white flower phenotype. So, the percentage of their offspring that will be white-flowered is 100%.
2. In a cross between a white-flowered plant (ww) and a plant that is heterozygous for the trait (Ww), the Punnett square would look like this:
| W | w |
---------------
W | WW | Ww |
---------------
w | Ww | ww |
From the Punnett square, we can see that 50% of the offspring will have the genotype Ww, which is heterozygous and exhibits the purple flower phenotype. So, the percentage of offspring that will have purple flowers is 50%.
3. In a cross between two plants heterozygous for the gene that controls flower color (Ww x Ww), the Punnett square would look like this:
| W | w |
---------------
W | WW | Ww |
---------------
w | Ww | ww |
From the Punnett square, we can see that 25% of the offspring will have the genotype WW, which is homozygous for purple flowers. So, the percentage of offspring that will be homozygous for flower color is 25%.
Remember, to solve these problems, you need to set up Punnett squares to determine the genotypes and phenotypes of the offspring.
A. Round seeds are dominant to wrinkled seeds
1. In a cross between Rr x m, the genotype of the parents are Rr and mm. To find the percentage of offspring that will be round, we can use a Punnett square:
Rm rM
Rr RrMm rRMm
rM rRm rrMm
Out of the four possible offspring genotypes, only one contains the round seed allele (RrMm). Therefore, 1 out of 4, or 25% of the offspring will be round.
2. In a cross between RR x Rr, the genotype of the parents are RR and Rr. Using a Punnett square:
Rr Rr
RR RRR RRr
RR RRR RRr
All four possible offspring genotypes contain the round seed allele. Therefore, 4 out of 4, or 100% of the offspring will be round.
3. In a cross between Rr x Rr, both parents have the genotype Rr. Using a Punnett square:
Rr Rr
Rr RRR RRr
Rr RRr rrr
Out of the four possible offspring genotypes, three contain the round seed allele (RRR, RRr, RRr). Therefore, 3 out of 4, or 75% of the offspring will be round.
4. A homozygous round seeded plant is crossed with a homozygous wrinkled seeded plant. The genotypes of the parents are RR and rr. All offspring will be heterozygous (Rr), so 0% of the offspring will be homozygous.
B. Tall plant is dominant to short plant
1. In a cross between ATT plant (homozygous tall) and tt plant (homozygous short), the genotypes of the parents are ATT and tt. All offspring will be heterozygous tall (AtT), so 100% of the offspring will be tall.
2. In a cross between Tt plant (heterozygous tall) and Tt plant (heterozygous tall), the genotypes of the parents are Tt and Tt. Using a Punnett square:
Tt Tt
Tt TT Tt
Tt Tt tt
Out of the four possible offspring genotypes, one contains the short allele (tt). Therefore, 1 out of 4, or 25% of the offspring will be short.
3. In a cross between a heterozygous tall plant (Tt) and a homozygous tall plant (TT), the genotypes of the parents are Tt and TT. All offspring will be heterozygous tall (Tt), so 0% of the offspring will be homozygous.
C. Purple flowers are dominant to white flowers
1. In a cross between two white flowered plants, both parents have the genotype ww. All offspring will be homozygous for the recessive white allele, so 100% of the offspring will be white flowered.
2. In a cross between a white flowered plant (ww) and a plant that is heterozygous for the trait (Ww), the genotypes of the parents are ww and Ww. Using a Punnett square:
Ww Ww
ww wW wW
ww ww ww
Out of the four possible offspring genotypes, three contain the dominant purple allele (wW, wW, ww). Therefore, 3 out of 4, or 75% of the offspring will have purple flowers.
3. In a cross between two plants, both heterozygous for the gene that controls flower color (Ww), the genotypes of the parents are Ww and Ww. Using a Punnett square:
Ww Ww
Ww WW Ww
Ww Ww ww
Out of the four possible offspring genotypes, two contain the dominant purple allele (WW, Ww). Therefore, 2 out of 4, or 50% of the offspring will have purple flowers.