Mechanism of Heredity Transmission - Effect of Heredity on Human Beings

Heredity means the passing of physical and mental traits from parents to children through genes. Every person receives 23 chromosomes from the mother and 23 from the father. Together, they make up the 46 chromosomes that determine a person’s inherited traits.

Even though detailed experiments on humans are difficult, studies on twins and families show that heredity influences how we look, how our bodies function, and sometimes how we think and behave.

1. Physical Features

• Heredity strongly affects features such as skin color, hair type, eye color, height, and facial shape.
• For example, curly or straight hair and the size of the nose or feet depend on the genes a person gets from their parents.
• Nutrition and climate may slightly change the appearance, but cannot alter these genetic foundations.

2. Anatomical Traits

• Our body structure (height, weight, hand size, etc.) depends largely on heredity.
• Twin studies by Newman, Freeman, and Holzinger (1937) found that identical twins look and grow more alike than non-identical twins, even when raised in different homes.
• Example: Children of tall or large-sized parents are usually taller and heavier.
• Some physical defects, like cleft palate or clubfoot, are found more in identical twins, showing the role of genes.
• However, the environment (food, exercise, health) also affects body growth.

3. Physiological Traits

• Heredity influences how our internal systems work, such as blood pressure, pulse rate, and breathing.
• Jost and Sontag’s study on twins showed that physiological functions are more similar in identical twins than in other siblings.
• This means genes affect how the nervous system and body organs function.
• Some people inherit a tendency to get high blood pressure or stress easily.

4. Age of First Menstruation

• The first menstruation (menarche) happens earlier or later, partly due to hereditary influence.
• Daughters of mothers with early menstruation often face the same pattern.
• Environment (nutrition and health) can also change this timing.

5. Longevity

• Life span is also partly hereditary.
• If parents or grandparents live long, their children may also have longer lives, provided they live in a healthy environment.

6. Physical Defects and Diseases

• Some diseases and defects run in families because of faulty genes.
• Examples: Diabetes, hemophilia, color blindness, hearing or visual defects, high blood pressure, and some cancers.
• Some illnesses, like ulcers or allergies, may depend on both hereditary and environmental factors.
• PKU (Phenylketonuria): A hereditary disorder where the child lacks an enzyme needed for normal metabolism, causing brain issues if untreated.
• Down Syndrome (Mongolism): An Extra chromosome (47 instead of 46) causes physical deformity and mental delay.

7. Mental Defects and Retardation

• Many intellectual defects result from inherited problems.
• Infantile amaurotic family idiocy (a rare hereditary disease) and PKU are genetic conditions that affect brain function.
• Chromosome defects like Down syndrome also cause intellectual disability.

8. Mental Disorders

• The role of heredity in mental disorders is still being studied.
• Kallman (1953) studied schizophrenia in twins and found that:
• Identical twins often both have schizophrenia, while fraternal twins rarely do.
• This suggests heredity plays some role, though environment and upbringing also matter.
• Modern psychology views mental illness as a result of both genes and environment.

• Heredity gives the foundation for development.
• Environment shapes what heredity provides.
• Both work together — one cannot function fully without the other.
• Twin and family studies are the main research methods in heredity studies.