The material basis of genetic variation and the mystery of human diversity

Most variations in humans are due to different combinations of genes inherited from parents. Each child receives half of their genes from their father and the other half from their mother. Although the number of genes each child receives is the same, the content differs, making each child a unique individual, different from their parents and siblings, thus creating individual differences. It is precisely because of variation that humanity has so many ethnic groups. People can easily distinguish one person from another in a crowd. Without variation, everyone would look the same, leading to numerous social problems. Besides physical differences, variation also includes variations in the basic building blocks of the body-proteins. Each person has their own unique proteins. Therefore, if skin or organs are transplanted from one person to another, rejection will occur because their proteins are different.

In addition, there is another type of variation: mutations in genetic genes. These mutations are often induced by environmental conditions and can be passed on to the next generation. Many gene mutations can result in genetic diseases.

In short, heredity and variation are two inseparable aspects of genetic phenomena. We inherit genetic material from our parents, ensuring that basic human characteristics remain unchanged over time. However, variations constantly occur during the process of inheritance, and each person develops and grows in different environments, thus creating the diverse human population.

**Youthful exuberance, the most vibrant of all things, the fiercest masculine spirit**

As the smoke dissipates with the morning light, the lake beside my pillow has dried up.

Expectation has degenerated into waiting, and I have bid farewell to the sudden.

When tear stains outline regrets, memories exaggerate sadness, and flowing water symbolizes the passage of time.

Finally, we no longer revel in life or go crazy for love.

However, the vibrant summer of youth is about to unfold day by day.

Mayday's song "Poetry of Post-Adolescence" uses beautiful language to record our youth: anticipation, waiting, farewell, regret, and memories all slowly unfold during each of our adolescence, constructing a passionate and grand period in our lives. Adolescence is the golden period of brain development and also the period with the strongest memory. During this time, teenagers' brains are highly excited, they are easily impulsive, and possess strong explosive power. At home, they always try their best to break free from parental control, discard their "crutches," and "walk" independently, feeling that they are already upright men.

If life is like the four seasons, then adolescence is the vibrant spring, a time of flourishing life. After entering puberty, males experience rapid physical growth and development, especially with the secretion of large amounts of male hormones and the gradual maturation of sexual function. They begin to develop secondary sexual characteristics. These changes in physical appearance and the onset of sexual maturity inevitably lead to subtle psychological changes and the emergence of special emotions. Sexual awareness quietly opens their tightly closed hearts, accompanied by excitement, curiosity, shyness, and doubt as they face the "challenge" from the opposite sex.

Adolescence is a crucial period of rapid growth and development, the second peak of growth and development after infancy. The age of onset, rate of development, and age of maturity vary greatly among individuals and may differ geographically or ethnically. Typically, males enter puberty between the ages of 12 and 19. Many physiological changes occur during puberty; besides the significant development of sex organs, the most obvious changes are rapid physical growth and the development of secondary sexual characteristics. Accompanying these physiological changes are also significant psychological and behavioral alterations. Adolescence is the transitional period from childhood to adulthood. The dividing line between adolescence and childhood is sexual maturation.

The characteristics of puberty development are mainly reflected in the tremendous changes in morphology, function, sex organs, and secondary sexual characteristics, especially the rapid development of the reproductive system, which gradually reaches maturity.

During puberty, males typically experience a rapid increase in height. Before puberty, they grow 3-5 centimeters per year. After entering puberty, the rate of height growth accelerates significantly. Males can grow up to 12 centimeters per year, while those who grow less often still grow 6 centimeters per year. Generally, adolescent males gain 5-6 kilograms per year, but some who develop rapidly can even gain 8-10 kilograms per year.

In addition, men's lung capacity and grip strength also increase with age. Lung capacity increases rapidly during puberty. The lung capacity of a 10-year-old child is 1400 ml, which increases to 2500 ml by the age of 14-15, and can reach about 4800 ml by young men around the age of 20.

When males enter puberty, their brain and nervous system functions rapidly mature, nerve cells further differentiate and mature, their thinking and comprehension abilities are enhanced, their responsiveness to stimuli improves, they can acquire a wider range of knowledge and skills, and their memory capacity is strengthened, enabling them to remember things more coherently and systematically. However, because the brain and nervous system are still developing and not fully mature, they lack stamina. Therefore, it is essential to cultivate perseverance and endurance during this period.

Now that we know about the changes in morphology and function in adolescent males, what changes occur in their male characteristics?

Male sexual characteristics include external genitalia and secondary sexual characteristics. During puberty, due to the influence of male hormones, males begin to experience voice changes, develop an Adam's apple, and grow facial hair, armpit hair, and pubic hair; these physical changes are called "secondary sexual characteristics." Sex organs, including the testes, epididymis, seminal vesicles, prostate, and penis, develop slowly before the age of 10, but accelerate during puberty. On average, males experience their first ejaculation around age 14. Generally, the length of a male penis increases rapidly after age 11, slows down slightly after age 16, and approaches adult size by age 18.

Related studies show that the first ejaculation in Chinese men occurs earlier in the south than in the north, and earlier in urban areas than in rural areas. Those aged 11-14 who experience ejaculation have significantly better physical, physiological, and athletic abilities than those who do not. At the onset of sexual development, about half of males also experience breast development, sometimes on one side, sometimes on both sides, with protruding nipples and a hard lump under the areola, which may be tender to the touch. This condition usually disappears after several months, and these are all normal phenomena.

During puberty, a male's body undergoes a transformative change, fundamentally due to the regulatory role of sex hormone secretion. During puberty, the secretion of male sex hormones increases significantly, with common sex hormones including gonadotropin-releasing hormone (GnRH), luteinizing hormone (LH), and follicle-stimulating hormone (FSH). This increase in sex hormone secretion is regulated by the hypothalamic-pituitary-gonadal axis. Gonadal enlargement is the first change to occur during puberty, preceding any other pubertal changes. Once the gonads begin to develop, various secondary changes induced by puberty follow.