SAT - Biology Subject Mendelian and Molecular Genetics Questions and Answers

Question 1

In a dihybrid cross between two pea plants heterozygous for both tallness (T) and purple flowers (P), what is the expected phenotypic ratio of the offspring? (Tall is dominant to short, and purple is dominant to white).

Accepted Answer

9 tall, purple : 3 tall, white : 3 short, purple : 1 short, white

Answer

This is a classic Mendelian dihybrid cross. When two parents that are heterozygous for two independent traits (TtPp x TtPp) are crossed, the offspring exhibit a 9:3:3:1 phenotypic ratio. This ratio represents the combination of the individual 3:1 ratios for each trait.

Question 2

Which of the following enzymes is responsible for unwinding the DNA double helix during replication?

Accepted Answer

Helicase

Answer

Helicase is the enzyme that separates the two strands of the DNA double helix by breaking the hydrogen bonds between complementary base pairs, creating the replication fork. DNA Polymerase synthesizes new DNA strands, DNA Ligase joins DNA fragments, and Primase synthesizes RNA primers.

Question 3

A red-flowered snapdragon is crossed with a white-flowered snapdragon, and all the offspring have pink flowers. This pattern of inheritance is known as:

Accepted Answer

Incomplete dominance

Answer

Incomplete dominance occurs when the heterozygous phenotype is an intermediate blend of the two homozygous phenotypes. In this case, the pink color is a blend of the red and white parental traits. Codominance would result in both red and white being expressed simultaneously (e.g., red and white spots).

Question 4

The lac operon in E. coli is a classic example of gene regulation. The operon is transcribed at a high level under which of the following conditions?

Accepted Answer

Low glucose, high lactose

Answer

The lac operon is an inducible system that is activated to metabolize lactose when it is available and glucose, the preferred energy source, is scarce. Low glucose levels lead to an increase in cAMP, which binds to CAP, enhancing transcription. The presence of lactose (as allolactose) removes the repressor protein from the operator, allowing transcription to proceed.

Question 5

A man with type AB blood and a woman with type O blood have a child. What is the probability that their child will have type A blood?

Accepted Answer

50%

Answer

The man's genotype is I^A I^B, and the woman's genotype is ii. The possible genotypes for their offspring are I^A i (Type A blood) and I^B i (Type B blood). Therefore, there is a 50% chance of having a child with type A blood and a 50% chance of having a child with type B blood.

SAT - Biology Subject Test Practice Test

SAT - Biology Subject Test Practice Test

SAT - Biology Subject Mendelian and Molecular Genetics Questions and Answers