HL 1 Biology

4.1 Genes, Chromosomes, Mutations


Genetics – study of variation and inheritance.

Gene – Basic unit of inheritance; a heritable factor that controls a specific character

  • 30,000-40,000 genes in humans
  • genome– the whole of the genetic information of an organism; all of the genes of an organism collectively
  • genes are made of DNA


Chromosomes – organized structure of DNA + protein found in cells

  • eukaryotic chromosome is made of DNA and protein.
  • typical animal/plant chromosome contains: about 1000 genes
  • arranged in linear sequence
  • gene locus – specific position of a gene on a chromosome
  • homologous – when 2 chromosomes have the same sequence of genes; chromosomes usually not identical (alleles differ)
  • allele – a form of a gene, differing from other alleles of the gene by a few bases at most and occupying the same locus as the other alleles of that gene

<<Chromosome numbers>>

  • eukaryote chromosomes – DNA molecules associated with protein
    • visible during mitosis/meiosis
    • condensed (become shorter and fatter by coiling process of histone proteins)
    • in body cells of most eukaryotes: 2 chromosomes of each type – 2 homologous chromosomes = DIPLOID
    • only one chromossome of each type passed on in a gamete
    • Gametes = HAPLOID
    • Meiosis: a special type of division of the nucleus to produce haploid gametes


Gene mutation – genes changing when passed from parent to offspring; a change to the base sewuence of a gene

Base mutation – when one base in a gnee is replaced by another base.

  • Results to diseases
  • e.g. sickle cell anemia



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