2024USABO竞赛临近~往年真题打卡练习（2）

Location of Photosynthetic Reactions: Which statement below is TRUE about the localization of the photosynthetic reactions within the chloroplast?

A.  The Calvin-Benson cycle occurs in the thylakoid interior.

B.  Photosystems I and II are built into the inner chloroplast membrane.

C.  Protons diffuse through ATP synthases from the outside to the inside of the thylakoids.

D.  Carbon fixation occurs in the stroma.

E.  ATP is formed in the interior of the thylakoids.

考察对于Photosynthesis在chloroplast内发生的位置和作用细节，Calvin cycle发生于stroma

故选择D，其余均不正确。

Which of the following is TRUE about CD4+ lymphocytes?

A. They possess MHC class II receptors on their surface.

B. They mature in the lymph nodes.

C. They interact with antigen presenting cells via T cell receptors.

D. They possess both B and Toll-like receptors.

E. They are agranulocytes and mature in the bone marrow.

CD4+ cell属于 Helper T cell 成熟于thymus 具有 MHC II receptor

故选A

Harry found a bunch of interesting plants growing in his backyard of varying heights. He recently learned about polygenic traits and additive genetics in his biology class. To test these theories, he crossed the tallest plant in his garden, which was 20 inches tall, with the shortest plant, which was 8 inches tall. All the F1 offspring from this cross were 14 inches tall. Harry then crossed two F1 plants, and the F2 generation had the distribution of heights shown below. How many genes are responsible for determining the height of Harry’s plants?

A. 1.

B. 2.

C. 3.

D. 4.

E. 5.

考察Quantitative trait，根据表格所示共有7种phenotypes以normal distribution的frequency构成，判断出从低到高为+的allele数量为0-6，因此最高的20 inch F2 genotype 应为+++/+++，

故一共3 pairs of allele，选C

Judson is studying the heredity of various traits in octopuses. First, he crosses a true breeding Blue Happy octopus with a true breeding Green Angry octopus and gets an F1 generation of 100% Blue Happy octopuses. When he crosses the F1 generation with itself, however, he gets the following offspring counts for each set of traits:

If the Blue/Green gene and Happy/Angry gene are at two separate loci on the same chromosome, which of the following numbers is closest to the recombination frequency between these two genes?

A. 0.06.

B. 0.12.

C. 0.18.

D. 0.24.

E. 0.30.

考察Crossing over相关计算，根据Recombination frequency = 

故选B

Viral variants, such as influenza or SARS-CoV-2 variants that infect humans pose a major public health threat because they can weaken or evade immune responses. Such a variation exemplifies evolution by natural selection. Which of the following best describes how viral variants evolve?

A. Viruses survive and replicate themselves, just like cellular organisms.

B. Certain viruses, such as RNA viruses, have high natural rates of mutations, and are also subject to recombination events that alter their genetic sequence.

C. Some viruses evolve ways to infect cells and replicate independently of cellular mechanisms such as transcription or translation.

D. All types of viral variants arise by recombination when they jump between different host species.

E. Viral variants usually arise when viruses are exposed to antibiotics and become resistant.

考察Viruses的变异机制，covid-19属于RNA virus，不具备cell structure并且依靠central dogma之内的机制进行reproducing，不具备bacterial metabolism不受antibiotics的inhibition，具备不同strain之间的recombination能力，

综上选择B

Dr. Zhao is studying a population of llamas using Hardy-Weinberg Equilibrium Principle. These llamas have a special gene that completely controls eye color, such that the dominant phenotypic trait is brown eye color, and the recessive phenotypic trait is blue eye color. There are initially 500 llamas, 480 of which have brown eyes and 20 with blue eyes. If, after 20 years, the llama population has expanded to a size of 2000 llamas while remaining in Hardy-Weinberg Equilibrium, how many llamas are expected to be heterozygous for the eye color gene?

A. 280.

B. 360.

C. 480.

D. 640.

E. 920.

考察Hardy-Weinberg Equilibrium，根据p2 + 2pq + q2 = 1，当 ，

2pq=640

故选D