Examples of the Sum Rule in Genetic Crosses

📚 Quick Study Guide

• ➕ The sum rule (also called the 'or' rule) applies when calculating the probability of mutually exclusive events. Mutually exclusive means that both events *cannot* happen at the same time.
• 📝 The formula is simple: If event A and event B are mutually exclusive, then the probability of A *or* B happening is: $P(A \text{ or } B) = P(A) + P(B)$.
• 🌱 In genetic crosses, this often applies when calculating the probability of a specific phenotype arising from different possible genotypes.
• 🧬 Remember to first determine the probabilities of each individual genotype that leads to the desired phenotype.
• 💡 Then, *add* those probabilities together to find the overall probability.
• ➗ Individual probabilities are often calculated using Punnett squares.

Practice Quiz

1. A plant can have purple flowers (P) or white flowers (p). Heterozygous plants (Pp) have purple flowers. If you cross two heterozygous plants (Pp x Pp), what is the probability that the offspring will have the genotype *either* PP *or* pp?
1. A) 1/4
2. B) 1/2
3. C) 3/4
4. D) 1


2. In a certain species of bird, the allele for black feathers (B) is dominant to the allele for white feathers (b). If two heterozygous birds (Bb) are crossed, what is the probability that an offspring will have *either* the BB genotype *or* the Bb genotype?
1. A) 1/4
2. B) 1/2
3. C) 3/4
4. D) 1


3. A cross is performed between two pea plants with the genotypes Tt and Tt, where T represents the dominant allele for tallness and t represents the recessive allele for shortness. What is the probability that the offspring will be *either* tall (TT) *or* short (tt)?
1. A) 1/4
2. B) 1/2
3. C) 3/4
4. D) 1


4. In guinea pigs, black fur (B) is dominant to brown fur (b). If you cross two heterozygous guinea pigs (Bb x Bb), what is the probability that the offspring will be *either* homozygous dominant (BB) *or* homozygous recessive (bb)?
1. A) 1/4
2. B) 1/2
3. C) 3/4
4. D) 1


5. Consider a monohybrid cross where one parent is homozygous dominant (AA) and the other is heterozygous (Aa). What is the probability that the offspring will have *either* the AA genotype *or* the Aa genotype?
1. A) 0
2. B) 1/4
3. C) 1/2
4. D) 1


6. Two plants are crossed. One is homozygous recessive for flower color (rr - white) and the other is heterozygous (Rr - red). What is the probability the offspring will be *either* Rr *or* rr?
1. A) 1/4
2. B) 1/2
3. C) 3/4
4. D) 1


7. In a certain flower, red (R) is dominant to white (r). If you cross a heterozygous (Rr) plant with a homozygous recessive (rr) plant, what is the chance the offspring will express *either* the dominant phenotype *or* the recessive phenotype?
1. A) 0
2. B) 1/4
3. C) 1/2
4. D) 1