Living organisms acquire their genes from their parents by inheriting chromosomes. The transmission of traits from one generation to to the next is called inheritance, or heredity. This process allows offspring to resemble their parents more than they do closely resemble related individuals of the same species.


Coded information is transfered from parent to offspring in the form of hereditary units called genes. Genes are segments of DNA. DNA is composed of a chain of nucleotide bases, which code for specific traits. Inherited information is passed on in the form of each gene's specific sequence of nucleotides. Genetic information can either be passed through asexual reproduction or sexual reproduction.

I. Asexual Reproduction

  • one parent produces genetically identical offspring by mitosis
  • DNA is copied and allocated equally to two daughter cells
  • genomes of the offspring are virtually IDENTICAL copies of the parent's genome
  • usually occurs within eukaryotes, or single-celled organisms

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II. Sexual Reproduction

  • results in greater variation than in asexual reproduction
  • two parents give rise to offspring
  • unique characteristics from both parents are combined
  • offspring produced will vary genetically from their siblings and both parents
  • result is cells with a haploid number of chromosomes ----

III. Fertilization

  • sexual life cycles differ in the timing of meiosis in relation to fertilization
  • multicellular organisms may be diploid (as in animals), may be haploid (as in most fungi), and may alternate between diploid and haploid generations (as in plants)

IV. Meiosis

While mitosis conserves chromosome count, meiosis reduces the chromosome number by half. This occurs to compensate for doubling of chromosomes that occurs during fertilization. In animals, meiosis occurs in the male and female sex organs, meaning in either the ovaries or the testes. Meiosis is is preceded by the replication of chromosomes. This single replication is followed by two consecutive cell divisions, known as Meiosis I and Meiosis II. These series of divisions result in four daughter cells, each with half as many chromosomes as the mother cell.

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Steps of Meiosis

  • Interphase: Before meiosis begins, genetic material is duplicated.
  • First division of meiosis
    • Prophase 1: Duplicated chromatin condenses. Each chromosome consists of two, closely associated sister chromatids. Crossing-over can occur during the latter part of this stage.
    • Metaphase 1: Homologous chromosomes align at the equatorial plate.
    • Anaphase 1: Homologous pairs separate with sister chromatids remaining together.
    • Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.
  • Second division of meiosis: Gamete formation
    • Prophase 2: DNA does not replicate.
    • Metaphase 2: Chromosomes align at the equatorial plate.
    • Anaphase 2: Centromeres divide and sister chromatids migrate separately to each pole.
    • Telophase 2: Cell division is complete. Four haploid daughter cells are obtained.


V. Sexual Life Cycles

Genetic variation among a population's members is the raw material for evolution by natural selection. Sexual reproduction and mutations generate this variation.

Genetic variation among offspring may occur in several events:
  1. independent assortment of chromosomes during meiosis I
  2. crossing-over between homologous chromosomes during meiosis I
  3. random fertilization of ova by sperm