Monohybrid Crosses


A genetic cross is a means of determining the genetic characteristics of potential offspring based on the genetic characteristics of the prospective parents

A monohybrid cross determines the allele combinations of offspring for one particular gene only (as opposed to dihybrid crosses)


Performing a Monohybrid Cross

Monohybrid crosses can be calculated according to the following steps:

Step 1:  Designate characters to represent the alleles 

  • Capital letter for dominant allele, lower case letter for recessive allele


Step 2:  Write down the genotype and phenotype of the parents 

  • This is the P generation (parental generation)


Step 3:  Write down the genotype of the parental gametes 

  • These will be haploid as a result of meiotic division


Step 4:  Use a Punnett grid to work out the potential gamete combinations 

  • As fertilisation is random, all combinations have an equal probability


Step 5:  Write out the genotype and phenotype ratios of potential offspring 

  • This is the F1 generation (first filial generation)
  • Subsequent generations through interbreeding labeled F2, F3, etc.


Note:  The genotypic and phenotypic ratios calculated are only probabilities


Examples of Monohybrid Crosses

Genotype and phenotype ratios can be determined for different patterns of inheritance using a monohybrid cross

It is important to note that these ratios reflect probabilities and do not guarantee actual proportions in offspring


Autosomal Dominance / Recessive

  • Choose a letter where the upper and lower case forms are easily distinguishable (e.g. E/e, A/a, B/b)
  • Use the capital letter for the dominant allele and the lower case letter for the recessive allele
  • Example:


Codominance

  • Choose a letter to denote the general trait encoded by the gene (capital = dominant, lower case = recessive)
  • Use different superscript letters (capitals) to represent the different codominant alleles
  • Example:


X-linked Recessive

  • Use a capital "X" to denote the X chromosome
  • Choose a superscript letter to represent the trait (capital = dominant, lower case = recessive)
  • Example: