Dihybrid Cross Punnett Square: Example and Calculation

🧬 Quick Study Guide

• 🌱 A dihybrid cross involves tracking two different genes with two alleles each.
• 🧮 Each parent can produce 4 different gametes based on the possible combinations of alleles.
• 📝 A 4x4 Punnett square is used to visualize all possible offspring genotypes.
• 📊 The phenotypic ratio for a dihybrid cross with two heterozygous traits is typically 9:3:3:1.
• 💡 Remember to determine the genotypes of the parents before setting up the Punnett square.
• ➕ Use the product rule to quickly calculate the probability of specific offspring genotypes. For example, if the probability of getting 'aa' is $\frac{1}{4}$ and the probability of getting 'bb' is $\frac{1}{4}$, then the probability of getting 'aabb' is $\frac{1}{4} \times \frac{1}{4} = \frac{1}{16}$.

🧪 Practice Quiz

1. A pea plant is heterozygous for both seed color (Yy) and seed shape (Rr). What are the possible allele combinations that can be present in a single pollen grain?
   1. YR, Yr, yR, yr
   2. YY, rr, Yy, Rr
   3. Y, R, y, r
   4. YyRr
2. In guinea pigs, black fur (B) is dominant to brown fur (b), and rough fur (R) is dominant to smooth fur (r). If a guinea pig heterozygous for both traits (BbRr) is crossed with a guinea pig with brown, smooth fur (bbrr), what proportion of the offspring will have brown, smooth fur?
   1. 1/16
   2. 3/16
   3. 1/4
   4. 9/16
3. In tomatoes, red fruit (R) is dominant to yellow fruit (r), and tall plants (T) are dominant to dwarf plants (t). A plant heterozygous for both traits (RrTt) is crossed with a plant that is homozygous recessive for both traits (rrtt). What is the probability of obtaining a plant with red fruit and dwarf height?
   1. 1/2
   2. 1/4
   3. 1/8
   4. 3/4
4. What is the expected phenotypic ratio of the offspring from a dihybrid cross between two parents heterozygous for both traits, assuming independent assortment?
   1. 1:1:1:1
   2. 3:1
   3. 9:3:3:1
   4. 1:2:1
5. In a certain species of flower, purple petals (P) are dominant to white petals (p) and long stems (L) are dominant to short stems (l). A plant with genotype PpLl is crossed with a plant with genotype ppll. What proportion of the offspring will have white petals and long stems?
   1. 1/2
   2. 1/4
   3. 3/4
   4. 1/8
6. Two plants with the genotypes AaBb and Aabb are crossed. What is the probability of obtaining an offspring with the genotype AAbb?
   1. 1/2
   2. 1/4
   3. 1/8
   4. 1/16
7. In dogs, black coat color (B) is dominant to brown (b), and straight fur (S) is dominant to curly (s). If two dogs heterozygous for both traits (BbSs) are mated, what is the probability of having an offspring with brown coat and curly fur?
   1. 1/16
   2. 3/16
   3. 9/16
   4. 1/4