【PEP Edition】Junior High Biology, Grade 8, Vol. 2: The Command Center of Life Traits

If we compare a living organism to a magnificent building, then the nucleus is its"Central Archive"or "Command Center." Here, life's blueprint is tightly organized and protected. The traits of an organism—whether it's your single or double eyelids, or the sweetness of a fruit—are all controlled by the instructions stored within.

Core Command Hierarchy

Chromosome: Substances in the cell nucleus that can be stained dark bybasic dyes; they are the primary carriers of genetic material.
DNA: The primary genetic material, characterized by its uniquedouble helix structure (as shown in Figure 7-12).
Gene: A segment of DNA withhereditary effects; it is the smallest unit controlling an organism's traits.

From Instructions to Expression

Imagine the nucleus as a large archive, where chromosomes are scrolls made of paper (DNA) and spools (proteins). The specific passages of text on the paper aregenes. Traits do not arise from nothing; they are the result of genes directing protein synthesis. In somatic cells, chromosomes exist in pairs, so genes also existin pairs. When genes are altered through special means (such as space mutagenesis), an organism's traits change accordingly.

Key Distinction: The Nature of Variation

Variation caused solely by the environment, without affecting the genetic material (genes), will not be passed to offspring. This is callednon-heritable variation. Only when the genetic material itself changes does the variation have the potential to be transmitted across generations.

QUESTION 1

Which of the following statements about the relationship between genes, DNA, and chromosomes is INCORRECT?

Each DNA molecule contains many genes.

Each chromosome contains many DNA molecules.

Each chromosome contains many protein molecules.

A gene is a segment of DNA with hereditary effects.

QUESTION 2

Which of the following statements about biological traits is INCORRECT?

Every organism has many different types of traits.

People always distinguish different organisms by their traits.

All biological traits are characteristics observable with the naked eye.

The closer the genetic relationship between organisms, the more similar traits they share.

QUESTION 3

Determine whether the following statements are correct: (1) Can the skin color difference between identical twin brothers (caused by the environment) be inherited? (2) Is a normal-colored Bengal tiger giving birth to a white cub a case of variation?

(1) Yes; (2) Yes

(1) No; (2) Yes

(1) No; (2) No

(1) Yes; (2) No

QUESTION 4

The father is left-handed, the mother is right-handed, and they have a right-handed child. Which of the following judgments is correct?

Right-handedness is the dominant trait (√); The child has no left-handed gene (×)

Right-handedness is the recessive trait (×); The child has no left-handed gene (√)

QUESTION 5

The chromosome numbers in the somatic cells of corn, rice, and horses are 20, 24, and 64, respectively. The number of chromosomes in their sperm cells are:

10, 12, 32; exist in pairs

10, 12, 32; no longer in pairs

20, 24, 64; no longer in pairs

Case Study: The Tragedy of Dream of the Red Chamber and the Science of Breeding

Analyzing Literature and Agricultural Production Through Genetic Principles

Scenario 1: In the novel "Dream of the Red Chamber," Lin Daiyu and Jia Baoyu are close relatives (cousins from the father's side). Scenario 2: Agricultural experts cross high-yield, lodging-prone wheat with low-yield, lodging-resistant wheat to breed a new variety that is both high-yield and lodging-resistant.

1. From a scientific perspective, is it appropriate for Baoyu and Daiyu to marry? Please explain your reasoning (approximately 150 words).

Answer:
It is not appropriate. From a genetic perspective, Baoyu and Daiyu are collateral relatives within three generations. Close relatives share more common ancestors, so the probability of them carrying the same recessive disease-causing genes is significantly higher than that of unrelated individuals. Although these disease-causing genes do not manifest symptoms in the heterozygous state, if close relatives marry, the chance that both pass the same recessive pathogenic gene to their offspring increases dramatically. When the offspring's genotype becomes homozygous recessive (aa), the genetic disorder will be expressed. Therefore, prohibiting consanguineous marriage aims to reduce the incidence of recessive genetic diseases and ensure the health of offspring. This scientific basis is also the foundation of the marriage law in China that prohibits such marriages.

2. What is the principle behind breeding the new high-yield, lodging-resistant wheat variety in Scenario 2?

Answer:
This utilizes the principle of cross-breeding. Through the process of sexual reproduction, the genes controlling "high yield" and "lodging resistance"—two desirable traits—are recombined into a single individual, thereby cultivating a new variety that combines the advantages of both parents. This variation is caused by the recombination of genetic material and constitutes heritable variation.