CBSE Class 10 Science Notes: Heredity and Evolution

Heredity: The Inheritance of Traits

Definition: Heredity is the process by which traits are passed down from parents to their offspring. It’s the reason why children often resemble their parents, inheriting characteristics like eye color, height, and even certain predispositions to diseases.

Role of Genes: Genes are the fundamental units of heredity. They are segments of DNA that contain the instructions for specific traits. These genes are organized on chromosomes. Each individual receives one set of genes from each parent, leading to a unique combination of traits.

Mendel’s Contributions: Unraveling the Secrets of Inheritance

Experiments: Gregor Mendel, often called the “father of genetics,” conducted groundbreaking experiments with pea plants. He carefully observed traits like flower color, seed shape, and plant height over multiple generations.

Dominant and Recessive Traits: Mendel discovered that some traits are dominant (expressed even when paired with a recessive trait), while others are recessive (only expressed when paired with another recessive trait). For example, in pea plants, the allele for purple flowers (P) is dominant over the allele for white flowers (p).

Monohybrid Crosses: A monohybrid cross involves tracking the inheritance of a single trait.

Mendel’s Laws (Simplified):

• Law of Segregation: Each individual carries two alleles for each trait, and these alleles separate during gamete formation (sperm or egg).
• Law of Dominance: In a heterozygous individual (carrying one dominant and one recessive allele), the dominant allele expresses itself.

Example: Consider a cross between a pea plant with purple flowers (PP or Pp) and a pea plant with white flowers (pp).
If the purple flower plant has genotype PP, all offspring will have purple flowers (Pp). If the purple flower plant has genotype Pp, there will be 50% chances that the offspring has purple flower(Pp) and 50% chances that they have white flowers(pp).

Sex Determination: The Chromosomal Blueprint

Chromosomes and Sex: Humans have 23 pairs of chromosomes. One pair, the sex chromosomes, determines the sex of an individual. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).

How Sex is Determined: The mother can only pass on an X chromosome. The father determines the sex of the offspring:

• If the sperm carries an X chromosome, the offspring will be female (XX).
• If the sperm carries a Y chromosome, the offspring will be male (XY).

Evolution: Change Over Time

Concept of Evolution: Evolution is the process by which species change over generations. It’s driven by natural selection, where organisms with advantageous traits are more likely to survive and reproduce, passing those traits on to their offspring.

Overview: Evolution explains the diversity of life on Earth and how different species are related. It happens over long periods.