A level Biology exam revision resources written by A level Examiners


Monohybrid inheritance is the most straightforward to understand, it is the study of the inheritance of one characteristic. The terms introduced in this section will be used throughout the topic of inheritance, so it is important to understand the different terms before moving on.

  • gene = a length of DNA which codes for one characteristic (e.g. eye colour)
  • allele = a form of a particular gene (e.g. the gene for eye colour has blue and brown alleles)
  • dominant = an allele which is always expressed
  • recessive = an allele which is only expressed when present in its homozygous state.
  • homozygous = when a diploid organism carries two identical alleles.
  • heterozygous = when a diploid organism carries two different alleles
  • parental generation = the organisms used at the start of a study of inheritance
  • first filial (F1) generation = the offspring of the parental generation
  • second filial (F2) generation = the offspring of the F1 generation
  • self cross = when the offspring are bred with themselves
  • gametes = sex cells (e.g. pollen or ova)
  • phenotype = the observed characteristics of an individual (e.g. brown eyed)
  • genotype = the genetic make up of an individual (e.g. one allele for brown eyes and one allele for blue eyes)

The way genetic crosses are displayed is fairly standard. The following example introduces the conventions followed.

Example: Height in pea plants

  • Mendel first interbred tall pea plants, and dwarf pea plant, to establish pure breeding parents.
  • He then crossed the tall plants with the dwarf plants
  • The plants of the F1 generation were all phenotypically tall
  • Mendel then crossed the F1 generation with themselves
  • He found that the F2 generation plants were 75% tall and 25% dwarf
  • Mendel concluded that the F1 plants must have contained the allele for dwarfism, but not expressed it so the allele for tall plants is dominant and the allele for dwarf plants is recessive.

These observations can be explained with the use of genetic diagrams.




The Punnett squares show the theoretical outcome of genetic crosses. In the event of an actual cross between two organisms, there is an equal probability that any of the outcomes can occur. This means that the ratios predicted by the Punnett's squares will probably only be achieved if large numbers of organisms are used. Mendel carried out experiments on monohybrid inheritance with approximately 1000 pea plants, and he found the ratios of the F2 generation were almost exactly 75% dominant characteristic and 25% recessive characteristic.

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