[PEP] High School Biology • Compulsory Vol. 2: Continuity and Variation of Life

The transmission of life across generations is no mere act of replication — it is an exquisitely choreographed dance unfolding at the microscopic level.减数分裂 (meiosis) is not only a process that halves the chromosome number, but also the fundamental architecture ensuring both continuity (stability) and diversity (variation) of life. In specific organs such as thetestes (specifically theseminiferous tubules) or theovaries, organisms carry out this remarkable process.

1. Macro-Level Necessity: Weismann's Prediction

As Weismann predicted: during the maturation of sperm and egg cells, a special process must halve the chromosome number; otherwise, chromosome numbers would undergo an exponential catastrophe across generations. In essence,meiosis is a specialized form of mitosis, ensuring genetic stability through one round of chromosome replication followed by two successive cell divisions.

2. The Interplay of Genes, Chromosomes, and the Environment

Confirmed by Morgan through his fruit fly hybridization experiments:genesare located on chromosomes and determinephenotypes. However, microscopic genetic processes are highly susceptible to macro-level environmental influences. For instance,tobacco smoke contains thousands of chemicals, many of which can disrupt meiosis and impair the formation of sperm and egg cells, thereby contributing to conditions such as bronchitis, hypertension, and even lung cancer.

3. Sources of Genetic Diversity

Thecrossing overof homologous chromosomes and the independent assortment of non-homologous chromosomes during meiosis produce offspring with diversegenotypes. For example, when breeders select wheat resistant to stripe rust in test fields (Fig. 1-10), they are essentially harnessing the genetic recombination generated by meiosis to screen for desirable phenotypes.

Case Study: Giant Panda Genetics

A giant panda's somatic cells have 42 chromosomes. Through meiosis, its sperm or egg cells contain only 21. This precise halving mechanism ensures each generation of pandas maintains 42 chromosomes, while genetic recombination gives every panda its unique "personality" and "appearance."

QUESTION 1

[Short Answer] If gametes were formed through mitosis — just like somatic cell proliferation — what would be the consequence?

QUESTION 2

[Multiple Choice] A giant panda's somatic cells have 42 chromosomes. Determine whether each statement is correct. A. Sperm produced by meiosis contain 21 pairs of homologous chromosomes B. The zygote develops into a mature individual through mitosis, maintaining 42 chromosomes C. Chromosomes replicate twice during meiosis to ensure accurate numbers D. Genetic diversity in pandas arises solely from the randomness of fertilization

QUESTION 3

[Fill in the blanks] Summary: Every organism maintains genetic stability while also exhibiting genetic diversity during reproduction. From the perspectives of both gamete formation and fertilization, provide a comprehensive summary of the reasons for genetic stability and diversity.

QUESTION 4

[Short Answer] What is the significance of sperm cell remodeling (spermiogenesis) for fertilization?

QUESTION 5

[Short Answer] The gene controlling red-green color blindness is located on the sex chromosomes and follows a recessive inheritance pattern. Why is this condition far more common in males than in females?