Science: Class 10 — Chapter 8: Heredity

8.1 Accumulation of Variation during Reproduction

Reproduction & Variation: Reproductive processes give rise to new individuals that are similar, but subtly different.
Asexual Reproduction: Generates very minor differences because of small inaccuracies in DNA copying (e.g., in bacteria).
Sexual Reproduction: Maximises the number of successful variations and generates greater diversity.
Survival Advantage: Depending on the nature of variations, different individuals have different advantages (e.g., heat-resistant bacteria survive heat waves). Selection by environmental factors forms the basis of evolution.

8.2 Heredity

Inherited Traits: Features passed from parents to offspring (e.g., free vs attached earlobes), though human populations exhibit a wide range of variations.
Heredity: The mechanism through which traits and characteristics are reliably inherited across generations.

Equal Contribution: Both the father and mother contribute practically equal amounts of genetic material (DNA), providing two versions of each trait to the offspring.
Dominant vs Recessive: Traits like tallness (T) are dominant and require only one copy to be expressed, while recessive traits like shortness (t) require both copies to be expressed.
Mendel's Experiments: Gregor Mendel crossed tall and short pea plants. The first-generation (F₁) progeny were all tall, while the second-generation (F₂) had a 3:1 ratio of tall to short plants.
Independent Inheritance: Breeding pea plants with two different traits (e.g., seed shape and colour) showed that traits like tall/short and round/wrinkled seeds are inherited independently.

Gene: A section of cellular DNA that provides information for making a specific protein.
Protein Control: Genes control characteristics (traits) by directing the synthesis of specific proteins or enzymes (e.g., enzymes that trigger growth hormones to make a plant tall).
Chromosomes: Gene sets are present as separate independent pieces called chromosomes. Each cell has two copies of each chromosome, one inherited from each parent.
Germ Cells: Specialized cells that carry only one set of chromosomes so that when male and female gametes combine, the normal number of chromosomes is restored in the zygote.

Environmental Cues: In some species (e.g., reptiles), sex is determined by environmental temperature during egg incubation. In snails, individuals can change sex.
Genetic Determination: In humans, sex is genetically determined. Out of 23 pairs of chromosomes, 22 are identical, while the 23rd pair consists of sex chromosomes.
Sex Chromosomes: Females have a perfect pair of X chromosomes (XX). Males have a mismatched pair: one normal-sized X and one short Y chromosome (XY).
Paternal Influence: The mother always contributes an X chromosome. The sex of the child depends on whether the father contributes an X chromosome (resulting in a girl, XX) or a Y chromosome (resulting in a boy, XY).