In humans, the genes for detached ear lobes, the ability to taste PTC, and tongue curling (the ability to curl the tongue into a U shape) follow simple dominant Mendelian inheritance. Detached earlobes are dominant to attached earlobes, PTC tasting is dominant to not being able to taste PTC, and tongue curling is dominant to being unable to curl the tongue. Two individuals who are heterozygous for all three traits have a child. What is the probability that the child will be homozygous dominant for all three traits?
- 1/8
- 1/16
- 1/32
- 1/64
Click for Explanation
This question tests your ability to calculate the probability of a possible genotype resulting from a trihybrid cross between two heterozygotes. One method to determine the probability is to make a Punnett square. An organism that is heterozygous for three genes can produce eight different gamete possibilities. Thus, resultant Punnett square would contain an 8×8 table with a total of 64 genotype combinations. A more efficient method is to calculate the probability of each event occurring separately and then use the rule of multiplication to determine the likelihood of all three events occurring at the same time.
Probability of obtaining an AA genotype from a cross between Aa and Aa = ¼
Probability of obtaining a BB genotype from a cross between Bb and Bb = ¼
Probability of obtaining a CC genotype from a cross between Cc and Cc = ¼
Total probability of all three possibilities = ¼ x ¼ x ¼ = 1/64
Therefore, D is the correct answer.
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