To solve this dihybrid cross problem, we need to determine the possible gametes for each parent and then fill in the Punnett square to find the offspring genotypes and phenotypes.
Step 1: Determine the Gametes
Both parents have the genotype
YyTt.
*
Y = Yellow seeds (Dominant)
*
y = Green seeds (Recessive)
*
T = Tall plant (Dominant)
*
t = Short plant (Recessive)
To find the gametes, we separate the alleles. Each gamete gets one letter for seed color and one letter for height. We can use the "FOIL" method (First, Outer, Inner, Last) or simply pair them up:
1.
Y and
T $\rightarrow$
YT
2.
Y and
t $\rightarrow$
Yt
3.
y and
T $\rightarrow$
yT
4.
y and
t $\rightarrow$
yt
So, the four gametes for both the male and female parents are:
YT, Yt, yT, yt.
Step 2: Fill in the Punnett Square
We place the male gametes across the top and the female gametes down the side. Then we combine them in the boxes.
| |
YT |
Yt |
yT |
yt |
| :--- | :---: | :---: | :---: | :---: |
|
YT | YYTT | YYTt | YyTT | YyTt |
|
Yt | YYTt | YYtt | YyTt | Yytt |
|
yT | YyTT | YyTt | yyTT | yyTt |
|
yt | YyTt | Yytt | yyTt | yytt |
Step 3: Determine the Phenotypes
Now we look at the traits based on the dominant/recessive rules:
* If there is at least one
Y, the seeds are
Yellow.
* If there is no
Y (only
yy), the seeds are
Green.
* If there is at least one
T, the plant is
Tall.
* If there is no
T (only
tt), the plant is
Short.
Let's count the results from the 16 squares:
1.
Yellow & Tall (Has Y_ and T_):
* YYTT, YYTt, YyTT, YyTt, YYTt, YyTt, YyTT, YyTt, YyTt
* Count:
9
2.
Yellow & Short (Has Y_ and tt):
* YYtt, Yytt, Yytt
* Count:
3
3.
Green & Tall (Has yy and T_):
* yyTT, yyTt, yyTt
* Count:
3
4.
Green & Short (Has yy and tt):
* yytt
* Count:
1
This creates the classic Mendelian ratio of 9:3:3:1.
Final Answer:
Gametes: YT, Yt, yT, yt (for both parents)
Punnett Square Genotypes:
Row 1: YYTT, YYTt, YyTT, YyTt
Row 2: YYTt, YYtt, YyTt, Yytt
Row 3: YyTT, YyTt, yyTT, yyTt
Row 4: YyTt, Yytt, yyTt, yytt
Phenotype of offspring:
*
9 Yellow seeds, Tall plants
*
3 Yellow seeds, Short plants
*
3 Green seeds, Tall plants
*
1 Green seeds, Short plants
Parent Tip: Review the logic above to help your child master the concept of dihybrid crosses worksheet.
