Question

genetic problems: monohybrid
for each of the following problems:
a. make a key
b. write out the cross
c. make a punnett square
d. give the resulting genotypes, phenotypes and ratios for both
you must do this for every cross in each question!

1. in pea plants, red flower (r) color is dominant over white (r). cross a red purebred with a white purebred.
2. in pea plants, red flower color is dominant over white. cross a hybrid red with a white.
3. in wheat plants, tall stalks (t) are dominant over short stalks (t). cross a homozygous dominant with a heterozygous.
4. in humans, brown hair color is dominant over blonde. cross a true-breeding brown haired woman with a true-breeding blonde haired man, then cross the f₁ hybrids to get the f₂ results.

Explanation:

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Problem 3:

Step1: Define allele key

Red (dominant): $R$, White (recessive): $r$

Step2: Write parental cross

Homozygous dominant ($RR$) × Heterozygous ($Rr$)

Step3: Create Punnett Square

	$R$	$R$
$r$	$Rr$	$Rr$

Step4: Calculate genotype/phenotype ratios

Genotype ratio: $\frac{2}{4} RR : \frac{2}{4} Rr = 1:1$
Phenotype ratio: $\frac{4}{4}$ red flowers = 100% red
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Problem 4:

Step1: Define allele key

Brown (dominant): $B$, Blonde (recessive): $b$

Step2: F₁ Parental cross

True-breeding brown ($BB$) × True-breeding blonde ($bb$)

Step3: F₁ Punnett Square

	$B$	$B$
$b$	$Bb$	$Bb$

Step4: F₁ Genotype/Phenotype

Genotype: 100% $Bb$; Phenotype: 100% brown hair

Step5: F₂ Parental cross

F₁ hybrids ($Bb$) × F₁ hybrids ($Bb$)

Step6: F₂ Punnett Square

	$B$	$b$
$b$	$Bb$	$bb$

Step7: F₂ Genotype/Phenotype ratios

Genotype ratio: $\frac{1}{4} BB : \frac{2}{4} Bb : \frac{1}{4} bb = 1:2:1$
Phenotype ratio: $\frac{3}{4}$ brown : $\frac{1}{4}$ blonde = 3:1
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Answer:

Problem 3:

• Key: Tall stalks (dominant) = $T$, Short stalks (recessive) = $t$
• Cross: $TT$ × $Tt$
• Genotypes: 1:1 ratio of $TT$ to $Tt$
• Phenotypes: 100% tall stalks (1:0 ratio)

Problem 4:

F₁ Generation:

• Key: Brown hair (dominant) = $B$, Blonde hair (recessive) = $b$
• Cross: $BB$ × $bb$
• Genotypes: 100% $Bb$
• Phenotypes: 100% brown hair

F₂ Generation:

• Cross: $Bb$ × $Bb$
• Genotypes: 1:2:1 ratio of $BB$ : $Bb$ : $bb$
• Phenotypes: 3:1 ratio of brown hair to blonde hair