Question

genetics practice problems

1. for each of the genotypes below determine what phenotypes would be possible.

purple flowers are dominant to white
pp ______
pp ______
pp ______
brown eyes are dominant to blue
bb ______
bb ______
bb ______

1. for each phenotype below, list the genotypes (remember to use the letter of the dominant trait)

straight hair is dominant to curly
______ straight
______ straight
______ curly
tail spikes are dominant to plain tails
______ spikes
______ spikes
______ plain

1. complete the punnett squares for each of the crosses listed below.

tall (t) plants are dominant to short (t).
tt x tt
punnett square with t and t at top, two blanks on left
what percentage of the offspring are tall? ____ short? ____
tt x tt
punnett square with two blanks at top, two blanks on left
what percentage of the offspring are tall? ____ short? ____

1. a homozygous tall plant is crossed with a short plant.

punnett square with two blanks at top, two blanks on left
what percentage of the offspring are tall? ____ short? ____

Explanation:

Step1: Solve Problem 1 (Phenotypes)

Dominant traits show if at least one dominant allele is present; recessive only with two recessive alleles.

• Purple (P) dominant to white (p):
• PP: Purple flowers
• Pp: Purple flowers
• pp: White flowers
• Brown eyes (B) dominant to blue (b):
• BB: Brown eyes
• Bb: Brown eyes
• bb: Blue eyes

Step2: Solve Problem 3 (Genotypes)

Use dominant trait letter; dominant phenotypes have 2 genotype options, recessive only 1.

• Straight hair (S) dominant to curly (s):
• Straight: $SS$, $Ss$
• Curly: $ss$
• Tail spikes (K) dominant to plain (k):
• Spikes: $KK$, $Kk$
• Plain: $kk$

Step3: Complete Tt x tt Punnett Square

Parent alleles: Tt gives T/t; tt gives t/t.

$$\begin{array}{c|cc} & T & t \\ \hline t & Tt & tt \\ t & Tt & tt \\ \end{array}$$

Count genotypes: 2 Tt (tall), 2 tt (short).

• Tall percentage: $\frac{2}{4} \times 100 = 50\%$
• Short percentage: $\frac{2}{4} \times 100 = 50\%$

Step4: Complete Tt x Tt Punnett Square

Parent alleles: Both Tt give T/t.

$$\begin{array}{c|cc} & T & t \\ \hline T & TT & Tt \\ t & Tt & tt \\ \end{array}$$

Count genotypes: 3 tall (TT, Tt, Tt), 1 short (tt).

• Tall percentage: $\frac{3}{4} \times 100 = 75\%$
• Short percentage: $\frac{1}{4} \times 100 = 25\%$

Step5: Complete TT x tt Punnett Square

Homozygous tall is TT (gives T/T); short is tt (gives t/t).

$$\begin{array}{c|cc} & T & T \\ \hline t & Tt & Tt \\ t & Tt & Tt \\ \end{array}$$

All offspring are Tt (tall).

• Tall percentage: $\frac{4}{4} \times 100 = 100\%$
• Short percentage: $0\%$

Answer:

1.

• PP: Purple flowers
• Pp: Purple flowers
• pp: White flowers
• BB: Brown eyes
• Bb: Brown eyes
• bb: Blue eyes

3.

• Straight hair: $SS$, $Ss$
• Curly hair: $ss$
• Tail spikes: $KK$, $Kk$
• Plain tails: $kk$

4.
Tt x tt:

$$\begin{array}{c|cc} & T & t \\ \hline t & Tt & tt \\ t & Tt & tt \\ \end{array}$$

• Tall: 50%
• Short: 50%

Tt x Tt:

$$\begin{array}{c|cc} & T & t \\ \hline T & TT & Tt \\ t & Tt & tt \\ \end{array}$$

• Tall: 75%
• Short: 25%

5.

$$\begin{array}{c|cc} & T & T \\ \hline t & Tt & Tt \\ t & Tt & Tt \\ \end{array}$$

• Tall: 100%
• Short: 0%