Chapter 8: Heredity
Introduction
Heredity is the transmission of characteristics from parents to offspring. It is responsible for similarities between generations and also introduces variations, which play a key role in evolution. The study of heredity and variation is called genetics.
The basic principles of heredity were first studied by Gregor Mendel, who is known as the Father of Genetics. His experiments on pea plants led to the formulation of the laws of inheritance.
Inheritance of Traits
Traits are characteristics or features of an organism, such as height, eye color, or flower shape. These traits are passed from one generation to another through genes, which are present in DNA.
A. Variation and its Importance
- Variation refers to the differences among individuals within a species.
- It occurs due to genetic recombination, mutation, and environmental factors.
- Variation is essential for natural selection and evolution.
Mendel’s Experiments
Mendel conducted experiments on pea plants to study the inheritance of traits. He chose pea plants because they had contrasting traits, short life cycles, and easy cross-pollination.
A. Mendel’s Laws of Inheritance
- Law of Dominance: One trait (dominant) masks the effect of another (recessive) when both are present.
- Law of Segregation: Each organism carries two alleles for a trait, which separate during gamete formation.
- Law of Independent Assortment: Different traits are inherited independently.
Diagram: Mendel’s Experiment on Pea Plant
Sex Determination in Humans
The sex of a child is determined by the chromosomes inherited from the parents.
- Males have XY chromosomes (can pass either X or Y chromosome).
- Females have XX chromosomes (can pass only X chromosome).
- If the child inherits X from father, it will be a girl (XX).
- If the child inherits Y from father, it will be a boy (XY).
Diagram: Sex Determination in Humans
Evolution and Inherited Traits
Some traits are inherited, while others develop due to environmental factors. Evolution occurs when beneficial traits become common in a population over generations.
A. Difference Between Inherited and Acquired Traits
- Inherited Traits: Passed from parents to offspring (e.g., eye color, hair type).
- Acquired Traits: Developed due to environment or lifestyle (e.g., bodybuilding, scars).
Diagram: Inherited vs. Acquired Traits
Speciation
Speciation is the process by which new species evolve from existing species due to genetic variations and environmental changes. It occurs due to natural selection, genetic drift, and geographical isolation.
Factors Leading to Speciation
- Genetic Drift: Random changes in the frequency of alleles in a population.
- Natural Selection: Favorable traits get passed on, helping species survive.
- Geographical Isolation: Physical separation prevents gene flow between populations.
Diagram: Process of Speciation
Evolution and Classification
Evolution explains how species change over time and how organisms are classified based on common ancestry.
A. Fossils as Evidence of Evolution
- Fossils are remains of organisms that lived millions of years ago.
- They show progressive changes in species over time.
- Example: Archaeopteryx, a fossil that shows a link between reptiles and birds.
Diagram: Fossil Evidence of Evolution
B. Homologous and Analogous Organs
- Homologous Organs: Same structure, different function (e.g., human hand and bat wing).
- Analogous Organs: Different structure, same function (e.g., bird wing and insect wing).
Diagram: Homologous vs. Analogous Organs
Human Evolution
Human evolution is the process of gradual development from ape-like ancestors over millions of years. It is supported by fossil evidence, DNA studies, and anatomical similarities.
Stages of Human Evolution
- Australopithecus: Early ancestors who walked on two legs.
- Homo habilis: First tool-making human species.
- Homo erectus: Used fire and had a larger brain.
- Homo sapiens (Modern Humans): Developed culture, language, and technology.