3.4  Inheritance

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Essential Idea:

The inheritance of genes follows patterns


  • Mendel discovered the principles of inheritance with experiments in which large numbers of pea plants were crossed
  • Gametes are haploid so contain only one allele of each gene
  • The two alleles of each gene separate into different haploid daughter nuclei during meiosis
  • Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the same allele or different alleles
  • Dominant alleles mask the effect of recessive alleles but co-dominant alleles have joint effects
  • Many genetic diseases in humans are due to recessive alleles of autosomal genes, although some genetic diseases are due to dominant or co-dominant alleles
  • Some genetic diseases are sex linked
  • The pattern of inheritance is different with sex-linked genes due to their location on sex chromosomes
  • Many genetic diseases have been identified in humans but most are very rare
  • Radiation and mutagenic chemicals increase the mutation rate and can cause genetic diseases and cancer


  • Inheritance of ABO blood groups
  • Red-green colour blindness and haemophilia as examples of sex-linked inheritance
  • Inheritance of cystic fibrosis and Huntington’s disease
  • Consequences of radiation after nuclear bombing of Hiroshima and accident at Chernobyl


  • Construction of Punnett grids for predicting the outcomes of monohybrid genetic crosses
  • Comparison of predicted and actual outcomes of genetic crosses using real data
  • Analysis of pedigree charts to deduce the patterns of inheritance of genetic diseases