Heredity

Key Notes:

Definition of Heredity:

  • Heredity is the process by which traits or characteristics are passed down from parents to their offspring through genes.
  • It is the reason why children often resemble their parents and have certain physical or behavioral traits.

Genes and Chromosomes:

  • Genes are the basic units of heredity, made up of DNA. They carry instructions for the development of traits.
  • Chromosomes are structures that contain genes. Humans have 46 chromosomes (23 pairs), with one set inherited from each parent.

Mendelian Inheritance:

  • Gregor Mendel, known as the “Father of Genetics,” discovered the basic principles of heredity through experiments with pea plants.
  • Mendel’s Laws:
    • Law of Segregation: Each organism carries two alleles for a trait, which separate during the formation of gametes (sperm and egg).
    • Law of Independent Assortment: Genes for different traits are passed independently of one another.

Alleles and Dominance:

  • Alleles are different versions of a gene. They can be dominant (expressed even if only one copy is present) or recessive (expressed only if two copies are present).
  • An individual can be homozygous (having two identical alleles) or heterozygous (having two different alleles) for a trait.

Genotype and Phenotype:

  • Genotype refers to the genetic makeup of an organism (e.g., TT, Tt, or tt).
  • Phenotype refers to the observable traits or characteristics (e.g., tall or short height).

Punnett Square:

  • A tool used to predict the probability of offspring inheriting certain traits.
  • It shows how alleles from each parent can combine during fertilization.

Sex Determination:

  • The sex of an offspring is determined by the combination of sex chromosomes.
  • Females have XX chromosomes, while males have XY chromosomes. The father’s sperm determines the sex of the child.

Variations in Traits:

  • Continuous Variation: Traits like height and skin color show a range due to the influence of multiple genes (polygenic inheritance).
  • Discontinuous Variation: Traits that fall into distinct categories, such as blood type.

Mutations:

  • Mutations are sudden changes in the DNA sequence that can affect genes and lead to new traits.
  • Some mutations can be harmful, beneficial, or have no effect.

Importance of Heredity in Evolution:

  • Heredity provides the genetic variation necessary for evolution.
  • Natural selection acts on variations, leading to the survival of organisms best adapted to their environment.

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