Punnett Squares
Predict the probability of offspring inheriting specific traits.
Learn
Using Punnett Squares
- Genotype: The genetic makeup (letters like BB, Bb, bb)
- Phenotype: The physical appearance or trait shown
- Homozygous: Two identical alleles (BB or bb)
- Heterozygous: Two different alleles (Bb)
- Dominant: Allele that masks another (capital letter)
- Recessive: Allele masked by dominant (lowercase letter)
Practice
Question 1: Cross Bb x Bb. What percentage of offspring will be homozygous dominant?
Answer
BB = 1/4 = 25% will be homozygous dominant.
Question 2: Cross Bb x Bb. What is the phenotypic ratio?
Answer
3 dominant : 1 recessive (3:1 ratio).
Question 3: Cross BB x bb. What are all possible genotypes of offspring?
Answer
All offspring will be Bb (100% heterozygous).
Question 4: If brown eyes (B) are dominant over blue (b), what genotype produces blue eyes?
Answer
Only bb produces blue eyes (homozygous recessive).
Question 5: Cross Bb x bb. What percentage will show the recessive phenotype?
Answer
50% will be bb and show recessive phenotype.
Question 6: Two heterozygous parents (Bb x Bb) have 4 children. How many would you expect to show the dominant trait?
Answer
Expected: 3 out of 4 (75%) would show dominant trait.
Question 7: What cross produces offspring that are all heterozygous?
Answer
BB x bb produces all Bb offspring.
Question 8: Cross BB x Bb. What is the genotypic ratio?
Answer
2 BB : 2 Bb, simplified to 1:1 ratio (50% each).
Question 9: A plant with purple flowers (Pp) crosses with white flowers (pp). What fraction will have white flowers?
Answer
1/2 will be pp and have white flowers.
Question 10: If tall (T) is dominant over short (t), and two Tt plants are crossed, what is the probability of a short plant?
Answer
P(tt) = 1/4 = 25% probability of short plant.